Nutrient Management Is a Vital Part of Successful Field Nursery Crop Production. MACRONUTRIENTS NITROGEN CYCLE Mineralization Nitrification Immobilization Sources of Nitrogen Loss Nitrate Loss FERTILIZER CLASSIFICATION Simple, Compound, and Complete Fertilizers SOURCES OF NUTRIENTS Soluble Nitrogen Phosphorus Potassium Magnesium Sulfur Calcium MICRONUTRIENTS MOBILE ELEMENTS IMMOBILE ELEMENTS FOLIAR ANALYSIS SOIL NUTRIENT ANALYSIS SOIL SAMPLING Soil Sampling Tools Composite Soil Sample Mailing the Composite Soil Sample SOIL NUTRIENT CONTENT VALUES SOIL TEST RESULTS SOIL NUTRIENT STATUS DETERMINED BY SOIL ANALYSIS SOIL pH Favorable pH for Many Plants pH and Nutrient Availability pH Test Kits LIME AND FERTILIZER RECOMMENDATIONS Something to Grow On http://www.cals.cornell.edu/dept/flori/growon/field.html (1 of 3) [10/07/2001 11:17:54 a.m.]
Transcript
Nutrient Management Is a Vital Part of Successful Field Nursery Crop Production.
MACRONUTRIENTS
NITROGEN CYCLE
Mineralization
Nitrification
Immobilization
Sources of Nitrogen Loss
Nitrate Loss
FERTILIZER CLASSIFICATION
Simple, Compound, and Complete Fertilizers
SOURCES OF NUTRIENTS
Soluble Nitrogen
Phosphorus
Potassium
Magnesium
Sulfur
Calcium
MICRONUTRIENTS
MOBILE ELEMENTS
IMMOBILE ELEMENTS
FOLIAR ANALYSIS
SOIL NUTRIENT ANALYSIS
SOIL SAMPLING
Soil Sampling Tools
Composite Soil Sample
Mailing the Composite Soil Sample
SOIL NUTRIENT CONTENT VALUES
SOIL TEST RESULTS
SOIL NUTRIENT STATUS DETERMINED BY SOIL ANALYSIS
SOIL pH
Favorable pH for Many Plants
pH and Nutrient Availability
pH Test Kits
LIME AND FERTILIZER RECOMMENDATIONS
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Nitrogen Application Rates
Phosphorus Application Rates
Potassium Application Rates
CATION EXCHANGE CAPACITY
Nutrient Reserve for Plant Uptake
CEC Values for Various Soil Textures
BUFFERING CAPACITY
EXCHANGE ACIDITY
LIMING TO INCREASE SOIL pH
Liming Materials
Effective Neutralizing Value
Tons of 100% E.N.V. Lime Required to Increase the Soil pH to 6.5
Modifying the Recommended Lime Application Rate
Example
General Lime Recommendations
Example
REDUCTION OF SOIL pH
SOLUBLE SALTS
Soluble Salt Accumulation
Salt Index
TIMING OF FERTILIZATION
Optimal Nutrient Absorption
Active Root Growth
Episodic Growth
Two Annual Growth Flushes
More Constant Growth
FALL FERTILIZATION
Application Rate
Disadvantages
Winter Damage
Possible Advantage
FERTILIZER PLACEMENT
Small Shade Trees and Widely Spaced Trees Planted in Rows
Mature Field Plantings and Closely Spaced Shrubs
FERTILIZER APPLICATION RATE
Form of Nitrogen
Soil Texture
Coarse Textured Soils
Fine Textured Soils
Plant Age and Size
Plant Species
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Higher Level of Maintenance
Strong, Stable Growth
LET IT GROW!
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Content, graphics, and design of Something to Grow On were produced by Amy Fay Kasica.
Funding for this project was provided in part by the Long Island Nurserymen's Association, Inc. and the New York State Nurserymen's Foundation, Inc.
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Keep in mind that some fertilizers may vary in analysis of specific nutrients.
All of these fertilizer sources contribute to a decrease in soil pH.
Other soluble nitrogen sources affect soil pH differently.
Additional soluble nitrogen sources
Additional sources of soluble nitrogen include calcium nitrate (15% N), potassium nitrate (13% N), and sodium nitrate (16% N).
Calcium nitrate and sodium nitrate contribute to an increase in pH; potassium nitrate has a neutral effect on pH.
Sources of phosphorus
Phosphorus is the second element provided in a complete fertilizer.
The amount of phosphorus in a fertilizer is expressed as the percent of P2O5. To determine the actual amount of phosphorus in a particular fertilizer, divide the percent of P2O5 by 2.3.
Sources of potassium
Potassium is the third major element provided in a complete fertilizer.
In the fertilizer analysis, the amount of potassium present is expressed in the form of K2O (potash). To determine the amount of actual K available in a fertilizer, divide the percentage K2O by 1.12.
The following are soluble sources of potassium:
Potassium nitrate contains 39.2% K
Potassium chloride (muriate of potash) contains 53.5% K
Potassium sulfate contains 44.6% K
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The amount of potassium present in a fertilizer may vary depending on the fertilizer source.
Sources of magnesium
Sources of magnesium include:
Dolomitic limestone (11% Mg)
Magnesium sulfate (epsom salts; 16% Mg)
Magnesium oxide (45% Mg)
The magnesium content of liming and fertilizer materials may vary depending on the source.
Effect of common magnesium sources on soil pH
Sources of sulfur
Fertilizers containing sulfur include:
Ammonium sulfate (23% S)
Calcium sulfate (15% S)
Magnesium sulfate (14% S)
Potassium sulfate (17% S)
Single superphosphate (14% S)
Elemental sulfur (30-99% S)
Calcium sulfate (gypsum) and magnesium sulfate (epsom salts) may be applied to the soil without an acidifying effect.
Ammonium sulfate applied in sizable quantities will decrease soil pH.
Keep in mind that some of these fertilizers may vary in analysis of specific nutrients.
Sources of calcium
Irrigation water may contain calcium.
Excess calcium (Ca) may inhibit plant growth and result in deficiency of other elements such as boron (B), iron (Fe), manganese (Mn), copper (Cu), or zinc (Zn). Surplus calcium reduces availability of potassium (K) and magnesium (Mg).
Periodically monitor the level of calcium in irrigation water through chemical analysis.
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Calcium should be incorporated into the soil prior to planting.
Dolomitic and calcitic limestone supply calcium and also raise the soil pH.
Calcium sulfate (gypsum - 22% Ca) may be applied to supply calcium without increasing the alkalinity of the soil.
Other sources of calcium include:
Triple superphosphate (13% Ca)
Calcium nitrate (19 % Ca)
Depending on the source, some materials may vary in the content of certain nutrients.
Macronutrients--a review:
Macronutrients, or macro elements, are required in greater amounts by plants than micronutrients.
Macronutrients include:
Nitrogen (N)
Phosphorus (P)
Potassium (K)
Calcium (Ca)
Magnesium (Mg)
Sulfur (S)
Micronutrients
Micronutrients, or micro elements, are required in smaller amounts by plants than macro elements.
Mobile elements
Because nitrogen, phosphorus, potassium, magnesium, and molybdenum are mobile in the plant, deficiencies of these elements are expressed on older, rather than more recent growth.
This is due to the movement of the elements from more mature growth to newer, juvenile tissue when these elements are in short supply.
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Immobile elements
Because sulfur (S), calcium (Ca), iron (Fe), manganese (Mn), boron (B), copper (Cu), and zinc (Zn) are immobile in the plant, deficiencies of these elements are usually first expressed on juvenile growth.
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Nitrogen Forms and Availability Fluctuate within the Soil
The nitrogen cycle
Nitrogen is essential for plant growth. Factors such as soil moisture, aeration, temperature, pH, and microbial breakdown of soil organic matter influence the availability of nitrogen to plants.
The availability of nitrogen increases through application of inorganic or organic fertilizers to the soil.
Nitrogen availability also increases through bacterial fixation of gaseous nitrogen (N2) to ammonium (NH4+) compounds which can be absorbed by plants.
Mineralization
Nitrogen availability to plants also increases through mineralization.
Mineralization is the process of decomposition and transformation of organic matter (crop residues, manure, etc.) to available forms of inorganic nitrogen (such as NH4+).
Nitrification
Two forms of nitrogen available to plants are nitrate (NO3-) and ammonium (NH4+) nitrogen.
Roots can absorb both of these forms, although many species preferentially absorb nitrate-nitrogen over ammonium-nitrogen. Although nitrate-nitrogen has a greater propensity towards leaching, ammonium-nitrogen is not as readily utilized by most landscape plants. Ericaceous plants are an exception.
Ammonium-nitrogen is often converted to nitrate-nitrogen by microorganisms before absorption through a process called nitrification.
Modification of NH4+ to NO3- depends on the temperature of the soil; transformation proceeds more quickly under warmer soil temperatures (above 50 F).
Nitrification occurs most efficiently when soil pH is between 5.5-6.5.
Nitrification can be completed within two to four weeks.
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Other factors decrease nitrogen availability to plants.
Immobilization is the process in which available forms of inorganic nitrogen (NO3- and NH4+) are converted to unavailable organic nitrogen.
Sources of nitrogen loss
Under conditions of low water availability and high pH, ammonium (NH4+) is converted to volatile ammonia (NH3). This gaseous ammonia is lost into the atmosphere.
Fertilizer runoff is another critical source of nitrogen loss.
Nitrate loss
Other sources of nitrogen loss include leaching of mineral nitrogen (NO3-, NH4+) beyond the root zone of the plant, and denitrification.
Denitrification is a process which involves the bacterial conversion of nitrate-nitrogen (NO3-) to nitrite (NO2-) to nitric oxide (NO), nitrous oxide (N2O), or nitrogen gas (N2) which is subsequently lost from the soil as it is released into the atmosphere.
Decomposition of organic matter low in nitrogen by soil microorganisms and subsequent absorption of N by these organisms contributes to the unavailability of nitrogen for plant uptake.
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Nutrient Analysis Is Key to Successful Plant Production
Foliar analysis
Foliar analysis is a valuable tool for identifying nutrient deficiency and toxicity symptoms.
Sample:
Older and most recent foliage from current season's growth.
Healthy leaves and those expressing nutrient deficiency/toxicity symptoms.
Soil nutrient analysis
Soil nutrient analysis, followed by an appropriate fertilization program is a key element to the successful field production of nursery crops.
Soil test results provide information regarding fertilization and/or liming recommendations and application rates which are necessary to enhance plant growth and quality.
With this information, growers may make informed decisions on fertilization and/or liming programs to best accommodate their needs.
Cultural practices, soil type, yield potential, and crop history influence lime and fertilizer recommendations.
Soil test kits may be purchased from a local Cornell Cooperative Extension agent or ordered from Cornell Nutrient Analysis Laboratories at Cornell University. The testing kit consists of instructions on sampling, an information sheet, plastic bag, cloth mailing bag, and an envelope.
The standard soil test for nursery crops will provide analysis of nitrate-nitrogen (NO3-N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), aluminum (Al), iron (Fe), manganese (Mn), zinc (Zn), organic matter content, and soil pH.
Soil sampling
A field should be divided into areas for sampling such that 5-10 acres is represented by each sample.
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Sections which differ in previous fertilization, manure, or liming practices, or which supported various crops in the past should be sampled individually.
Sample areas which have dissimilar soil colors and crop growth response separately.
Atypical areas should be avoided when collecting soil samples, such as old fence lines, old lime or manure piles, windbreaks, snow fences, wet sites, dead and back furrows, regions near lime-rock roads, and borders between bottomland and gradients.
Soil sampling tools
When collecting soil samples, use a soil probe or auger, if possible. These tools facilitate sampling uniform depths and quantities of soil.
If these tools are unavailable, a spade or shovel may be used.
Composite soil sample
Soil subsamples should be combined thoroughly to make up a representative sample for each area.
Samples should be placed in a clean plastic container, as metal receptacles may lead to erroneous results and paper bags may contaminate the soil.
Mailing the composite soil sample
After thoroughly mixing the composite sample, seal two-thirds to one pint of the sample in the plastic bag provided with the soil testing kit.
This sample should be enclosed in the cloth bag and included in the mailing envelope with the completed information sheet.
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Nutrient Analysis Results Provided in Soil Test Report
Soil nutrient content values
Soil nutrient content values, derived from soil analysis, are expressed in pounds per acre.
These values, indicated in a soil test report, reflect the amount of the element extracted from the sample using a specific extraction procedure.
These values do not indicate the total amount of the element existing in the soil or the amount available to the plant at a particular soil pH.
Soil nutrient extraction procedure
The extraction method utilizes the Morgan solution, a sodium acetate solution with a pH of 4.8 at a concentration of 1 normal (N).
Different extraction procedures utilized by other laboratories may remove varying amounts of the element from the soil sample; therefore, it is important to compare extracted soil nutrient content values only with values classified by the Cornell Nutrient Analysis Laboratory.
Soil test results
This is an example of results obtained from soil nutrient analysis. Soil pH is also determined by soil analysis.
In order to evaluate the nutrient levels extracted by the soil test, soil nutrient content values are categorized into five different levels ranging from very low to very high, as noted in the report.
Other information provided by soil analysis is also indicated in the soil test report.
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Additional information in soil test report
In addition to soil nutrient content values, the exchange acidity and percentage of organic matter in the soil are indicated in the soil test report.
Organic matter contributes to the acidity of the soil as do humus, soluble salts, carbon dioxide, and exchangeable aluminum (Al).
Soil nutrient status determined by soil analysis
The soil nutrient content values are categorized into five different levels. These levels include very low (VL), low (L), medium (M), high (H), and very high (VH).
Iron and manganese are categorized as normal (N) or excess (E).
Soil test values based on analysis from Department of Soil, Crop, and Atmospheric Sciences, Cornell University. Soil test values may differ among laboratories due to different testing procedures.
Soil pH
Soil pH is a measure of the acidity or alkalinity of a soil on a scale of 0 to 14.
A pH below 7 indicates acidity, a pH above 7 is evidence of alkalinity, and a pH equal to 7 signifies neutrality.
Soil pH is a measure of the concentration of hydrogen ions on soil particle surfaces and in the soil solution.
Soil pH range favorable for many plants
pH and nutrient availability
Relationship between soil pH and nutrient availability for mineral soils.
Source of figure: Pettinger, N. A. 1935. Useful Chart for Teaching the Relation of Soil Reaction to Availability of Plant Nutrients to Crops. Virginia Agri. Ext. Bul. 136, 1-19.
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It is very important to monitor soil pH to detect any pH concerns before plant health is affected detrimentally.
With a soil pH test kit, you can easily determine soil pH.
The normal range pH kit detects soil pH in the following spectrum: 5.0 < pH < 7.2.
The wide range pH kit detects soil pH on higher and lower ends of the pH scale than the normal range kit. The wide range kit identifies the following soil pH range:7.2 < pH < 5.0.
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Suggested Application Rates Provided in Soil Test Report
Lime and fertilizer recommendations
Lime and fertilizer recommendations are also provided in a soil analysis report.
Nitrogen application rates
Selected recommended nitrogen application rates for existing nursery stock
To calculate the actual application rate for a particular fertilizer, divide the recommended application rate (lbs. of N per acre) by the percentage of nitrogen in the fertilizer.
Example: For evergreens, if using a fertilizer with a 12 - 4 - 4 (N: P2O5: K2O) analysis, divide the minimum recommended fertilizer application rate of 45 lbs. N/A by 0.12 = 375; the maximum recommended fertilizer application rate of 70 lbs. N/A divided by 0.12 = 583.
The actual fertilizer application rate ranges from 375 - 583 lbs. N/A for a 12 - 4 - 4 fertilizer.
Remember to adjust the fertilizer application rate according to specific soil nutrient analysis recommendations, soil texture, and plant response.
Phosphorus application rates
Application of fertilizers high in phosphorus should take place prior to planting in the field.
Recommended quantities of phosphorus to be added to the soil prior to planting are based on preplanting soil test results
If a preplanting application of fertilizer does not take place, phosphorus containing fertilizers should be added to the soil every year.
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Potassium application rates
Preplant application of potassium is recommended.
These potassium application rates are based upon preplanting soil test results. However, if potassium is not incorporated into the soil prior to planting, these rates may be applied after planting.
Yearly applications of potassium may be required if it is not implemented before planting nursery stock or if the soil is naturally low in potassium supplying power.
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Cation Exchange Capacity (CEC) quantifies the ability of a soil to provide a nutrient reserve for plant uptake.
CEC is the sum of exchangeable cations (positively charged ions) soil can adsorb per unit weight or volume and is usually measured in milligram equivalents per 100 g (meq/100g).
Soil provides nutrient reserve
A large cation exchange capacity is indicative of a soil with a high nutrient holding capacity which can retain nutrients for plant uptake between fertilization periods.
A high CEC aids in nutrient retention against leaching during irrigation and provides a buffer from abrupt fluctuations in soil salinity and pH.
Cations bind loosely to negatively charged sites on soil particles until they are absorbed by plant roots or exchanged for other cations in the soil solution.
Typical cation exchange capacity values for various soil textures
CEC values for various soil textures
Notice that sands do not have a high potential to retain vital plant nutrients; however, organic soils and clays have the ability to retain nutrients to a greater degree.
Source of table: Halbrooks, M. C. 1990 (June). Nutrition of Container and Field-Grown Nursery Crops. Ext. Bul. 138. Cooperative Extension Service, Clemson University.
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Buffering capacity
Buffering capacity is the ability to withstand rapid pH fluctuation. The greater the buffering capacity, the greater the quantity of acid or base which must be incorporated with a material to alter the pH.
Soil types having low buffering capacities include sandy soils containing little clay or organic matter.
Soils exhibiting a high buffering capacity are usually composed of large quantities of mineral clay and organic matter.
Soils with a high buffering capacity necessitate a greater amount of lime to raise the pH than soils with a low buffering capacity.
Exchange acidity
Exchange acidity is a measure of the amount of hydrogen (H+) ions adsorbed to soil particles which can be replaced by another cation.
Exchange acidity is a measure of buffering capacity.
Liming to increase soil pH
Liming increases soil pH.
Through application of lime to the soil, hydrogen ions (H+) are exchanged for calcium or magnesium (Ca2+ or Mg2+) ions on cation exchange sites and the acidity of the soil is neutralized.
Lime migrates slowly through the soil.
Additionally, free hydrogen ions (H+) are taken out of solution resulting in an increase in hydroxyl ions (OH-), contributing to an increase in pH.
The quantity of lime necessary to raise soil pH is dependent upon several factors.
Effective neutralizing value
The ability of a liming material to modify the soil pH within a year is referred to as the effective neutralizing value (E.N.V.).
Calcium carbonate, the reference standard, has an E.N.V. of 100.
If the E.N.V. of a liming material is less than 100, the recommended quantity of lime to apply must be adjusted accordingly.
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Tons of 100% E.N.V. lime
Remember that the exchange acidity, soil pH, and lime application rate recommendations are given in the soil test report.
Rates indicated in the table are tons of 100% E.N.V. lime required to increase the soil pH to 6.5 and are based on an eight inch plow depth.
Source of rates: Lime application rates are from Department of Soil, Crop, and Atmospheric Sciences, Cornell University.
For example, based on a soil pH of 5.3 and an exchange acidity of 14, the recommended application rate of 100% E.N.V. limestone per acre is 5.6 tons, which is indicated in the table.
Modifying the recommended lime application rate
If the plow depth is greater or less than 8 inches, first modify the recommended lime rate according to plow depth.
In such situations, increase or decrease the lime application rate by 0.12 for each extra inch.
The actual amount of lime to apply can be calculated by dividing the recommended lime rate by the E.N.V. of the liming material.
Example: Adjusting the Recommended Lime Application Rate
General lime recommendations
Exchange acidity is not usually calculated during soil analysis when initial soil pH exceeds 6.0.
In such circumstances, or when soil test reports are not accessible, General Lime Recommendations may be used.
These recommendations, based on soil texture, are for increasing the soil pH to 7.0 and are calculated for an eight inch plow depth.
Source of rates: Lime application rates are from Department of Soil, Crop, and Atmospheric Sciences, Cornell University.
An example using general lime recommendations
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The concentration of dissolved ions in the soil water extract is referred to as soluble salts.
Sources of soluble salts include dissolved elements in irrigation water and fertilizer materials which have been incorporated into the soil.
De-icing salts used during the winter are another source of soluble salts.
Soluble salt accumulation
Excessive concentrations of soluble salts may accumulate, especially during summer months, due to increased transpiration, insufficient leaching of salts from the soil solution, and over-fertilization.
Extremely high salt concentrations may inhibit water uptake and lead to chlorosis, scorching of foliage, premature leaf drop, dieback, and inhibition of growth.
Salt index
In order to quantify soluble salts originating from fertilizer materials, a fertilizer salt index has been established.
The salt index indicates the potential of a fertilizer to prevent water absorption from the soil solution by plant roots.
The index indicates the potential of a fertilizer to prevent water uptake by the roots compared to that of an equivalent weight of the standard, sodium nitrate, which has been assigned a salt index value of 100.
Salt index values of common fertilizer sources
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If a fertilizer has a high salt index, the likelihood for desiccation injury due to salt accumulation is greater than if a fertilizer has a low salt index.
Fertilizers with salt index values greater than 100 increase the potential to prevent water absorption by roots from the soil solution more than sodium nitrate.
Such fertilizers have a tendency to cause water to move out of, instead of into, the root cells.
Fertilizers with salt index values greater than 20 should be worked into the soil prior to planting or applied to the soil surface followed by sufficient irrigation. This will reduce the potential of salt injury or "fertilizer burn" attributed to concentrated areas of fertilizer material.
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Apply Fertilizers at Time of Optimal Nutrient Absorption
Timing of fertilization
Fertilizers including nitrogen, phosphorus, potassium, magnesium, and calcium are most optimally applied prior to planting which allows for adequate implementation into the root zone.
Soils with high cation exchange capacities have the ability to retain larger amounts of cations such as K+ (potassium), Ca2+ (calcium), and Mg2+ (magnesium) than soils with low CECs.
Preplant incorporation will allow adequate amounts of these necessary nutrients to be retained.
Optimal nutrient absorption
After a plant has become established, it is important to apply fertilizers at the time of optimal nutrient absorption by the plant, which is dependent upon the growth pattern of the species.
Fall fertilization: Application rate
Any late summer or fall fertilizer applications should be reduced to half of what was incorporated during summer applications.
This allows for adequate cold acclimation, increases in root and shoot dry weight, and root elongation to take place.
Fall fertilization: Disadvantages
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Some research has indicated that nutrients from fertilizers applied in the fall often leach beyond the root zone, contributing to groundwater contamination and an ineffective use of fertilizer materials. Because of this, fall application of soluble nitrogen sources is not recommended.
Fall fertilization: Winter damage
A reduction in the amount of nutrients applied in late summer or fall will decrease the potential of excessive growth late in the season which may predispose the plant to greater winter damage.
Fall fertilization: Advantage
May allow for adequate nutrient concentrations at the onset of spring growth.
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Plants absorb nutrients most effectively when roots are most actively growing, yet shoots are quiescent, i.e. after a growth flush but prior to lignification and bud set on the current season's growth.
By applying nutrients when shoots are not actively flushing, more energy is available for root growth and nutrient uptake.
Fertilization may also take place prior to bud break while the plant appears to be inactive.
Episodic growth
Some plants which exhibit episodic growth, such as forsythia, may benefit from a more continual fertilization program based on soil analysis and plant growth response.
Any spring fertilizer applications should be timed to occur two to three weeks prior to the initiation of new growth.
Two annual growth flushes
Woody plants which have two annual growth flushes should be fertilized twice per year.
Each fertilization should comprise only one half of the recommended rate when making two applications per year.
The first application may be made just prior to bud break and the second in early to mid-June.
Split fertilizer applications:
May meet the needs of additional growth.
Provide adequate nutrition for plants exhibiting root development throughout the growing season.
Minimize risk factors such as leaching, denitrification, and volatilization.
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Fertilize According to Soil Test Report Recommendations
Small shade trees and widely spaced trees planted in rows
Fertilizer materials are most effective when applied to the estimated root zone of the plant.
If soil test results indicate the addition of nitrogen or phosphorus to the soil for established nursery stock, these nutrients may be applied to the surface of the soil.
Potassium is not readily mobile in the soil. If post-plant application of potassium is indicated by soil analysis results, potassium should be incorporated into the upper 8 to 10 inches of the soil to facilitate adsorption by plant roots.
When fertilizing trees which are planted in rows at a spacing of greater or equal to six feet, it is best to keep the nitrogen application restricted to the vicinity of the estimated root zones either in bands placed adjacent to the liners/tree rows or in circles around shade tree liners.
Perhaps by confining nitrogen application to this area, root development may be promoted closer to the row and fertilization of weeds growing between rows may be minimized.
Fertilization of small shade trees may be done in bands or circles.
Mature field plantings and closely spaced shrubs
If a field is planted with closely spaced shrubs or mature field plantings, the fertilizing material may be broadcast over the field, especially if the fertilizing material contains essentially phosphorus.
If potassium application is recommended in the soil test report, incorporate this element into the upper eight to ten inches of the soil.
Uniform application of fertilizer materials is vital to prevent injury from the accumulation of concentrated fertilizer salts.
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Modify Fertilizer Application Rate Based on Soil Texture
Soil texture
The cation exchange capacity of a soil, which is influenced by the soil texture (the blend of the mineral fraction of the soil: sand, silt, and clay) affects the fertilizer application rate.
Coarse textured soils
Coarse textured soils such as sandy or silty soils have a low CEC and do not have the capability to retain large amounts of nutrients.
Therefore, recurrent fertilizer applications at reduced rates are appropriate.
Fine textured soils
Fine textured clay soils and soils containing substantial amounts of organic matter have the potential to adsorb significant quantities of nutrients.
These soil types may be fertilized less frequently at increased fertilizer rates.
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Plant Age and Size Affect Fertilizer Application Rate
Plant age and size
The fertilizer application rate should be modified according to the age and size of the plant.
Because young plants or recently transplanted trees and shrubs are more susceptible to root damage than are more mature specimens, fertilizer application rates should be lowered.
Lowering fertilizer application rates for new plantings will prevent the development of an unbalanced root : shoot ratio.
After a plant has developed a strong root system, which may be noted by an accelerated rate of growth, nutrient applications, specifically nitrogen, may be increased.
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Adjust Fertilization Rate According to Plant Species
Plant species
Species of genera such as Cornus, Tsuga, and Rhododendron bear shallow, fibrous root systems which are quite vulnerable to fertilizer salt damage.
Recurrent applications at reduced fertilizer rates are appropriate.
Plant species: Higher level of maintenance
Ulmus pumila, Acer saccharinum, and members of the genera Fraxinus, Lonicera, Salix, Ligustrum, and Forsythia exhibit a rapid growth rate.
If nitrogen application accommodates plant demand, a higher level of maintenance will be necessary for these plants.
Plant species: Strong, stable growth
Quercus rubra, Acer saccharum, and members of the genera Viburnum and Thuja respond to heavy fertilization by developing strong, stable growth, not necessitating substantial increases in maintenance.
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In conjunction with appropriate site selection, scheduled soil and foliar analyses, correct pH maintenance, and practices which complement mineralization while reducing runoff, leaching, volatilization, denitrification, and immobilization, a carefully planned and properly implemented nutrient managementprogram can improve field nursery crop production to produce gems such as this!
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the key to growing healthy nursery cropsin containers
Nutrient management is a challenge for producers ofcontainer-grown nursery crops. Proper management of macro- aswell as micro nutrients is essential for the successful productionof vigorous woody plants. This set of web pages describes thenutrients needed by nursery crops, the particular challengesassociated with growing nursery stock in containers, andstrategies for addressing the nutrient demands of woodyornamental plants.
The problems:
Most soilless media used in container nursery operationsprovide very few, if any, nutrients.
●
Most containers possess excellent drainage characteristicswhich necessitate frequent watering; this frequent wateringoften leaches applied fertilizers.
●
Low cation exchange capacities of container media, andlimited container volumes, increase the need to applynutrients on a regular basis throughout the growing season.
●
The solution:
A combination of fertilization techniques can ensurenutrient availability to containerized nursery stock.
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Contents
preplant fertilization●
post-plant fertilization●
controlled-release fertilizer materials●
liquid fertilizers●
timing of fertilization●
analysis of container media●
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Success ingrowing nursery cropsdepends on a goodstart. Ensure the rightnutrients are present,and adjust media pHprior to planting, sothat nutrients will beavailable as soon asthe plants are ready toabsorb them.
Preplant Fertilization
getting off to the right start
Preplant fertilization allows for throrough incorporation ofnutrients at rates sufficient for plant growth. By preparing yourmedia and containers before planting, you ensure that nutrientsare present and in available forms as soon as plant roots are ableto absorb them.
Nutrients not only must be present, they must also be in formswhich the plants can use. Adjusting the pH prior to planting willensure that nutrients are available for uptake by your plants.
Excessive fertilizerapplication can injureplant roots, negativelyaffecting plant growth
Preplant Fertilization
Macro Elements
elements required in greater amounts byplants
Correct fertilization allows for throrough incorporation ofnutrients at rates sufficient for plant growth. Make sure fertilizermaterilals are pre-mixed into the media at appropriate rates. Usecaution, though; excessive fertilizer application can injure plantroots and negatively affect plant growth. Additional applicationsof nutrients will most likely be necessary later in the growingseason as frequent irrigations leach nutrients from the media.
Nitrogen●
Phosphorus●
Potassium●
Ratios of N-P-K●
Calcium and Magnesium●
Associated Tables
NitrogenNitrogen is the first element in a complete fertilizer.
Incorporate a controlled-release fertilizer at or soon afterthe time of planting.
●
Nitrogen supplements may be necessary during thegrowing season through application of liquid fertilizers orby top-dressing granular or controlled-release fertilizers.This is particularly true later in the growing season whencontrolled-release fertilizers may lose their nutrientsupplying power.
●
When using a hardwood bark based mix as the containermedia, controlled-release fertilizers can be used at the rateslisted in the accompanying tables. Refer to Table 3 forsuggested pre-plant fertilizer applications for pine bark
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Nitrate-nitrogenleaches more readilyfrom container media
thanammonium-nitrogen
media.
Note that preplant addition of a wetting agent is oftennecessary when organic amendments such as bark and peatare part of the container media. Water is often repelled byvarious types of bark, particularly pine bark.
●
If preplant controlled-release fertilizers cannot beincorporated into the media, some formulations may beuniformly applied to the surface of the media. However,fertilizer materials applied to the surface may be lostthrough heavy rains or spillage.
●
Associated Tables
PhosphorusIncorporate phosphorus into the media before planting andsupplement, when necessary, with liquid,controlled-release, or granular fertilizers containing thisnutrient.
●
In the past, sources such as superphosphate or treblesuperphosphate have been recommended for incorporationinto the media to supply phosphorus. However, becausecontainer media usually does not contain mineral soil,phosphorus from superphosphate readily leaches from thesoilless mix immediately after potting.
●
Incorporate a complete fertilizer that supplies adequateamounts of available phosphorus; this method of supplyingphosphorus is as effective as pre-incorporatingsuperphosphate. Superphosphate and treblesuperphosphate can be eliminated from preplant fertilizerapplications if subsequent post-plant fertilizations containadequate levels of phosphorus.
●
Because potassium promotes the transformation ofphosphorus into compounds necessary for plantdevelopment, providing adequate amounts of potassiumlessens detrimental consequences of high phosphoruslevels.
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PotassiumPotassium is required at a higher concentration thanphosphorus but a lower concentration than nitrogen for thegrowth of healthy nursery crops.
●
Supply potassium before planting through acontrolled-release fertilizer or complete granular fertilizer.
●
Supplement during the growing season, as necessary, witha controlled-release, complete granular, or liquid fertilizer.
●
Ratios of N-P-KParticular ratios of Nitrogen, Phosphorus, and Potassium (N-P-K)have been shown to promote optimum growth of numerouscontainer-grown plants. According to Ingestad (1973), manyspecies perform well when fertilized with an N-P-K ratio of 5-1-3to 8-1-4. Volden (1979) observed that genera including Berberis,Betula, Cotoneaster, and Acer grow well when supplied with anN-P-K ratio of 6-1-5. Although these ratios have been observedto promote good growth, they are not exclusive of other fertilizerratios. Many different N-P-K ratios may be used with favorableresults.
Calcium and MagnesiumOften supplied by media amendments, particularly thosethat contain hardwood bark, and irrigation water.
●
Calcium and magnesium sulfates also have the ability tosupply adequate levels of these elements.
Calcium sulfate, or gypsum, can be incorporatedinto the media to provide calcium and sulfur.
❍
Little or no change in pH occurs through theaddition of calcium sulfate, particularly if the mediapH is between 5.5 - 7.5.
❍
●
Incorporating dolomitic limestone prior to planting nurserycrops supplies the media with calcium and magnesium;however, liming container media to adjust the pH may beunnecessary if all essential nutrients are applied andremain in the correct balance. After dolomitic limestone isincorporated into the media, ammonium is readily
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transformed to nitrate. In addition, if the soilless mixcontains pine bark, ammonium-nitrogen availabilitydecreases as a result of bark adhesion. Consequently,nitrate-nitrogen becomes the predominant form ofavailable nitrogen.
Post-plant fertilization is an integral part of any nutrient managementprogram. Post-plant fertilization programs can be implemented by using oneof the following:
a controlled-release fertilizer,●
a controlled-release fertilizer supplemented by a soluble nitrogenfertilizer, or
NitrogenThe following suggested fertilization rates are given as an indication of whereto begin the post-plant fertilization program. Application regimes for thecritical macronutrient nitrogen can have effects on the availability of othernutrients in the media. Rates should be adjusted according to plant response.
Application of nitrogen sources
Apply controlled-release fertilizers to container-grown nursery cropsaccording to the higher label rates; conversely, apply lower rates to mostmembers of the genus Rhododendron. When applying controlled-releasefertilizers in combination with soluble nitrogen fertilizers, apply according tothe lower label rates. If controlled-release or granular fertilizers are notavailable, weekly applications of 250-300 ppm soluble nitrogen have beensuggested (Gilliam and Smith, 1980a). Keep in mind that container nurserystock fertilized once a week requires more concentrated levels of fertilizerthan nursery stock fertilized daily.
For example, in order to provide container-grown woody plants withsufficient nitrogen, a controlled-release fertilizer may be top-dressed alone orin combination with weekly supplemental additions of a soluble nitrogenfertilizer (Gilliam and Smith, 1980a) at 150-200 ppm N.
When fertilizingwith nitrate-nitrogen,
the acidity of themedia solution should
be maintained toensure the solubility of
iron and othermicronutrients such as
manganese, copper,and zinc
Nitrogen sources and media pH
In a well-aerated mix, ammonium-nitrogen is usually quickly converted tonitrate-nitrogen, making addition of either ammonium- or nitrate-nitrogensources suitable.
Be aware of the effect different nitrogen sources have on media pH.Urea and ammonium-nitrogen sources such as ammonium-nitrate,ammonium sulfate, and ammonium phosphate decrease media pH
●
nitrate-nitrogen sources such as potassium nitrate and calcium nitrateincrease media pH.
●
Changes in media pH can affect the solubility of other elements.Since nitrate-nitrogen increases the pH of the media solution, thesolubility of iron is reduced.
●
If the concentration of iron is substantially reduced, plant growth maybe inhibited.
●
Thus, when fertilizing with nitrate-nitrogen, the acidity of the mediasolution should be maintained to ensure the solubility of iron and othermicronutrients such as manganese, copper, and zinc.
When liquid feeding with nitrate-nitrogen, application of chelated ironmay help avoid iron deficiency.
●
Nitrogen sources have effects on other nutrientlevels
The type of nitrogen carrier affects the form and rate of nitrogenavailable in the media solution.
For example, applying urea to pine bark media results in rapidavailability of ammonium-nitrogen in the media solution.
●
The ammonium-nitrogen produced then undergoes transformation intonitrate-nitrogen through the process of nitrification, which occurs mostreadily at a pH range of 7.0-8.0.
●
The nitrification process acidifies the media solution, increasing thesolubility of salts and availability of ions such as magnesium, calcium,manganese, and zinc.
●
Because cations such as Ca2+, Mg2+, and K+ are exchangeable bases,they can replace H+ on the cation exchange complex, resulting infurther acidification of the media solution.
●
Further increase in the concentration of Ca 2+, Mg2+, K+ as part of thecation exchange complex attracts hydroxyl ions (OH- ), increasing thepH of the media solution.
●
Consequently, from an initial application of ammonium-nitrogen, a complexchain of events follows that leads to the availability of nitrate-nitrogen andother essential elements. In addition, keep in mind that frequent irrigationsmay induce the leaching of nitrogen, especially in the nitrate form, from themedia solution.
Literature CitedFoster, W. J., R. D. Wright, M. M. Alley, and T. H. Yeager. 1983. Ammonium adsorption on a pine-barkgrowing-medium. J. Am. Soc. Hortic. Sci. 108: 548-551.
Gilliam, C. H. and E. M. Smith. 1980a. Fertilization of container-grown nursery stock.. Ext. Bul. 658.The Ohio State University.
Gilliam, C. H. and E. M. Smith. 1980b. How and when to fertilize container nursery stock.. AmericanNurseryman 151(2): 7, 117-127.
Halbrooks, M. C. 1990. Nutrition of container and field-grown nursery crops.. Ext. Bul. 138.Cooperative Extension Service, Clemson University.
Ingestad, T. 1979. Mineral nutrient requirements of Pinus silvestris and Picea abies seedlings. . Physiol.Plant Pathol. 45: 373-380.
Rader, L. F., Jr., L. M. White, and C.W. Whittaker. 1943. The salt index--A measure of the effect offertilizers on the concentration of the soil solution.. Soil Science 55: 201-218.
Thomas, S. and F. B. Perry, Jr. 1980. Ammonium-nitrogen accumulation and leaching from an all pinebark medium.. HortScience 15: 824-25.
Tinus, R. W. and S. E. McDonald. 1979. How to grow tree seedlings in containers in greenhouses. .USDA For. Serv. Gen. Tech. Rep. RM-60.
Volden, S. 1979. Effects of fertilizing, liming and growth medium on the growth of container-grownplants.. Forsk. Fors. Landbruket 30: 479-497.
Wright, R. D. and A. X. Niemiera 1987. Nutrition of Container-Grown Woody Nursery Crops.. In J.Janick (Ed). Horticultural Reviews 9: 75-101. New York: Van Nostrand Reinhold Co.
PotassiumPotassium levels may decrease during the growing season for several reasons:
controlled-release materials may lose their ability to provide thenutrient,
●
changes may be implemented in the soluble fertilization program, or●
levels of the nutrient injected through the irrigation system may change.●
If necessary, top-dress the media with soluble or controlled-release potassiumfertilizers or apply supplemental potassium via soluble carriers in theirrigation system. Apply fertilizer sources such as potassium sulfate,potassium chloride, or potassium nitrate to maintain adequate potassiumlevels during the growing season. Potassium nitrate also providessupplemental nitrogen. If potassium chloride is applied, regulate soluble saltand chloride levels closely.
Micro ElementsMicronutrients are most available when the media 5.0 < pH <6.5. Maintaining the correct pH reduces the need forsupplemental post-plant micronutrient applications. Always basefertilizer application rates on media analysis results andrecommendations. Recommended rates may need to be adjustedbased on the fertilizer formulation; follow the manufacturer'sinstructions.
Fertilizer materialsMaintaining an adequate level of nutrients in the soilless mediasolution is the key objective of the fertilization program. Ascontainer crops increase in size, increase nutrient levels to meetelevated plant nutrient requirements. This can be achieved byincreasing the fertilization rate and/or frequency of application.
Simple and complete fertilizers
A complete fertilizer, such as 10-10-10, contains nitrogen,phosphorus, and potassium.
●
A compound fertilizer contains two of the nutrients.●
A simple fertilizer contains only one of the elements N, P,or K. For example, a fertilizer material with the analysis of0-20-0 contains only phosphorus.
Types of dry fertilizers include both granular (e.g., 10-10-10 and15-15-15) and controlled-release fertilizers. A wide variety of dryfertilizer materials are available for container nursery cropfertilization. Although applying dry fertilizer materials does notrequire a substantial equipment investment, labor costs may besignificant if multiple applications per year are necessary.
Granular fertilizers
Granular nitrogen sources can be incorporated into the mediabefore planting or top-dressed post-planting. If a pre-plantfertilization program was not implemented, nitrogen, phosphorus,and potassium can be applied through granular fertilizers.
Because nitrogen, phosphorus, and potassium are subjectto loss through leaching, complete granular fertilizersmay be applied at frequent intervals during the growingseason to supply container-grown nursery stock withrequired nutrients.
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These fertilizers can supplement a liquid feeding system ora controlled-release system which may fail to deliver
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Supplements of magnesium and calcium sulfates helpmaintain magnesium and calcium at viable levels.
●
Micronutrients should also be provided. For example,Micromax can be top-dressed on the media along with acomplete granular fertilizer such as 10 - 10 - 10.
●
Keep in mind that uniform application of high analysis granularfertilizers may prove difficult in container nursery fertilizationprograms because of the small quantities of fertilizer materialrequired. Always apply fertilizer materials according to mediaanalysis results and recommendations.
Controlled-Release FertilizersControlled-release fertilizers meter the release of nutrients overtime.
Fertilizer selection and application●
Controlled-release fertilizer classification●
Mechanism of nutrient release●
Application methods●
Factors such asmedia pH,
temperature, moisture,coating thickness, and
bacterial activityregulate the release of
nutrients fromcontrolled-release
fertilizers.
Fertilizer selection andapplicationAlthough controlled-release fertilizers should be appliedaccording to the manufacturer's instructions, rates may need to beadjusted based on the following factors:
● age and size of the plant,● environmental conditions, and● plant species.
Some adjustment of fertilizer application rates can be made asgrower experience with these products increases. When selectinga controlled-release fertilizer, note the length of time required forinitial nutrient release and the duration of nutrient availability.
Factors such as media pH, temperature, moisture, coatingthickness (if coated), and bacterial activity regulate the release ofnutrients from controlled-release fertilizers.
Under some circumstances controlled-release fertilizers maynot provide nutrients at the levels required by plants for theentire release period as indicated on the product label. Forexample, elevated ambient air temperatures may result in therelease of nutrients at a faster rate than would occur at lowertemperatures, exhausting the nutrients within thecontrolled-release fertilizer earlier than expected. In such cases,supplemental liquid fertilization and/or surface application ofgranular inorganic controlled-release fertilizers may be necessary
organic, synthetic isobutylidene diurea (IBDU) and urea-formaldehyde(UF);
●
sulfur-coated synthetic organic (urea) and inorganic fertilizers; and●
inorganic, resin-coated fertilizers.●
controlled-release organicfertilizers
Organic fertilizers
Although they may be fairly expensive, organic fertilizers are typified by theirlow water solubility, prolonged nutrient release rate, extended residual,and reduced likelihood of causing fertilizer injury.
Natural organic fertilizers have animal or plant origins, while other organicfertilizers are synthetic in nature.
Depending on the temperature and pH of the media, organic nitrogen (NH2)
from these controlled-release materials is converted to ammonium (NH4+) by
microorganisms. Subsequently, ammonium is transformed into nitrate (NO3-),
a process which is also pH- and temperature-dependent. The rate at whichthese transformations occur meters the release of nitrogen for plant uptake.
Resin or sulfur-coated controlled-release materials
Controlled-release fertilizers in the form of sulfur- or resin-coated granulesinclude osmocote and sulfur-coated urea (SCU). Urea-formaldehyde isavailable in tablet form, allowing for ease of placement in the containermedia.
The rate of nitrogen release from organic fertilizers depends onthe pH, moisture, and temperature of the media asmicroorganisms convert organic nitrogen to available forms ofinorganic nitrogen through the process of mineralization.
Nitrogen release from Milorganite, an example of anactivated sludge-based product, is optimal at 6 < pH < 7and increases with temperature and media moisture. Theoperative period of this organic controlled-release fertilizeris two to four months.
●
types of controlled-releasefertilizers
Resin-coated fertilizers
Resin-coated fertilizers such as Osmocote, High-N, and Sierrarelease nitrogen according to coating thickness, temperature (therate of release increases with warmer temperatures), and thepresence or absence of moisture. Although moisture is necessaryfor the release of nutrients from plastic-coated controlled-releasefertilizers, the amount of moisture present does not have a majoreffect on the rate of release from the encapsulated fertilizer.Media pH and microorganism activity do not influence the rate ofnitrogen release from encapsulated fertilizers. Physically marringor disrupting the resinous fertilizer coating before or afterincorporating into the media hastens the release of nutrients fromthe coated granules.
Poly-S is a controlled-release fertilizer which has a dual coating;sulfur-coated urea and ammonium phosphate are encapsulated inplastic. The rate of nitrogen release from Poly-S is temperaturedependent; higher temperatures accelerate the process.
Nitrogen release from sulfur-coated urea depends on factors thatcontribute to the deterioration of the sulfur coating; increasedmedia moisture and temperature erode the coating, expeditingnutrient release. Coating thickness also influences the rate ofnutrient availability; thicker coatings retard the rate of nutrient
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The rate of nitrogen release from urea-formaldehyde (U.F.)depends on the structure of the nitrogen source. Because shortercarbon-chained urea-formaldehyde molecules are more solubleand more easily assimilated by microorganisms than longerchained U.F. molecules, nitrogen is more quickly released fromshorter chains. Sufficient media moisture and oxygen, mediatemperatures exceeding 55°F, and 5.5 < pH < 6.5 also promotethe release of nitrogen from this water insoluble nitrogen source(Gilliam and Smith, 1980a). Urea-formaldehyde can also be usedwhen nursery stock is overwintered. Encapsulatedcontrolled-release fertilizers often rupture when frozen, releasingthe total content of soluble material at one time, causing highsoluble salts levels in the root zone and subjecting the roots tofertilizer injury. This would not happen with a U.F. basedfertilizer.
Isobutylidene diurea (IBDU) , a urea-based controlled-releasefertilizer, releases nitrogen with increasing media moisture andtemperature. Fertilizer particle size and media pH influence therate of nutrient release; smaller particle sizes and low pHaccelerate the process.
MagAmp (magnesium ammonium phosphate) is a completecontrolled-release fertilizer containing magnesium. Watersolubility of MagAmp is minimal but sufficient enough toprovide N, P, and Mg at levels that meet plant needs. Potassiumhas been added to this material, making it a complete fertilizer.Table 8 lists several controlled-release fertilizers which may betop-dressed or incorporated into the container media.
Remember to adjust fertilization rates according to plant age andsize, species, and concentration of nutrients in the fertilizer.Always apply nutrients in a way so as to minimize leaching.Controlled-release fertilizers are most effective at plantingtime. For controlled-release fertilizers with an effective period of8-9 months, an early spring application is appropriate; if thefertilizer has an effective period for only 3-4 months, an earlyspring application should be supplemented by a summerfertilization. Reduce fertilization rates after July 1.
If controlled-release fertilizers are used during overwintering,monitor media temperatures which may increase substantially in
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warm polyhouses.
Warmer temperatures may trigger the release of excessiveamounts of nitrogen and contribute to more rapid solubilityof nutrients, resulting in fertilizer injury to the plants andaccumulation of excessive levels of soluble salts duringperiods of reduced irrigation.
●
Very cold temperatures can result in freezing ofencapsulated controlled-release fertilizer forms, disruptingthe resinous coat and releasing excessive amounts ofnutrients into the rootzones of plants.
Application MethodsMethods of applying controlled-release fertilizers include top-dressing andincorporation into the media, or dibbling. The latter is a method in which thefertilizer material is placed in a hole in the container media, allowing thefertilizer to remain at a more controlled, reduced temperature.
Dibbling is especially common when using resin-coated fertilizers whosemechanism of release is regulated by temperature and moisture.
Fertilizers applied byoverhead irrigationoften fall between
containers, increasingthe risk of runoff,
groundwatercontamination, and
other negativeenvironmental
impacts.
Liquid fertilizersLiquid fertilization, also called fertigation, uses very solublenutrient carriers which are dissolved in water, formingconcentrated liquids which are injected into irrigation systems.Nutrients can be applied through trickle or overhead systems, andcan vary in concentration and composition.
Liquid fertilization poses advantages and disadvantages.Advantages include uniform distribution and relative ease ofapplication. Liquid fertilization programs can be finely tuned tothe nutritional requirements of containerized nursery stock,unlike other fertilizers whose nutrient release may not matchplant needs. Fertilizer application may be automated, decreasinglabor costs.
Disadvantages of fertigation include the inefficiency ofapplication when overhead irrigation systems are used, and theinitial expense of proportioning equipment.
Trickle and Spray
Trickle irrigation systems are noted for their efficiency, reducingthe likelihood of nutrient migration from the site of application.Overhead irrigation is a much less efficient delivery method,since much of the fertilizer applied falls between containers. Thisincreases the risk of nutrient migration from the site ofapplication, resulting in a negative impact on the environment.
Liquid fertilizers may be applied at several different fertilizerconcentrations. Stronger concentrations (250-400 ppm N) can beapplied with each watering. Reduce application rates for youngor newly transplanted nursery stock, slow-growing narrow-leafevergreens, and members of the genus Ericaceae. Well-drainedmedia require more frequent applications than poorly drainedmedia.
Proportioning Devices
Liquid fertigation can be accomplished through a proportioningdevice that injects concentrated fertilizer solutions into the
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irrigation system. Although the preliminary cost of equipment ishigh, automation of this kind prevents excessive ongoing laborcosts.
Two basic types of proportioners are available, those that operateby positive displacement and those that employ the Venturisystem.
Displacement proportioners:Introduce a calculated amount of concentrated fertilizerstock solution into the irrigation system to mix with ameasured amount of irrigation water.
●
Constant water flow rate must be maintained to preservethe correct ratio of water to fertilizer concentrate.
●
Irrigation water must be separated from the fertilizer stocksolution to prevent dilution of the fertilizer concentrate.
●
Venturi-type proportioners:Allow a measured amount of concentrated fertilizersolution to be mixed into the irrigation line.
●
In order to maintain a constant fertilizer : water ratio atvarious pressures, the rate of fertilizer injection shouldcoincide with the irrigation water flow rate.
●
Venturi-type proportioners with regulating valves allow forconstant dispensing of the fertilizer even when changes inthe irrigation pressure line exist.
●
Proportioners should be chosen which accommodate the needs ofthe nursery. Calculate the capacity of the irrigation system,dilution ratio, solubility of the fertilizer to be used, and determineif a mobile proportioner is required before choosing aproportioner.
Using a soluble fertilizer to prepare the stock solutionallows for proper functioning of the proportioner.
To enhance solubility, dissolve and stir the fertilizer in hotwater (@ 150 degrees F).
●
Prepare the stock solution in a plastic or fiberglasscontainer.
●
Use dyes in the fertilizer solution to accurately monitor therate of mixing concentrated fertilizer solution with theirrigation water and fertigation itself (Gilliam and Smith,1980a).
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Foliar FertilizationFoliar fertilization is primarily used to apply nitrogen and microelements. Urea can be applied safely at 5 lbs/100 gallons of water(Gilliam and Smith, 1980b) without burning the foliage. Foliarsprays should only be used temporarily in cases of micronutrientdeficiencies or when an immediate plant response is necessary.Foliar fertilization should not be relied upon as a permanentsolution to nutrient deficiency problems. Measures should betaken as soon as possible to correct the problem(s) which inducednutrient deficiencies.
Timing of FertilizationPlants absorb nutrients most effectively when roots areactively growing, yet shoots are quiescent. By applyingnutrients when shoots are not actively growing, more energy isavailable for root growth and uptake of nutrients. In addition,research indicates that nutrient uptake is elevated after intervalsof withholding fertilizer. Keep in mind, however, that some plantspecies grow continuously. Apply reduced amounts ofcontrolled-release, liquid, or granular fertilizers at frequentintervals to maintain adequate levels of nutrition throughout thegrowing season.
When fertilizing with controlled-release materials, apply atthe time of planting and use a material with a total releaseperiod of at least 8-9 months. If incorporating acontrolled-release fertilizer which has a release period of only 3-4months, supplement an early spring application with atop-dressing in July to extend the nutrient availability to 8-9months. After August 1, controlled-release fertilizers should onlybe applied at half the growing season's recommended rate.
Some research indicates that late summer and fallfertilizations often promote spring growth which is attributedto elevated nitrogen concentrations and nutrient reserves in planttissues. In addition, the cold tolerance of plants fertilized in latesummer and fall appears to parallel that of plants supplied withreduced rates of fertilizer. Under-fertilization can promotenutrient deficiencies which contribute to a lack of cold hardiness.Late season over-fertilization will also likely result in freezinginjury to late season vegetative growth. Any fall fertilizerapplications should be made when media temperatures are abovefreezing; nutrient absorption does not occur when mediatemperatures are below 40°F. Late summer and fall fertilizerapplications should be reduced to one-half the amount appliedearlier in the season. Fertilization at this time enhances rootgrowth, cold hardiness, and provides nutrient reserves for aspring growth flush. For containerized plants protected during thewinter under one or two layers of poly, fertilization may extendto October 1. If microfoam or low levels of heat provide winterprotection, fertilization may extend later into the fall.
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Containerized plant materials are usually covered in Upstate NewYork during early to mid-November, while on Long Island inlater November, depending on the onset of freezing temperatures.
Because solubility of some controlled-release fertilizersdepends on media moisture or high temperatures, applyingcontrolled-release fertilizers during the winter months mayinduce soluble salt accumulation, particularly under elevatedpolyhouse temperatures. Fertilizer elements released during thisperiod of reduced watering contribute to salt accumulation in thecontainer media, resulting in plant injury. Monitor soluble saltslevels, and avoid heavy late season applications of fertilizersincluding controlled-release formulations. Controlled-releasefertilizers may be more safely applied after April 1 whenirrigation is frequent and polyhouse temperatures are reduced.
Analysis of Container MediaAnalyze the nutrient content of the soilless media solution beforeplanting, after planting, and on a regular basis to monitor changesin nutrient availability. Adjust the fertilization programappropriately, based upon test results. Because container media isconfined to a limited volume and nutrients are subject to leachingthrough frequent irrigation and low cation exchange capacities,test media regularly for pH, nutrient content, and soluble saltslevels. Monitor the level of micro elements throughout thegrowing season, especially when incorporating highly solubleliquid fertilizers which may decrease micronutrient availability.
Cornell Nutrient Analysis Laboratories provide servicesincluding analysis of foliage and container media, fertilizers,hydroponics solutions, and irrigation water used in theproduction of container-grown ornamentals, fruit, vegetable, andgreenhouse crops. Complete the information sheet forCommercial Greenhouse and Nursery Container Mixes, availablethrough local Cornell Cooperative Extension field staff, andreturn with each sample.
The information sheet provides important information about thecrop, media and site characteristics, fertilization materials andapplication frequency, and sample identification. Thisinformation, together with the analysis results, will be assessedby the Cornell Nutrient Analysis Laboratory to provide nutrientmanagement recommendations.
Optimal and actual nutrient values, pH, and soluble saltsinformation is provided by the lab. Use consistent samplecollection methods each time container media is sampled. Onerecommended sampling method involves collecting the sampleafter fertilizing and subsequently irrigating with a definedvolume of water. Take representative core samples from at leastten containers of the group of plants to be tested; core samplesinclude the entire container depth, excluding the uppermostquarter inch. Mix all core samples together for laboratoryanalysis. Obtain bags to mail the composite sample through localCooperative Extension personnel or from the Cornell NutrientAnalysis Laboratory. Send samples and the appropriate analysis
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Take samples 40 days after planting nursery stock, during themonth of July, and the first week of October. Results willprovide essential information required to monitor and, ifnecessary, modify the fertilization program.
The Cornell Nutrient Analysis Laboratories uses the leachatemethod to prepare samples for nutrient analysis. Containernursery crop mixes can be analyzed preplant and post-plant forthe following:
nitrate-nitrogen●
ammonium-nitrogen●
phosphorus●
potassium●
calcium●
magnesium●
boron●
copper●
manganese●
molybdenum●
zinc●
iron●
sodium●
aluminum●
cobalt●
cadmium●
chromium●
nickel●
lead●
soluble salts, and●
pH.●
After receiving results of media analysis, compare media nutrientlevels with the recommended nutrient ranges that correspond tothe extraction procedure used to obtain the values. Media nitratelevels are critical to plant establishment. Nitrate concentrations inthe leachate should be checked regularly in order to produce thebest crop possible, yet prevent ground water contamination.
Values below the optimal or normal ranges may be anindication of critically low nutrient, pH, or soluble saltslevels. Nutrient values above these ranges do not necessarilyindicate excessive levels in the media solution. Other factors suchas soluble salts levels, pH, media type, plant species, and the timeat which the media sample was taken influence the interpretationof actual sample values.
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yMillisiemens per centimeter. mS/cm is a unit of conductivity. 1 Siemens = 1000 millisiemens = 1 mhozppm = parts per million.Source: Leachate extract values based on recommended container media solution levels by the CornellNutrient Analysis Laboratory, Department of Floriculture and Ornamental Horticulture, CornellUniversity.
nursery plants, alwaystest your soil for theseelements, and monitormedia pH levels over
the course of thegrowing season.
Micro Elements
nutrients required in smallerconcentrations, but vital to plants
Preplant fertilization programs should include the incorporationof micro elements, including iron, manganese, boron, copper,zinc, and molybdenum, into the media if micronutrient levels aretoo low. Micronutrient availability depends on the form of theelement, media pH, and the element's propensity for precipitationor leaching. Micronutrients are most available when the media5.0 < pH > 6.5. Media pH greater than 7.0 results inmicronutrient unavailability, while media pH less than 4.5 leadsto micronutrient toxicity.
The type of media used influences theneed for supplemental micronutrientfertilization.
Organic media does not require the same level ofmicronutrient application as sand-based media. Asorganic media decomposes, certain amounts ofmicronutrients are released into the media solution,becoming available for plant uptake. If the organic mediasolution is kept within a pH range of 4.4 - 5.6, andnitrogen, phosphorus, potassium, calcium, and magnesiumare maintained in the correct concentrations, supplementalmicronutrient applications may not be as critical as in othertypes of media containing inorganic amendments.
●
Always use a soil test to monitormicronutrient levels and to gauge toxicity.
Applications of municipal compost can elevatemicronutrient concentrations to lethal levels. Ifmunicipally composted sludge is a component of themedia, monitor micro element levels closely to preventmicronutrient toxicity.
●
Some irrigation waters may contain boron in high●
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Micronutrients can be applied by pre-plant incorporation,top-dressing, or injection through irrigation.
Always base fertilization rates on soilless media testresults. If media analysis results recommend addition ofmicronutrient fertilizers, follow manufacturer'srecommended application rates. Even if micronutrients aresupplied by supplemental fertilization, their availabilitymay decline because of high media pH, micronutrientprecipitation, or leaching; however, sulfate-basedmicronutrient fertilizers usually maintain theirmicronutrient release capabilities in organic mediathroughout the growing season.
●
Application rates may have to be modified, dependingon the formulation of the fertilizer. For example, solublemicronutrient fertilizers should be applied at lower ratesthan controlled-release fertilizers to prevent micronutrienttoxicity. Fertilizer formulations containingcontrolled-release sources of nutrients can be applied athigher rates than more soluble forms since they meter therelease of nutrients over time. This table lists examples offertilizers which can be applied to supply micro elementsto containerized nursery stock.
●
Because some controlled-release fertilizers containmicronutrients, they represent an alternative means ofproviding these important elements. Soluble sulfates,complexes adsorbed to clay particles, and chelatedmaterials are other fertilizer forms that supplymicronutrients. Chelated forms, if applied to foliage,should serve only as a temporary means of supplyingmicro elements.
●
Make sure micronutrient fertilizers have been trialedon woody ornamental species before using them in yournursery. It is important to incorporate correct amounts ofmicronutrients to prevent damage to nursery stock;applying excessive amounts of micro elements can lead toaccumulation of micronutrients at toxic levels in themedia. Most manufacturers of soluble trace elements makespecific recommendations which should be followed.However, Tinus and McDonald (1979) found the following
●
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concentrations beneficial for container-grown forestspecies: 0.01 ppm molybdenum, 0.02 ppm copper, 0.5 ppmboron,0.05 ppm zinc, 0.5 ppm manganese, and 4.0 ppmiron. Wright and Niemiera (1987) reported that manywoody ornamental species fertilized with similar nutrientconcentrations injected through the irrigation systemresponded favorably.
pH is a measure of acidity or alkalinity on a scale of 0 to 14, with7 being neutral. The pH of the media affects many elements,including nutrient availability, plant nutrient uptake, andmicroorganism activity. Because media pH affects many factors,it is important to maintain proper pH throughout the growingseason.
Considering the complexity of factors involved, therecommended pH for container media is between 5.5 and 6.5.Ericaceous crops usually require a slightly more acidic mediaenvironment; however, do not allow the pH to fall below thevalue of 4.5 for members of this plant family.
Regular media pH testing will allow you to make informeddecisions if media pH adjustment is necessary. Test each mediacomponent individually and test samples of the blended productto make sure unexpected pH shifts do not occur.
Nutrient availability●
Microorganism activity●
Nitrification●
Increasing media pH●
Decreasing media pH●
Nutrient availability
Nutrients tend to be more available when the medium issomewhat acidic. Calcium and magnesium are more readilyadsorbed from organic media when pH < 5.2, while most othernutrients are readily adsorbed when pH ranges between 4.0 and5.2.
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In addition to affecting nutrient absorption, the pH of organicmedia influences the activity of microorganisms. Bacteria aremore prevalent at pH greater than 5.5, while fungi are most activeat pH less than 5.5.
Nitrification
Media pH also influences the process of nitrification, whichoccurs most readily at a neutral pH (7.0). During nitrification, theammonium-nitrogen cation (NH4
+ ) is transformed to the
nitrate-nitrogen anion (NO3- ). Nitrate-nitrogen has a greater
potential to leach from the soilless media solution thanammonium-nitrogen.
Limestone, whenmixed with
ammonium-nitrogenfertilizer, produces
ammonia. Because ofthis, do not apply
Increasing media pH
If pH adjustment is necessary, apply liming materials prior toplanting. Media analysis results and specific crop needs dictatehow much adjustment of the media pH is necessary. Thecomposition and pH of the container media, pH and bufferingcapacity of the irrigation water, and plant species to be growninfluence the amount of lime required to raise the media pH.
Dolomitic limestone raises the pH of the containermedia while supplying calcium and magnesium.However, limestone, when mixed withammonium-nitrogen fertilizer, produces ammonia.Because of this, do not apply ammonium-nitrogen for 7-14days after liming container media.
●
Apply hydrated lime as an alternative to dolomiticlimestone. This provides quicker results because of thesmaller particle size of hydrated lime and is especiallyuseful for raising the media pH after planting nurserystock. Hydrated lime, or limewater, can be prepared a rateof one pound of hydrated lime per 100 gallons of waterand applied as a drench to the root zone. (Gilliam andSmith, 1980b).
●
Be aware that hydrated lime may cause severe injury toyoung nursery stock. This material should only beapplied to established crops and under circumstances
●
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ammonium-nitrogenfor 7-14 days after
liming container media
where the soluble salts level is less than 90; application ofhydrated lime significantly increases the level of solublesalts in the media.
When selecting a liming material, keep in mind that thecoarseness of the material affects the rate of pH change.More finely ground materials react more quickly thancoarser materials. Materials are graded according to thepercent that passes through a 100-mesh screen; a materialwith a 75% grade is suitable for raising media pH. Inaddition, determine the amount of calcium carbonate andmagnesium carbonate present in the liming material.Dolomitic lime containing 35 to 40% magnesiumcarbonate and 45 to 60% calcium carbonate can be used tolime container media.
●
In order to supply calcium or magnesium withoutaltering the media pH, incorporate calcium sulfate(gypsum) or magnesium sulfate (Epsom salts),respectively. The courseness of the liming material affectsthe rate of pH change.
●
Decreasing media pH
Gilliam and Smith (1980a) recommend lowering the pH ofcontainer media by injecting phosphoric acid into the irrigationwater at a rate of 2 oz./100 gallons of water. Keep in mind thatphosphoric acid will also supply the macronutrient phosphorus.Alternatively, these researchers also suggest that iron sulfate canbe applied as a drench at the rate of 1 oz./2 gallons of water.Application rates of phosphoric acid and iron sulfate may differdepending on the buffering capacities of the container media.Apply ferrous sulfate drenches cautiously; because the acidifyingeffect is more rapid, plant damage may result and the pH changeis only temporary.
prevent saltaccumulation, as longas the irrigation wateritself is not a source of
harmful ions.
Soluble saltsFertilizer materials dissociate in water into their constituentpositively or negatively charged particles, or ions. Theconcentration of ions in soil water extract is referred to as solublesalts. Sources of soluble salts include dissolved fertilizer andother substances in irrigation water, or fertilizer materials whichhave been incorporated into the soilless mixes. Salt levels mustbe managed to prevent damage to plants.
Irrigation water can be a source of chloride, sodium,and if high enough in concentration, boron, which canaccumulate to hazardous levels in container media andplant tissue. Chloride and sodium are taken up by plantroots or absorbed through the leaves; symptoms of toxicityinclude foliar necrosis, marginal scorch, and prematureleaf drop. Elevated levels of boron and othermicroelements can also induce toxicity.
●
Routinely monitor soluble salt levels by analyzingirrigation water. Regulate media salt levels by selectingproper media and fertilizer materials with low salt indices,and leaching accumulated salts from the container media.
●
Exercise caution when applying preplant fertilizers sothat soluble salts caused by elevated nutrientconcentrations do not reach toxic levels in the media.Gilliam and Smith (1980a) recommend potting nurserystock within two to three days if controlled-releasefertilizers are incorporated into the container media. Thiswill prevent the accumulation of lethal levels of solublesalts by activation of the release mechanism of thecontrolled-release fertilizer, particularly if air temperaturesexceed average conditions.
●
Routine irrigation of the media helps prevent saltaccumulation. Dry media contains less available waterthan moist media to dilute the fertilizer salt concentration,making roots more prone to salt injury. If soluble saltspersist even with frequent irrigation, supplementalirrigations can be effective in leaching salts from thecontainer media. Leach the media with twice the containercapacity of water to remove excess accumulated soluble
●
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salts. The container capacity is the amount of water themedia will retain after irrigation to the point of saturationand draining.
salt indices of fertilizers
Soluble saltsLearn salt index values of fertilizer carriers to avoid problemsassociated with high salt levels.
A fertilizer having a high salt index increases thelikelihood of desiccation injury.
●
Materials having low salt index values pose a much lowerrisk of inducing such problems.
●
Factors such as the maximum amount of water the soil canhold (field or container capacity), the rate of fertilizationand water application, and quality of irrigation waterinfluence the effect that fertilizer salt content will have onplants.
z Indicates % N in nitrogen carriers, % potassium oxide in potassium carriers, % magnesium oxide inmagnesium carriers including dolomite, and % calcium oxide in calcium carbonate and gypsum.y Calculated by dividing salt index by number of fertilizer units (fertilizer analysis).Source: Rader, L.F., Jr., M. White, and C.W. Whittaker. 1943.
Media: Rooted in SuccessUnderstanding the general characteristics of growing media, as well asthe specific attributes of the media you are using, will enable you toget the most out of your nursery. This set of pages describes theattributes of media and how to manage media for success.
Media is composed of solid, liquid, and gaseous components.
Solid materials usually constitute 33-50% of the media volume.Spaces, or pores , between the solid particles are filled with airor water. As water moves through container media, it is retainedby smaller pores, but drains through larger pores.
●
The second fraction of the media, the liquid portion, consists ofnutrients, organic materials, dissolved gases, and water.
●
The third media phase consists of gaseous materials includingoxygen and carbon dioxide. Although media oxygen levels varyfrom 0-21%, a concentration of at least 12% oxygen is necessaryfor root initiation to occur. Roots of most plants fail to grow in amedia atmosphere containing less than 3% oxygen. The carbondioxide content of the media may range from 0.03% to 21%;however, very high carbon dioxide contents may be detrimentalto plant health (Bilderback, 1982).
●
Understanding the attributes of each of these media components, aswell as the interactions between these components, is essential for thesuccessful operation of a nursery. The following pages describeconcepts and provide illustrations of how to effectively manage yourmedia and keep your crops rooted in success.
Contents:
Ions in the Media●
Percent Base Saturation●
pH and Soluble Salts●
Porosity●
Bulk Density●
Media Drainage●
Absence of Pathogens and Pests●
Rewettability●
Organic Media●
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Cation exchange capacities forvarious growing media
amendments and selected media
Ions
Cation exchange capacity
Cation exchange capacity (CEC) quantifies the ability of media toprovide a nutrient reserve for plant uptake. It is the sum ofexchangeable cations, or positively charged ions, media can adsorb perunit weight or volume. It is usually measured in milligram equivalentsper 100 g or 100 cm3 (meq/100 g or meq/100 cm3, respectively).
A high CEC value characterizes media with a high nutrient-holdingcapacity that can retain nutrients for plant uptake between applicationsof fertilizer. Media characterized by a high CEC retains nutrients fromleaching during irrigation. In addition, a high CEC provides a bufferfrom abrupt fluctuations in media salinity and pH.
Important cations in the cation exchange complex in order ofadsorption strength include calcium (Ca2+) > magnesium (Mg2+) >potassium (K+) > ammonium (NH4
+), and sodium (Na+).Micronutrients which also are adsorbed to media particles include iron(Fe2+ and Fe3+), manganese (Mn2+), zinc (Zn2+), and copper (Cu2+).
The cations bind loosely to negatively charged sites on media particlesuntil they are released into the liquid phase of the media. Once they arereleased into the media solution, cations are absorbed by plant roots orexchanged for other cations held on the media particles.
Anion exchange capacity
Some media retains small quantities of anions, negatively chargedions, in addition to cations. However, anion exchange capacities areusually negligible, allowing anions such as nitrate (NO3
-), chloride
(Cl-), sulphate (SO4-), and phosphate (H2PO4
-) to leach from themedia.
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Literature CitedBartok, J. W., Jr. 1985. Media mixing systems offer efficiency, variety. Greenhouse Manager4(8):108-110, 112-113.
Barnett, J. P. 1977. Effects of soil wetting agent concentration on southern pine seed germi- nation.Southern Journal of Applied Forestry 1(3):14-15.
Barnett, J. P. and J. C. Brissette. 1986. Producing southern pine seedlings in containers. Gen. Tech.Rep. SO-59. New Orleans, LA: USDA Forest Service, Southern Forest Experiment Station.
Bilderback, T. E. 1982. Container soils and soilless media. In: Nursery Crops Production Manual.Raleigh, NC: North Carolina State University, Agricultural Extension Service.
Bunt, A. C. 1988. Media and mixes for container-grown plants. Boston: Unwin Hyman.
Digat, B. 1988. The bacterization of horticultural substrates and its effects on plant growth. ActaHorticulturae 221:279-288.
Handreck, K. A. and N. D. Black. 1984. Growing media for ornamental plants and turf. Kensington,NSW, Australia: New South Wales University Press.
Hoitink, H. A. 1980. Composted bark, a lightweight growth medium with fungicidal prop- erties. PlantDisease 64(2):142 - 147.
Hoitink, H. A. and H. A. Poole. 1976. Composted bark media for control of soil-borne plant pathogens.International Plant Propagators' Society Combined Proceedings 26: 261- 263.
Judd, R. W. Jr. 1984. Making soilless mixes not without its problems. Greenhouse Manager 3(2):135,137.
Kusy, W. 1989.Beware of hidden costs in mixing your own media. Greenhouse Manager 8(5):134, 136,138, 141.
Landis, T. D. 1990. Containers and growing media, Vol. 2, The container tree nursery manual.Agric. Handbk. 674. Washington, DC: U.S. Department of Agriculture, Forest Service 41 - 85.
Landis, T. D., R. W. Tinus, S. McDonald, and J. Barnett. 1990. The biological component: Nurserypests and mycorrhizae, Vol. 5, The container tree nursery manual. Agriculture Handbk. 674.Washington, DC: U.S. Department of Agriculture, Forest Service.
Mastalerz, J. W. 1977. The greenhouse environment. New York: John Wiley & Sons, Inc..
Moore, G. 1987. Perlite: Start to finish. International Plant Propagators' Society Combined
Media: Literature Cited
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Pawuk, W. H. 1981. Potting media affect growth and disease development of container- grownsouthern pines. Res. Note SO-268. New Orleans, LA: USDA Forest Service, South- ern ForestExperiment Station.
Peck, K. 1984. Peat moss and peats. Hummert's Quarterly 8(3):1, 4-5.
Perlite Institute. 1983. Typical chemical and physical properties of perlite. Tech. Data Sheet 1- 1. NewYork.
Pokorny, F. A. 1987. Available water and root development within the micropores of pine bark particles.J. of Environmental Horticulture 5(2):89-92.
Pokorny, F. A. 1979. Pine bark container media: an overview. International Plant Propagators'Society Combined Proceedings 29:484-495.
Stewart, N. 1986. Production of bark for composts. International Plant Propagators' SocietyCombined Proceedings 35: 454-458.
Swanson, B. T. 1989. Critical physical properties of container media. American Nurseryman 169(11):59-63.
Ward, J., N. C. Bragg, and B. J. Chambers. 1987. Peat-based composts: their properties defined andmodified to your needs. International Plant Propagators' Society Combined Proceedings 36:288-292.
Whitcomb, C. E. 1988. Plant production in containers. Stillwater, OK: Lacebark Publications.
Wilson, G. 1985. Effects of additives to peat on the air and water capacity. Acta Horticulturae 172:207-209.
Wolffhechel, H. 1988. The suppressiveness of sphagnum peat to Pythium spp. Acta Horticulturae,221:217-222.
Percent base saturationThe concentration of potassium, magnesium, and calcium expressed asa percentage of the cation exchange capacity is referred to as thepercent base saturation. Values for percent base saturation should bewithin the range of 1-5%, 10-15%, and 60-80% for potassium,magnesium, and calcium, respectively. Media nutrient analysisrecommendations for the application of these nutrients are establishedfrom the ratios of potassium, magnesium, and calcium to each other inaddition to the quantity of these nutrients present in the media.
Media pHMedia pH is a measure of the acidity or alkalinity of a substrate, with apH = 7 indicating a neutral pH. Measured on a logarithmic scaleranging from 0 to 14, a pH > 7 denotes alkaline media and a pH < 7signifies acidic media. The acidity of media is determined by theconcentration of hydrogen ions [H+] on media particles and in themedia solution. The chemical composition of media particles, the ratioof media components in the mix, and irrigation and fertilizer practicesaffect the pH of growing media. Container media can increase 0.5 -1.0 pH units during the growing season as a result of alkalineirrigation water.
Microorganism activity
The pH of organic media influences the activity ofmicroorganisms; bacteria are more prevalent at pH > 5.5, while fungiare most active at pH < 5.5. Nitrification occurs most readily at aneutral pH, contributing to the transformation of theammonium-nitrogen cation (NH4+) to the nitrate-nitrogen anion(NO3-); this increases the potential for nitrogen leaching from thesoilless media solution.
Nutrient availability
Micronutrient availability is optimal at media 5.0 < pH < 6.5.However, because these nutrients are furnished through fertilization,pH regulation is not as crucial with container-grown nursery crops as itis with field-grown woody ornamentals. It is usually unnecessary tomodify the container media to a pH greater than 6.5 for most woodyplant species if sufficient levels of nutrients are available; forEricaceous crops, the media pH should not exceed the value of 5.5.
Soluble salts
Because media is restricted to a limited container volume, ions fromdissolved fertilizers and irrigation water can accumulate and contributeto high soluble salts levels in the media water extract. Media, fertilizermaterials, and irrigation water sources should be selected to minimizesoluble salts buildup; in addition, media solution soluble salts levelsshould be monitored regularly.
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Consult the Cornell Cooperative Extension publication NutrientManagement: The Key to Growing Healthy Nursery Crops foradditional information on soluble salts and pH management.
Buffering Capacity
Buffering capacity is the ability of media to withstand rapid pHfluctuations. Media with a high buffering capacity requiresincorporation of a greater quantity of acid or base to alter the pH thanmedia with a low buffering capacity. Media characterized by lowbuffering capacities include sandy mixes containing little organicmatter, while media exhibiting high buffering capacities are usuallycomposed of greater quantities of organic matter such as peat moss,bark, sawdust, composted sewage sludge, or spent mushroom compost.Select a container media with as high of a buffering capacity aspossible to alleviate unexpected pH fluctuations.
Low initial fertility
Nutrient levels can be more accurately monitored in mediacharacterized by minimal inherent nutrient value than in purchasedand prepackaged media containing pre-incorporated fertilizermaterials. Low initial media fertility affords the grower the opportunityto develop a fertilization program targeted towards fulfillment of thenutrient requirements associated with the developmental stage of thespecies in production.
Some types of media may render certain nutrients unavailable forplant uptake. Use of media such as sawdust or bark that is notadequately composted can lead to the unavailability of nitrogen forplant absorption as microorganisms break down these materials andassimilate the nitrogen for their own use. Vermiculite can inhibitabsorption of phosphorus and iron; likewise, certain kinds of pine barkcan eliminate iron from the media solution.
media pore space is a criticalphysical characteristic whichinfluences nutrient cycling by
growing plants
PorosityThe amount of pore space in container media is a critical physicalcharacteristic which influences water and nutrient absorption andgas exchange by the root system. Pore space is related to theshape, size, and arrangement of media particles. Aerationporosity and water-holding capacity are two critical physicalattributes of container media.
Total porosity reflects the total pore space present in growingmedia; it represents the percentage of the container media volumewhich is not occupied by solid media particles. Porosity isdetermined by media particle size and the extent to which theparticles can be compressed. Total porosity is the sum of theaeration and water-holding porosity of media and shouldcomprise over 50% of the container media volume.
Irrigating media to the point of saturation fills the total porespace with water. As the media drains by the force of gravity,smaller pores remain filled with water while larger poresempty and fill with air. When all water has drained from thelarge pores, the amount of water remaining in the medium's smallpores is referred to as container capacity. Aeration porosity iscomprised mainly of the large pore spaces, macropores, whichdrain water freely as a result of gravitational forces and remainfilled with air after media saturation and drainage.
For adequate gas exchange, aeration porosity should constitute atleast 15%, but ideally, 20-35% of the media volume. Waterretaining micropores should comprise 20-30% of the mediavolume. Water held in even smaller pores is not easily extractedby the plant. Conditions under which these very small spaces arethe only pores retaining water often result in some stomatalclosure and wilting. As the media dries and water is availableonly from the smallest pores, significant wilting can occur.
For sufficient gas exchange, drainage, and water-holdingcapacities, the proper proportion of macropores tomicropores is necessary. The type of container media mix useddetermines the amount of macropores and micropores in themedia. In addition, the size arrangement of pores is important in
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the ultimate water-holding capacity of the mix. A peat-sand mixcontains a greater number of large and medium sized pores than abark-sand mix. Media containing the greatest amount ofmedium-sized pores has the potential to hold more readilyavailable water.
The movement of air, water, and nutrients within media depends onthe media structure, or the geometrical shape of the media, and theporosity of the media. Good media structure provides sufficient mediaaeration, drainage, and water necessary for plant growth.
Determining media porosity
To ensure sufficient media porosity, it is essential to determine totalporosity, aeration porosity, and water-holding porosity. Porositycan be determined through the following procedure:
With drainage holes sealed in an empty container, fill thecontainer and record the volume of water required to reach thetop of the container. This is the container volume.
●
Empty and dry the plugged container and fill it with the growingmedia to the top of the container.
●
Irrigate the container medium slowly until it is saturated withwater. Several hours may be required to reach the saturationpoint, which can be recognized by glistening of the medium'ssurface.
●
Record the total volume of water necessary to reach thesaturation point as the total pore volume.
●
Unplug the drainage holes and allow the water to freely drainfrom the container media into a pan for several hours.
●
Measure the volume of water in the pan after all free water hascompleted draining. Record this as the aeration pore volume.
●
Calculate total porosity, aeration porosity, and water-holdingporosity using the following equations (Landis, 1990):
●
Total porosity = total pore volume / container volume
Media composed of largerparticles has a greater percentage
of aeration porosity
Media composed of smallerparticles has a greater percentage
of water-holding porosity.
Factors influencing porosity
Media porosity is influenced by several factors including:
the particle size of the separate constituents,●
the particle size of the media mixture, and●
particle attributes.●
Particle size
Media composed of large particles has a higher percentage of totalporosity than a medium composed of smaller particles. Larger particlesize also contributes to a greater percentage of aeration porosity thanwater-holding porosity.
In addition, particle size of the media mixture influences mediaporosity. Because media particles are various sizes, the combinedvolume of two or more types of media is less than the sum total of theinitial volumes. Large pore spaces created by large particles are filledby smaller particles when mixed together. Mixtures containing bothlarge and small particles have less aeration porosity, or large porespace, than media composed of only large particles.
Particle attributes
Particle attributes influence the ratio of aeration to water-holdingporosity. Chemical and physical media attributes affect particleshrinkage, breakage, and compression. Decomposition, compaction,and breakage of media alter the original media porosity. As smallermedia particles decrease in size and fill larger pore spaces, pore sizedecreases; as a result, the water-holding porosity near the containerbottom increases.
Saturation of available pore space forms a perched water table. Smallerpore space reduces media drainage as water is held more tightly in thesmaller pores. As the root system of the plant grows, large pore spacesbecome filled, decreasing media aeration porosity. Consider thesefactors when selecting media and monitor the media to detect anyundesirable changes.
Changes in media components
Media particles undergo many changes that affect porosity.Uncomposted or improperly composted media particles such assawdust or bark shrink as the particles deteriorate. Peat moss expandsand contracts when it is subject to wetting and drying cycles. Althoughvermiculite compresses and breaks easily during mixing, bark andperlite withstand compression to a greater degree. The asymmetrical
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shape of perlite allows for large pore sizes in a mix. Sand and pumiceare composed of many particle sizes; small particles often accumulatetowards the bottom of the container, reducing porosity and elevatingthe perched water table.
Internal porosity
Some media constituents consist of internal pore spaces that affect thebulk density, water-holding and nutrient-holding capacities ofcontainer media. Water held inside pine bark can be absorbed by plantsonly if roots penetrate individual particles. However, if rootpenetration does not occur, the internal water is not available forabsorption. Sphagnum peat moss and vermiculite also have highinternal porosities.
Bulk densities and porosities ofselected media for containerized
nursery stock
Bulk density and Physical SupportMedia bulk density is the weight per unit volume that includes solidparticles and pore spaces. Individual particles' arrangements, bulkdensities, and compaction qualities contribute to the total bulk densityof the media.
Bulk density often represents a good estimation of total porosity;however, since water-holding and aeration porosity are closely relatedto the individual particle organization and ratio of micropores tomacropores, these latter two properties cannot accurately bedetermined by the media bulk density. Media constituents that differ inparticle size have higher bulk densities as a mix, lower nutrient andwater-holding capacities, and less air porosity than media composed ofsimilar particle sizes.
Bulk density is measured in grams per cubic centimeter (g/cm3),kilograms per cubic meter (kg/m3), pounds per cubic foot (lbs/ft3), orpounds per cubic yard (lbs/yd3).
Consider dry and wet bulk density values when selecting containergrowing media. Dry bulk density is calculated from oven-dried mediaand is an important factor in media transport. Wet bulk density affectsthe ease of handling and shipping of irrigated container nursery crops.
Absorptive properties
Absorptive properties of media affect wet and dry bulk densities.Although peat moss has a relatively low dry bulk density, oncesaturated, the bulk density increases considerably. A saturated mediacomposed of only peat moss is quite heavy.
Bulk densities determine the stability of container-grown nurserycrops. The dense particle nature and high bulk density of sandprovides ballast to keep top-heavy nursery crops upright in windyconditions. As a rule of thumb, mixes should weigh 40-75 lbs/ft3. As acomparison, sand weighs approximately 100-120 lbs/ft3, while soilweighs approximately 80-100 lbs/ft3 .
Physical support
Mixes should provide ballast and an environment in which the plantcan establish a strong root system. The weight per unit volume of the
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Soluble SaltsThe total concentration of dissolved elements in the soillessmedia leachate is a measurement of soluble salts, also known assalinity. Periodically monitor soluble salts concentrations toprevent toxic accumulation in the media.
Soluble salts accumulate from over-fertilization, insufficientleaching, and as deposits from irrigation water. For optimal plantgrowth, soluble salt concentrations must be within a specifiedrange, and higher concentrations may predispose the plant todesiccation, resulting in chlorosis, defoliation, stem dieback, leafburn, or inhibition of growth. Be especially cognizant of solublesalts levels during the growing season when frequent fertilizationand increased transpiration rates increase the susceptibility ofcontainer-grown plants to salt damage. At the other extreme, lowsoluble salts levels caused by excessive watering or insufficientfertilization can induce chlorosis of mature foliage or inhibitgrowth.
On-site Testing and Sample Preparation
Although soluble salts analysis is performed through a distilledwater extract by the Cornell Nutrient Analysis Laboratories inconjunction with nutrient testing, frequent monitoring of medialeachate can be done at the nursery. According to Halbrooks(1990), the following procedure can be used:
Water the media to container capacity 12-14 hours prior tosample collection to ensure sufficient media moisture.
●
Irrigate the container media with distilled water so that50-75 milliliters of leachate can be collected from thebottom and side holes of the container.
●
For containers one quart, one gallon, or three gallons insize, irrigate with 75, 150, or 350 mls of water,respectively, to yield the desired quantity of leachate.
●
After the media drains for approximately five minutes,analyze the collected leachate for soluble salts content and
●
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If samples are not analyzed immediately after collection,refrigerate the leachate until use.
●
Using a Salt Bridge to Monitor SolubleSalts Levels
Regular monitoring of soluble salts levels helps curb plant injuryfrom excessively high or low salt concentrations. Soluble saltslevels can be monitored with an apparatus called a solu-bridgewhich is available through nursery supply companies. Afterrunning an electrical current through the leachate, theconductivity of the solution can be calculated, indicating the levelof soluble salts in the media solution. The conductivity of thesolution increases with the concentration of soluble salts.
Conductivity readings are often taken on an extract of aspecified ratio of media to water volume, i.e. 1 : 2 dilutionratio. In such cases, to calculate the sample's soluble saltsconcentration, multiply the solu-bridge reading by thedilution factor.
●
For example, when using a dilution ratio of 1: 2 (one partmedia to 2 parts water by volume), multiply thesolu-bridge reading by two (Gilliam and Smith, 1980a).
●
To ensure the solu-bridge is operating correctly, Halbrooks(1990) recommends routinely calibrating the instrument bypreparing a standard solution of 0.01 N KCl (0.7456 g KClper 1 liter distilled water) to measure the conductivity. Thesolution should have a conductivity reading of 1.41mmhos/cm.
●
In addition to salts monitoring, frequently monitor pH at thenursery site. Portable pH meters allow for ease of pHdetermination on media leachate samples. By properlymonitoring and managing pH, nutrient deficiency and toxicityproblems may be alleviated.
Foliar AnalysisConduct foliar analysis approximately once per year to monitorfertilization programs and to diagnose nutrient toxicities ordeficiencies. By periodically analyzing the leaf nutrient content,any rise or decline in acceptable nutrient levels can be controlledbefore the onset of toxicity or deficiency symptoms. In addition,excessive or in sufficient fertilization may be avoided.
Foliar analysis can help to montor fertilization programs, butshould be used in conjunction with other methods of assessingnutrient availabilities. Because several factors affect tissuenutrient concentrations, optimal foliar nutrient levels have notbeen accurately determined for many species. Nutrient uptake isinfluenced by media pH, as pH affects the cation exchangecapacity of the media and nutrient availability. Fertilizationprograms also affect the tissue nutrient content; the addition ofcertain elements affects the rate of absorption of other nutrients.Since optimal foliar nutrient levels have not been established orstandardized, avoid regulating fertilization programs based solelyon tissue nutrient content.
However, tissue nutrient analysis is a valuable tool foridentifying nutrient toxicities or deficiencies. Analyze healthyand unhealthy tissue concurrently since analysis values representactual amounts of nutrients taken up by plants.
Because some nutrients are mobile (N, P, K, Mg, Cl, andMo), deficiency symptoms usually appear first on oldergrowth.
●
Deficiencies of immobile elements, such as Ca, S, B, Fe,Mn, Zn, and Cu are usually initially manifested on juvenilefoliage.
●
As a result of the mobility of some nutrients within the plant,erroneous values may occur from sampling only mature orjuvenile foliage. For a more reliable analysis, collect young andold foliage from the plant at the time the last foliage of the seasonfully expands. Analysis of the mature and juvenile foliage at thistime provides a good comparison of nutrient levels at both stages
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For plants which display more than one growth flush peryear, sample foliage before a growth flush, but after budexpansion. Samples should represent healthy leaves and thoseexpressing nutrient deficiency/toxicity symptoms. Whensampling a group of plants representing the same species or thosegrown under comparable conditions, take composite samples torepresent the entire group, each consisting of 30-100 recentlymatured leaves. When sampling narrow-leaved plants, collect 50shoot tips two inches in length; for broad-leaved specimens,analyze 30-100 leaves (Gilliam and Smith, 1980a). Toxicitysymptoms may also be identified in this manner.
Media drainageSufficient media drainage is critical for optimal plant growth. Therate at which water drains from container media depends on the poresize and cohesive and adhesive forces between the water and containermedia. Media depth and pore size affect the height of the perchedwater table which is created when water saturates media pore spaces. Ifcoarse materials like gravel or sand are placed in the bottom of thecontainer, the smaller pores in the media above this layer will retainwater until pressure forces the liquid downward. Water accumulationabove these coarse materials elevate the perched water table.
If small pores are prevalent in the bottom layer of container media,water will pass through the larger pores above this layer fairly quicklyand saturate the base layer, potentially creating an atmosphere too wetfor vigorous root growth (Swanson, 1989). Avoid media saturation inthe upper or lower layers of the container by thoroughly mixing themedia.
Drainage, or hydraulic conductivity, is the rate at which water flowsthrough the media.
Drainage is affected by the height of the container. Containers thathave identical heights but different diameters have similar drainagecharacteristics when the same media is used in both. In general, waterretention of container media decreases as the height of the watercolumn increases. Media in a tall container characterized by a greaterdepth drains more readily than the same media in a short containerwith a shallower media depth. Media in a short container remainswetter than the same media in a tall container because of a lack ofdrainage; use a deeper container to improve media drainage.
When coarse material is placed at the bottom of a container, the heightof the column is shortened, altering the drainage pattern. However,addition of coarse materials to the container bottom aids in drainage byconstructing larger pore spaces.
Media capacity exists when large pore spaces do not contain any freewater after drainage. Water is retained only in small pore spaces byadhesive and cohesive forces. After drainage, such a situation exists inthe upper portion of container media.
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As an indication of sufficient water retention, media should absorb twoinches of water per hour without runoff. In addition, if one quart ofwater can flow through media in a one gallon container perminute, adequate drainage exists.
Container media should also have the capability to retain sufficientamounts of water for root uptake. The ability of one cubic foot ofmedia to retain three gallons of water is indicative of sufficientmedia moisture retention (Swanson, 1989).
Absence of pathogens and pestsPathogenic fungi, non-beneficial insects, weed seeds, and somenematodes must be eliminated from media before use in containernurseries. Some materials, such as perlite and vermiculite, are alreadysterile as a result of elevated temperature treatment during theproduction process.
Certain types of media have the ability to suppress disease organisms.For example, some species of composted hardwood bark can suppressdeleterious effects of Pythium, Phytophthora, and Thielaviopsis rootrots, Rhizoctonia damping-off and crown rot, Fusarium wilt, andcertain diseases caused by nematodes. Pythium and Phytophthora rootrots have been repressed by composted pine bark, however,Rhizoctonia has not (Hoitink, 1980). Southern pine seedlingsinoculated with Pythium and Fusarium had a higher death rate in apeat-vermiculite mix than in a mix that included pine bark (Pawuk,1981). Ericaceous plants have shown significant growth increaseswhen the primary mix component was composted hardwood barkrather than peat. Composted bark suppresses plant pathogens by:
discouraging the proliferation of pathogenic organisms,●
fostering organisms antagonistic to many plant pathogens, and●
Composting reduces the presence of plant pathogens. Thecomposting process, quantity and source of nitrogen added, andtemperature of the stack affect the pathogen suppressiveness of theresultant media.
Composting also exterminates chemical inhibitors present inhardwood and some softwood barks. Because these toxic substanceswhich are often present in fresh bark may be lethal to young plants it isimportant that the correct duration of time and composting temperaturebe employed to eliminate these materials. Antagonisticmicroorganisms and naturally occurring chemicals with fungicidalattributes curtail the incidence of root pathogens. In addition, thequantity of wood present in the bark influences pathogen suppression.Elevated wood contents decrease Phytophthora control; similarly,mixes containing more than 50% Canadian peat as a substitute for
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composted hardwood bark exhibited increased incidence of root rots(Hoitink, 1980). On the contrary, some Sphagnum peat lots cansuppress root rot and damping-off caused by Pythium spp. Suppressionmay be attributed to the presence of antagonistic fungi in some of thesepeat lots (Wolffhechel, 1988).
Media selectionEvaluate several factors when selecting container media. Althoughcost is an important consideration in the selection of container media,it should not be the sole factor upon which selection is based. Becausecontainer media plays an important role in the successful establishmentand growth of nursery crops, it should be carefully selected with thefollowing factors in mind:
uniformity●
reproducibility●
dimensional stability●
bulk density●
wettability●
availability●
ease of storage●
Many factors to consider are closely related. Availability ofcomponents affects media price and uniformity of the mix. Materialsmanufactured in one geographic region may be costly to transport toanother location. In addition, shipping costs may be substantialbecause of high media bulk densities. Demand for a particularcomponent may also affect the cost, perhaps limiting the availability ofthe material.
Maintain consistent media composition once a suitable mix isidentified for growing particular nursery crops. Uniformity andreproducibility of media components and the resultant mix are vital insupporting healthy plant materials. Altering any media componentsaffects the entire growing system and may require modifying wateringand fertilizing practices, which involves a significant amount of timeand effort. If a mix is contributing to the production of quality plantmaterials, is economically feasible for your operation, and is readilyavailable, stick with it!
Structural integrity and ease of storage
Container media should be characterized by structural integrity andease of storage. Materials which require composting but have not yetundergone the process may decompose and lose the critical attributesfor which they were chosen. Wood chips, sawdust, and bark may beused after composting along with other composted organic
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constituents; an exception is pine bark, which does not need to becomposted before use. Keep in mind that peat moss varies in its stateof decomposition; knowledge of this is important to maintainreproducibility of a mix. Sterile media should be stored in closedplastic containers to eliminate contamination with unwanted pests orweed seeds. This may not be possible for large operations; in suchcases, a fully enclosed structure may reduce potential mediacontamination by pathogens or weed seeds.
RewettabilityThe ability to rewet media is important, especially for materials suchas peat moss and pine bark, which tend to be hydrophobic upon drying.The wettability of pine bark is increased by composting. A wettingagent, or surfactant, may be necessary during media mixing toprovide adequate moisture for plant growth; in addition, more frequentirrigation may be required. Although they enhance the wettability ofmedia, the effectiveness of surfactants may decrease over time; inaddition, some wetting agents are phytotoxic to woody plant materials.
Organic mediaThe major types of organic media used in container-crop horticultureare peat moss, spent mushroom compost, and bark. Other organic andinorganic additives are also often added to these media.
Peat Moss
Peat is produced when incompletely decayed plants, including speciesof sedges, grasses, and mosses, accumulate under cool temperaturesand conditions of decreased oxygen and nutrient levels.
Different types of peat moss vary in their degree of decomposition.Plant species, climate, and quality of water affect the distinctcharacteristics of peat moss. Four horticultural classifications of peatmoss exist:
sphagnum,●
hypnum,●
reed-sedge, and●
peat humus●
Sphagnum peat moss
Sphagnum peat moss, derived from the genus Sphagnum, contains atleast 90% organic matter on a dry weight basis. In addition, this peatmoss contains a minimum of 75% Sphagnum fiber, consisting ofrecognizable cells of leaves and stems. Approximately 25 species ofSphagnum exist in Alberta, Canada and 335 species are presentthroughout the world. Sphagnum fuscum is an important speciesbearing many desirable traits. Sphagnum grows in northern coolregions and is also located in peat bogs found in Washington, Maine,Minnesota, and Michigan.
Many pores are present in the leaves of sphagnum; when used asgrowing media, as much as 93% of the water occupying this internalpore space is available for plant uptake (Peck, 1984). After draining,sphagnum peat can hold 59% water and 25% air by volume.Sphagnum is usually characterized by an acidic pH, low soluble saltscontent, structural integrity, and the ability to serve as a nutrientreserve (Landis, 1990).
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Although peat mosses are classified into four different groups,variation may exist within any one type of peat moss. Peats of thesame classification often differ notably in quality, and even peats fromthe same bog taken from separate layers can possess different chemicaland physical properties.
Sphagnum peat moss is classified as light or dark peat, based on itscolor. Light peats are characterized by a large amount of internal porespace, 15-40% of the pore space comprises aeration porosity Darksphagnum peat does not display the elasticity of light peat and isusually not as long lasting.. Dark sphagnum peat moss maintains twicethe cation exchange capacity of light peats, yet does not possess asmuch total or aeration porosity. An associated table lists generalcharacteristics of sphagnum peat moss.
Hypnum peat moss
Hypnum peat moss, a second category of peat, is found in the northernUnited States and tends to break down more quickly than sphagnummosses. Although hypnum peat moss is usually less expensive thansphagnum peat, it may contain plant pathogens or weed seeds as aresult of the conditions under which it was produced. To meet thecriteria of this classification, the oven dry weight of hypnum peat mustbe comprised by over 90% organic matter; 50% of this must representplant material from the genus Hypnum. Container tree seedlings shouldnot be grown in media consisting of large quantities of hypnum peat;however, this type of peat is often used as a suitable media componentfor acid-intolerant crops.
Reed and sedge peat
Sedges (Carex spp.), reed grass (Phragmites), grasses, rushes, andother marsh plants are represented in this category of peat moss. Anoven dry sample consists of at least 33% of these plant materials on adry weight basis. This category of peat moss is not very suitable forgrowing media; it quickly decomposes, is characterized by a fineparticle size and low fiber content, and is less acidic than sphagnum.Sedge peats often contain more plant nutrients than sphagnum; thisresults from nutrients leaching through mineral soils into thedeveloping layers of peat. This type of peat often has a higher cationexchange capacity per unit weight, appears darker in color, andmaintains a lower water-holding capacity than sphagnum peats(Landis, 1990).
Peat humus
Because the plant materials which comprise this category of peat areextensively decomposed, it is difficult to distinguish among the
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individual components. Peat humus, often composed of reed-sedge orhypnum peat moss, contains less than 33% peat fiber. Since it oftencontains clay and silt and does not increase drainage or aeration, peathumus is not usually recommended as a component for containermedia (Landis, 1990).
Harvesting
The texture of peat is affected by the method in which it was harvestedand processed. The technique used to harvest peat depends on climateand bog characteristics, such as the presence of tree stumps in the bog.Peat is harvested from bogs by hydraulic mining or block cutting.
Peat compacts more through hydraulic mining. In this harvest method,peat is shredded and removed from the bog by dredging. The peat'saeration porosity is decreased.
Peat derived by the block-cutting method is cut in slabs as it isexcavated from the bog and shredded to a coarse texture. Block-cutpeat has a higher total porosity and aeration porosity and holds moreavailable water than mined peat; however, water-holding porosity islower in block-cut peat as opposed to mined peat(Wilson, 1985).
Spent mushroom compost
Spent mushroom compost can be incorporated to represent 25 - 50% ofthe mix volume. This organic material is often aged nine to twelvemonths before use. It is characterized by a pH > 7, high potassium,phosphorus, and salt levels, and contains adequate amounts of traceelements and calcium.
Spent mushroom compost continues to decompose and compact in thecontainer, decreasing air pore space and increasing the water-holdingcapacity of the mix. Adding pine bark or perlite enhances the airporosity. Although this material has a high buffering capacity,applying iron sulfate contributes to reducing the pH (Bunt, 1988).
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Hardwood bark mustbe composted before
use in containermedia.
Bark
Bark is often used as a media component to increase the air porositywithin a mix. Some bark fragments contain up to 43% internalporosity, from which roots can absorb water if penetration of theparticle occurs (Pokorny, 1987). Pine bark, which is acidic in nature,also has a low initial fertility-- an important characteristic of growingmedia. Composted bark has a higher cation exchange capacity thanraw bark and represses pathogenic fungi (Hoitink, 1980).
Several bark particle sizes have been recommended for mediacomposition. Suggested formulations for container-grown cropsinclude:
a mix characterized by 25-33% of the pine bark particles lessthan 0.5mm in size;
●
peat moss based media containing 25-50% pine bark; or●
media containing various bark particle sizes attained by using ahammermill with a screen size of 2 - 2.5 cm.
●
The use of bark in container media offers both advantages anddisadvantages. Bark which has not been composted properly inducesnitrogen deficiency problems; however, composted bark withsufficient nitrogen fertilizer added during the process should not posethis problem. Bark from alder, poplar, maple, and oak are prone todecay as a result of a high cellulose content; plants grown in mediacontaining these barks may experience nitrogen deficiency as theconstituents rapidly decompose. Because of this, it is necessary to addmore supplemental nitrogen to hardwood rather than softwood barkbefore or during composting to preclude nitrogen deficiency(Bilderback 1982). Hardwood bark breaks down three times morequickly than softwood bark. Continued decomposition of compostedhardwood bark media during the growing season increases thewater-holding capacity and decreases the air porosity of the mix. Inaddition, hardwood bark seems to repress nematodes and rootpathogens more effectively than softwood bark; fungicidal inhibitorsand antagonistic organisms present in composted hardwood barkcontribute to this repression. Some barks contain organic or inorganictoxins, including high levels of monoterpenes, phenols, or manganesethat may prove harmful to plants. Phenolic compounds in fresh barksare especially toxic to young nursery crops. Tree species, age, time ofharvest, soil type, and geographical region are factors that affectphytotoxicity. Bark derived from older trees, lower portions of the tree,or removed during winter months tends to be more phytotoxic thanbark removed from younger trees, upper portions of the tree, or duringspring months. In addition, obtaining bark of uniform quality andparticle size is often difficult.
The characteristics of softwood and hardwood bark are quite different.
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Some softwood bark can be used without composting; hardwood barkmust be composted before use or phytotoxicity may ensue. Aging andcomposting bark is usually an effective way to eradicate toxins. Freshpine bark repels water to a greater extent than aged pine bark orcomposted hardwood bark; to increase the moisture content of pinebark, soak it under a sprinkler system. Although pine bark has a lowerwater-holding capacity than peat moss, it holds a greater amount ofavailable water for the plant (Pokorny 1979). Avoid water stress innewly planted nursery crops by watering regularly, particularly duringthe 30 days after planting.
Many plant materials appear to grow well in fresh pine bark (Self andPounders 1974). Fresh pine or softwood bark usually has an initial pHrange of 4.0 - 5.0; as pine bark ages, the pH does not increaseappreciably. To increase the pH of pine bark, add 4 - 15 lbs. ofdolomitic limestone per cubic yard; within a few weeks the pH of themedia should equilibrate to a suitable planting pH (Bilderback 1982).Aged pine bark is often favored over fresh pine bark by growers; thismay be attributed to a more desirable particle size distribution in theformer (Pokorny, 1975).
Recently harvested hardwood bark is usually characterized by a pH of5.2 - 5.5. Lime should not be added to hardwood bark mixes; as thebark ages or is composted, the pH may exceed 7.0 as a result of thenatural calcium content of the bark. To avoid magnesium deficiency inhardwood bark mixes, incorporate one pound of magnesium sulfateinto each cubic yard of mix. If a bark-sand mix is desired, add a lowpH sand to decrease the pH of composted hardwood bark media(Bilderback, 1982).
Peat additives
Additives incorporated into young sphagnum peat tend to decrease thetotal porosity of the media. For example, the addition of fine sand toyoung sphagnum peat appreciably decreases media aeration porosityand total pore space. Research results indicate that the addition ofpolyacrylamide gel to young sphagnum peat increases thewater-holding porosity but decreases the aeration porosity of themedia. Addition of a wetting agent to the peat contributes to the easeof drainage and reduces the surface tension, increasing media airporosity (Wilson, 1985).
Other organic media
Sawdust, wood chips, spent mushroom compost, rice hulls, and barkmay be used for container media. Compost wood residues, includingsawdust, before using as a growing media. Because of the high carbonto nitrogen ratio (C:N) of these materials, it may be necessary to add
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nitrogen during composting. This will help prevent nitrogen deficiencyand phytotoxicity. Because hardwood sawdust decomposes morequickly than pine sawdust, approximately 1% more nitrogen by weightis required to compost the former material (Mastalerz, 1977). Althoughaged sawdust requires less nitrogen for decomposition, incorporationof nitrogen is essential for complete adjustment of the C:N ratio priorto mixing and potting. Keep in mind that phytotoxicity may result fromtreating wood with preservatives; consequently, avoid using thesematerials.
Chemical properties of sawdust vary widely. Sawdust from walnut,redwood, western redcedar, or incense-cedar may contain substancestoxic to plants; even sawdust derived from different tree species withina genus may contain varying levels of toxins. High salt concentrationspresent in sawdust derived from trees harvested in seaboard regionscan be harmful to plants; inconsistent particle size may pose a problemin the formulation of uniform media (Landis, 1990). Trial mediacomponents on a small scale to ensure compatibility withestablishment of healthy nursery crops before incorporating them intothe mix for large scale use.
Inorganic mediaMaterials such as vermiculite, perlite, and sand represent the inorganicfraction often used in container media formulations. These materialsgenerally increase the aeration porosity and drainage yet decrease thewater-holding porosity of media. Inorganic components are usuallyinert materials characterized by a low cation exchange capacity.
Vermiculite Grades
Vermiculite
Vermiculite is a commonly used inorganic media component which ismined in the U.S. and Africa. This mineral, comprised of analuminum/iron/magnesium/silicate mixture, is excavated as a materialcomposed of thin layers. Processing includes heating the vermiculite totemperatures upwards of 1000°C, which converts water trappedbetween the layers of the material into steam. The production of steamresults in a pressure that expands the material, increasing the volumeof the pieces 15 to 20 times their original size. Vermiculite is sterilebecause of these high heating temperatures used during processing.
Vermiculite is characterized by a high water-holding capacity as aresult of its large surface area:volume ratio, a low bulk density, nearlyneutral pH, and a high cation exchange capacity attributed to its platystructure. Because it compacts readily when combined with heaviermaterials, vermiculite is sometimes recommended more forpropagating material than container media.
Vermiculite gradually releases nutrients for plant absorption; onaverage it contains 5-8% available potassium and 9-12% magnesium.This inorganic media component can adsorb phosphate--some ofwhich remains in an available form for plant uptake--but cannot adsorbnitrate, chloride, or sulfate. Vermiculite can fix ammonium into a formthat is not readily available for plant absorption. This fixed nitrogen isgradually transformed to nitrate by microorganisms, making itavailable for plant uptake.
Vermiculite is manufactured in four different grades, differentiated byparticle size. Insulation grade vermiculite and that which is marketedfor poultry litter (which has not been treated with water repellents) hasbeen used with some success. Vermiculite which has been treated withwater repellent, such as block fill should not be used as growingmedia. Because vermiculite tends to compact over time, it should be
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incorporated with other materials such as peat or perlite to maintainsufficient porosity. It should not be used in conjunction with sand or asthe sole media component, because as the internal structure ofvermiculite deteriorates, air porosity and drainage decreases (Landis,1990).
The particle size of vermiculite influences the water-holding andaeration porosity of the material. Although grade classification is basedupon particle size, each grade is represented by a range of particlesizes. Note that grades consisting of larger particle sizes have a higheraeration porosity and lower water-holding porosity than gradesconsisting of a smaller range of particle sizes. Properties of the fourvermiculite grades are shown in an associated table.
Perlite Classifications
Perlite can retain twoto four times its dry
weight in water.
Perlite
A mineral of volcanic derivation, perlite is a second inorganiccomponent which may be used in formulating container mixes.
This chemically inert material is extracted in New Zealand, the U.S.,and other countries and is usually mined by scraping the earth'ssurface. The processing method includes a grinding and heat treatment(up to 1000‰C) which results in very lightweight, white sterilefragments. As the ore is heated, internal water escapes as steam,resulting in the expansion of the material.
Perlite has a very low cation exchange capacity, low water-holdingcapacity (19%), and neutral pH. The closed-cell composition of perlitecontributes to its compaction resistance, enhances media drainage, andheightens the aeration porosity of peat-based media (Bilderback 1982).Because perlite contains only minute amounts of plant nutrients, liquidfeeding is a practical mode of fertilization. Be aware of possiblealuminum toxicity in acidic media (pH < 5).
The very low levels of fluoride perlite contains is not likely to poseplant health problems. Any soluble fluoride present in a mediacharacterized by 6.0 < pH < 6.5 will precipitate out of the media withexcess calcium from sources such as gypsum, limestone, or calciumnitrate.
Although perlite has several positive attributes, it also has drawbacks.Perlite consists of many fine fragments which, when dry, can lead tolung or eye irritation. In addition, because water clings to the surfaceof perlite, it may tend to float in the presence of water (Landis, 1990).
Perlite contains, on average, 47.5% oxygen, 33.8% silicon, 7.2%aluminum, 3.5% potassium, 3.4% sodium, 3.0% bound water, 0.6%iron and calcium, and 0.2% magnesium and trace elements (PerliteInstitute, 1983). Although a uniform categorization of perlite does not
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exist, individual producers of this inorganic component assign gradelevels. Perlite classifications for horticultural use are listed in anassociated table. This inorganic media amendment is sometimesrecommended for use only in propagation media because of its lowbulk density and tendency to compact.
In comparison with sand, polystyrene, or pumice, perlite has thegreatest inner total porosity. Coarse perlite is characterized byapproximately 70% total porosity, 60% of which is aeration porosity.Perlite can retain two to four times its dry weight in water, which ismuch greater than that of sand and polystyrene, yet much less than thewater-holding capacity of peat and vermiculite (Moore, 1987).
Sand
Sand has been used as an inorganic media component to add ballast tocontainers. Some sands contain calcium carbonate which may raisemedia pH undesirably. A rise in pH may lead to nutrient deficiencies,particularly of minor elements such as iron and boron. A few drops ofdilute hydrochloric acid or strong vinegar may be added to sand to testfor carbonates; if bubbling and fizzing result, carbonate is present as aresult of carbon dioxide production. Sand used for container mediashould have a 6 < pH < 7. Sand maintains good drainage, a lowwater-holding capacity, and a high bulk density when usedindependently of other materials. Because of its shape and size, sandcan obstruct pore spaces, decreasing drainage and aeration, instead ofimproving porosity. Various sand particle sizes have beenrecommended for container media use, including ranges of 2-3 mm or0.05 - 0.5 mm (fine sand) in size (Landis, 1990). In addition, anotherrecommendation suggests that 60% of the particles be within 0.25-1.0mm range, and 97% be greater than 0.1 mm and less than 2 mm(Swanson, 1989). Uniformity coefficients assigned to sand mixturessignify the amount of sand which is within a certain size range; acoefficient < 4 is evidence of a homogeneous sand mixture (Swanson,1989). If the correct grade of sand is used, the wettability of the mediais enhanced.
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Calcined clays
When fired at high temperatures, some clays, fuel ash, and shales formstable compounds that possess low bulk densities and internalporosities of 40-50%. Though calcined clays alter the physicalattributes of media in a positive way, they also decrease the level ofwater-soluble phosphorus in the mix. Because calcined clays arecharacterized by a high cation exchange capacity, fertilizer applicationrates may need to be modified if calcined aggregates are incorporatedinto the media mixes (Bunt, 1988).
Pumice
Pumice is produced as volcanic lava cools; escaping steam and gascontribute to its porous nature. This alumino-silicate material containspotassium, sodium, magnesium, calcium, and slight amounts of iron.Pumice can absorb K, Mg, P, and Ca from the soil solution and renderit available for plant absorption later (Bunt, 1988).
offers no nutritivevalue to the plant andis not prone to decay.
Plastics: Foam Materials
Expanded polystyrene flakes
This chemically neutral material may be used with other types ofmedia amendments, such as peat, to enhance the physical properties ofa mix. Because of its large size and closed pore structure, polystyrenecan improve the aeration porosity of media while decreasing itswater-holding capacity. Expanded polystyrene flakes should not besterilized with steam, chloropicrin, or methyl bromide.
Because of their inability to attract nutrients, it may be necessary tobegin a liquid fertilization program sooner than normal when usingpolystyrene flakes in a mix. This material tends to float duringirrigation and cling to other materials and surfaces during blending(Bunt, 1988).
Urea-formaldehyde foam resins
This low bulk density material, composed of an open arrangement ofpores, can imbibe 50-70% of its volume in water. This materialcontains 30% nitrogen by weight, which it gradually releases as theresins slowly decompose. Because of the slow rate ofurea-formaldehyde resin decay, the nitrogen contained in thissubstance does not contribute measurably to available nitrogen forplant use. Freshly produced urea-formaldehyde foam resins maycontain less than 2.5% formaldehyde which should be thoroughlyevaporated before incorporating the foam resins into container mixes.In addition, this media amendment lacks any other nutrients, ischaracterized by a pH near 3.0, and often comprises 20 - 50% of themix volume (Bunt, 1988).
Polyurethane foam
A porous material of low bulk density, polyurethane foam can absorb70% of its volume in water. This material is characterized by a pH near7.0, offers no nutritive value to the plant, is not prone to decay, and isavailable in flake form for use in container media. It has beensuggested that this material either be heated to 100‰C for 120 minutesor be rinsed in ethanol followed by water to ensure any phytotoxicsubstances are eliminated from the foam before use (Bunt, 1988).
Media: Plastic inclusions
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University of California mixes consist of five distinct materialcombinations comprised of either peat, sand, or both components towhich any of six fertilizer formulations may be incorporated. Threepopular mix formulations are the University of California mixes C, D,and E. UC Mix C, composed of 50% peat and 50% sand by volume, iswidely used in the container nursery industry. Mix D consists of 75%peat moss and 25% sand by volume and UC Mix E consists of 100%peat. Steam sterilization of these mixes is suggested (Bunt, 1988).
Cornell Peat-lite mixes
Developed at Cornell University, these mixes consist of peat andperlite or peat and vermiculite to which various fertilizer formulationsmay be added. Peat-lite Mix A consists of 50% sphagnum peat and50% vermiculite by volume. Peat-lite Mix B consists of 50%sphagnum peat and 50% perlite by volume. Keep in mind that thesemixes may be too light for outdoor use. A non-ionic wetting agent maybe incorporated into the Cornell Peat-lite mixes first by adding thewetting agent to a small amount of vermiculite and then working it intothe mix. Alternatively, the wetting agent may be used at the rate of85cm3 per 30-60 liters of water to moisten each cubic meter of mix(Bunt, 1988).
Other mixes, such as the Pennsylvania State and Oklahoma StateUniversity mixes are also available. The Penn State mix is based onsphagnum peat; the Oklahoma State University mix contains 3 partsground pine bark: 1 part peat: 1 part sand by volume. Various fertilizerformulations have been recommended for addition to these mixes(Landis, 1990).
Custom mixingCorrect media mixing is imperative for successful nursery cropproduction. Uniformity of the mix is essential to avoid potentialdrainage, aeration, and plant growth problems. The formation ofaggregates in the media, enhanced by the addition of water, helpsmaintain mix homogeneity (Bartok, 1985). Peat, when lightlysqueezed, should dribble water slightly if properly moistened. Perliteand vermiculite may be moistened with water to alleviate dust, butother container mix ingredients should remain dry. For greater ease ofblending peat-based media in a mixer, incorporate dry components intomoistened peat (Bartok, 1985).
When mixing media, uniform quantities of materials should be addedto produce a consistent, final product from batch to batch. Differentmixing techniques result in varying degrees of product uniformity.Using a shredder may result in unwanted separation of heaviercomponents and a rotary tiller may not mix the bottom layer ofconstituents. However, drum, concrete, and bin mixers achieve mixturehomogeneity through a rolling action (Bartok, 1985).
Mixing can be accomplished by blending in batches or through acontinuous flow process. Blending in batches, by hand, or in a mixerproduces a defined amount of media. Continuous flow systems aresuitable for producing large quantities of media. As an alternative tomechanical mixing, media can be blended on a sanitary pad with handtools--up to 6 cubic feet of media can be blended at one time using thismethod. When blending ingredients by hand, all media constituentsshould first be mounded on top of each other. The stack should bethoroughly turned and moistened during blending to facilitate thewettability of the media.
When machine mixing, a variety of options are available. With a beltmixer, each constituent is fed onto a conveyor, where it is transportedinto a revolving receptacle for blending with other media components.Paddle mixers combine ingredients in a stationary drum; paddlesaffixed to the inside of the drum stir media constituents together(Landis, 1990). Small batches can be blended in mobile cement ormortar mixers with 3-6 cubic foot capacities or in modified concretemixing trucks with 5-11 cubic yard capacities. Single batch mixers canyield 0.25 to 12 yd3 of mixed media per hour, while continuous flowmedia blending procedures can produce up to 50 cubic yards per hour(Bartok, 1985). Other machinery such as soil shredders, grinders, and
Media: Custom Mixes for Container Media
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auger mixers are not recommended for media mixing because of theextensive particle deterioration that results from these processes (Judd,1984). It is important that aseptic conditions be preserved duringmixing; all media constituents should remain sterile from start tofinish.
Mixing duration
The duration of mixing is critical for the formation of a successfulproduct. Overmixing organic matter like peat may result in a mix thatis too finely ground, leading to compaction and decreasing aerationand drainage. When mixing peat and vermiculite, separate fibersshould remain distinguishable and these materials should maintainsome resiliency. When mixing by machine, ensure mixing time is notexcessive. Overfilling the mixer, blending when the components aretoo wet, or mixing too long may result in an undesirable product.Many mechanical mixers require only three or four minutes tosufficiently blend materials when filled to 75% capacity (Whitcomb,1988). A blending time of two to four minutes works well when usingdrum or hopper mixers (Bartok, 1985), but mixing media longer thanfive minutes has been shown to diminish particle size (Landis, 1990).
Consider many additional factors when deciding upon mixing methodsand equipment. The location of equipment should allow for ease ofmedia blending and storage. Small mobile mixers, large stationarymixers, and continuous flow operations have different spacerequirements. Energy demands should be considered, as electric or gaspowered engines are available. Smaller mixers usually operate on 2-3horsepower, while larger machines may require 5-10 horsepower(Bartok, 1985). Other machinery which may be incorporated into themixing system includes wetting attachments, steam pasteurizinginjectors, and screens to sift out clumps of media.
Consider all costs
When producing custom mixed media, be aware of all costs involvedwith the processes. Keep in mind that blended media may shrink up to25% by volume when water is added; factors such as duration ofblending, quantity of water added, and the structural stability of themedia constituents affect the volume loss of final product. Weigh thecost of purchasing commercially produced media against the cost ofcustom producing a comparable volume. Factor in shrinkage,subsequent loss of raw materials, and consider labor costs beforedeciding to custom mix media (Kusey, 1989).
Avoid overcompaction
Overcompaction of container media detrimentally affects crop growth.
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In addition to decreasing total porosity which affects aeration anddrainage, media compaction also inhibits root growth. Mediacompaction may be expressed in symptoms such as browning of roots,leaf drop, foliar chlorosis, and necrosis. Symptoms resembling nutrientdeficiency, drought stress, or overwatering may also be attributed tocompaction. Media compaction inhibits adequate nutrient absorption,often resulting in iron chlorosis and other maladies. In addition, rootsin compacted media are more vulnerable to pathogen attack as a resultof the stress of compaction (Landis, 1990).
Product and shipping costs of prepackaged, commercially preparedmedia may initially be high; however, long term success with plantproduction, substrate uniformity, and freedom from pests mayoutweigh the initial purchasing costs (Bilderback, 1982). Productinconsistency and poor quality of some custom mixed mediacontribute to grower costs during the growing season. Initial costsassociated with custom media mixing can be increased by poor planthealth or required weeding as a result of improper media sterilization.Evaluate the possible consequences and concealed expenses of custommixing before deciding to follow such a course of action. Nursery size,availability of media constituents, and mixing equipment are all factorsaffecting the decision to custom mix media or use prepackagedcommercially prepared media.
Combining media components
When deciding which media components to include in a custom mix,keep in mind the aeration and nutrient requirements of the specificcrops to be grown. Individual characteristics of media components donot necessarily have an additive effect when combined into a mix.Media porosity is determined by particle size and distribution of theindividual components in relation to one another.
Avoid incorporatingfertilizers beforepasteurizing or
sterilizing media.
Preplant incorporation of materials
Fertilizers
Because ideal container media is characterized by low inherentfertility, it is necessary to carefully develop a fertilization program thatwill fulfill the specific nutrient requirements of the nursery crops.Factors such as plant age, species, and time of year affect nutrientrequirements of many nursery crops. To avoid elemental deficienciesor toxicities, fertilize with adequate nutrient concentrations andincorporate fertilizers uniformly. Base nutrient management programson media analysis results and recommendations. Consider any inherentmedia nutrient supplying power when formulating a nutrientmanagement program. See the other documents in the resourceNutrient Management: The Key to Growing Healthy Nursery Cropsfor more information.
Surfactants
Surfactants are chemical agents that enhance the wettability of organicmaterials such as pine bark and peat moss. Some wetting agents maybe harmful to woody plants; researchers have found that application ofsurfactants at some of the suggested rates may result in plantphytotoxicity. Consult the following references for more information:Ward et. al, 1987; Whitcomb, 1988; Barnett and Brissette, 1986;Barnett, 1977; and Pokorny, 1979.
Mycorrhizal fungi inoculum
Mycorrhizae are soil fungi associated with plant roots and increasenutrient and water absorption by the roots. The fungal hyphae extendbeyond the root zone, expanding the area from which nutrients andwater may be absorbed. Certain mycorrhizae also produce growthregulators that stimulate feeder root development. Some mycorrhizalfungi produce antibiotics that are lethal to certain root pathogens.Barriers formed through root association with mycorrhizal fungi mayalso aid in defending against pathogens. Mycorrhizal associationdecreases drought stress, yet increases salt tolerance and rootregeneration. Mycorrhizal fungi can be introduced into the mediaduring mixing to shorten the length of time for reinoculation to occur.Consult the following references for additional information and
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possible inoculum sources: Landis et al., 1990 and Digat, 1988.
Pasteurization and sterilization
Sterilizing media eradicates all living organisms from the componentswhile pasteurizing eliminates only some microorganisms from media.The presence of some fungi, bacteria, and actinomycetes may bedesirable in container media; in such cases pasteurizing the media isrecommended to eliminate injurious organisms while retainingbeneficial organisms. Steam may be used to sterilize or pasteurizemedia, depending on the treatment temperature. During pasteurization,media is heated to temperatures of 140°F to 177°F (60°C - 82°C) for atleast half an hour.
Some inorganic media constituents, such as perlite and vermiculite, aresterile as a result of their processing method. Organic components,including peat, may not be sterile after processing. Commerciallyprepared media is usually free of insects, pathogens, and weed seeds ifcontamination has not occurred during storage. However,custom-mixed container media may require pasteurizing before using.Avoid incorporating fertilizers before pasteurizing or sterilizing media;if fertilizers are present, the high temperatures used during theseprocesses could lead to soluble salt accumulation and associatedproblems.
Rooted in successCarefully consider the many factors that define suitable containermedia before making a selection. In addition to chemical and physicalproperties of media components, review other important characteristicsduring media evaluation:
media uniformity,●
reproducibility,●
wettability,●
bulk density,●
stability,●
availability, and●
ease of storage.●
Trial media on a small scale to resolve any potential problems beforeadopting the mix for widespread use. Selecting container media that isappropriate for a particular nursery crop operation lays the foundationfor producing woody plants rooted in success.
AcidicA term describing materials with a pH less than 7.
AlkalineA term describing materials with a pH greater than 7. Asynonym of "basic".
Anion exchange capacity
A measure of the ability of a medium to adsorb negative ions.Most anion exchange capacities of growing media arenegligible; this allows some nutrients such as nitrate andphosphate to leach readily from the soil.
Buffering capacityThe ability of a medium to withstand rapid pH fluctuations.
Cation exchange capacity
Often abbreviated CEC, a measure of the ability of a medium toadsorb positive ions, or cations. Many nutrients which arenecessary for plants are in this form.
Controlled-release Fertilizer
An additive to soils or growing media which meters the releaseof nutrients over time.
Ericaceae
Genus of plants which includes azaleas and rhododendrons,often cultivated for ornamental purposes, which prefer stronglyacidic soils.
FertigationLiquid fertilization; a type of post-plant fertilization which usessoluble fertilizers to deliver nutrients to plants.
KPotassium.
Macronutrient
An element required in proportionately larger amounts by plantsand vital for healthy plant growth. Macronutrients includeNitrogen (N), Potassium(K), and Phosphorus (P), as well as
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Calcium (Ca), Magnesium (Mg), and Sulfur (S). cf.Micronutrient.
MediaPlural of medium. Soil and soil-like environments in whichplants are grown. Often contain both organic (e.g., peat moss)and inorganic (e.g. sand) additives.
Micronutrient
An element required in proportionately smaller amounts byplants but still important for healthy plant growth.Micronutrients include Iron (Fe), Manganese (Mn), Zinc (Zn),Copper (Cu), Boron (B), and Molybdenum (Mo). cf.Macronutrient.
N, NitrogenA Macronutrient essential to plants' development and growth.
N-P-KA way of classifying fertilizers. Three numbers which denote theratio of Nitrogen, Phosphorus, and Potassium in a fertilizer.
PPhosphorus. A Macronutrient essential to plants' developmentand growth.
pH
A measure of the concentration of hydrogen ions in a substance.pH of 7 is neutral, while pH < 7 is acidic and pH > 7 isconsidered alkaline. Media pH regulates the availability ofnutrients to plants.
PotassiumA Macronutrient essential to plants' development and growth.Chemical symbol K.
ProportionerDevice used to regulate the flow of liquid fertilizers into anirrigation system.
Porosity
The degree to which growing media has contains open spaces.These spaces, or pores, are essential to holding water(water-holding porosity) and allowing gas exchange (aerationporosity) in plants' root zones.
SCUSulfur-coated urea. A granular, controlled-release fertilizer.
Salt index
The potential of a fertilizer to prevent water absorption from thesoil solution by plant roots.
Surfactant
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A chemical added to media components to enhance theirwettability, or ability to retain water.
These pages were created in 1997 from three bulletins written byAmy Fay Kasica.
Edited by Amy Fay Kasica & George L. GoodAdditional editing & world wide web design by Robert D. Scott
Hard copies of this document, other reference materials related tothe care and production of ornamental plants, and slide sets aboutthe cultivation of ornamental plants may be ordered from:
Department of Floriculture and Ornamental HorticultureCornell UniversityIthaca, NY 14853-5905
