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SoilChapter 10
Soil Fertility ManagementPages 311 – 355
“Fertilizers, liming soils and irrigation are easily manipulated factors for increasing plant growth.”
pg. 311
When large amounts of plant nutrients are harvested from soils in the form of crops . . .
Fertilizers are commonly used to replenish lost nutrients in most all agricultural situations
Low cost fertilizers can yield good yield results
Excess use of fertilizers can yield pollution problems in ground or surface water
Soil fertility problems are typically easy to solve
GoalsThe goal is for minimum fertilizer
inputs - reduce costs- maximum yields - increase profits- reduce pollution- increase health and aesthetics of plants
Limiting factors to fertilization
Goal to spend less than 20% of all production costs to increase yields up to 50%
But fertilizer input may not be profitable when:
Limiting factors to fertilization
Water is a limiting factorGrowth hindered by insects, soil pH,
plant diseases, extreme temperatures
Increased yields has less market values than fertilizer costs
Replenishing nutrientsNitrogen can be replenished by
recycling crop wasteOr by growing legume crops and
recycling them
Replenishing nutrientsSoil minerals lost through crop
harvest are typically replenished by the addition of those minerals as mineral fertilizers
Replenishing nutrientsGood soil fertility improves yieldsPlant diseases can be linked to low
soil fertilityGood soil fertility can be linked to
favorable aesthetics of landscapes
PollutionPoor fertility management practices
can yield pollution problemsNitrate and phosphate fertilizers
misapplies can produce contamination of water
PollutionNitrate fertilizers Percolate through the soil Contaminate ground water Making it undrinkable
PollutionPhosphate fertilizers Runoff into ground water Causes algal blooms Kills aquatic wildlife populations
Are there benefits . . . To the use of fertilizers?The more vegetation we grow –
crops, trees, shrubs and grass we grow . . .
There are benefitsMore CO2 is removed from the
atmosphereMore plant cover over the soil surface
means:Less erosion of soil moving downslopeLess dust in the atmosphereLess runoff into bodies of water
There are benefitsThe more crops grown on stable
landscapesThe less pressure to grow crops on
fragile landscapes
Land managementWas typically done on a “large scale”Entire fields were treated alike“Medium scale” management
subdivided fields into smaller section“Site-specific” management or
“precision” management divides fields up into many smaller sections
“Site-specific” management
Can be based on the smallest sections of land and their individual requirements
“Site-specific” management
Continuous observations are made of: plant growth . . . Vigor . . .weed growth . . .water requirements Can be used to determine management
practices for each section
Soil samplingSoil samples should be
representative of the area in which samples are being taken
Composite samples at a given depth often provide the most uniform results
Soil samplingSamples can still be misleading
depending on relief of the area and previous grading or leveling
Topsoil depths may vary greatly and over different subsoils
Sampling depths . . .Vary depending on the tests to be
performed
Sampling frequenciesFor landscaping . . . Sampling should be done at the
beginning of any new, or redo, landscape project
Also a good idea to sample when starting new maintenance jobs
Sampling frequenciesFor agriculture . . .Annually with new land Every 2 to 3 years once established
Sampling . . .Inspect the areas to be sampled for
differences in soil characteristicsLook for obvious changes in texture,
structure, color, depth of topsoil, etc.
Sampling . . .Samples should be composites of
similar soilsIn some cases composite samples
can be separated from front to back yards
Samples may also be separated by the intended plantings
Sampling . . . The best advice is to “follow the
suggestions given by the laboratory that will be doing the tests.”
Inform the testing facility as to what types of plant materials will be grown
If there is a planting list – provide that to the testing facility
Soil testingIf we expect to obtain the maximum
growth response from added elemental nutrients . . .
All other essential nutrients must be in adequate amounts . . .
Amounts that will not injure plants
Soil testingSoil testing can be expected to
provide:Soil pHLevels of soluble saltsAvailable phosphorus (P)Available potassium (K)
Testing soil acidityBest done using pH electrode testing
equipmentpH test strips and pH color kits are
often inaccurate Tests are made using a saturated
soil solution of 1:1 or 1:2 soil:water suspension
Testing for nitrogen Due to its volatile nature Nitrogen is difficult to test forRecommendations are often based
on goals . . . Ex. 1 lb of nitrogen per 1,000 sq.ft.
for turf
Testing for phosphorus & potassium
Used to determine crop responses to fertilizers
Many sols are showing depleted levels of phosphorus and potassium
PhosphorusMuch of the phosphorus in soils is
unavailable to plantsIt must be placed directly into the
root zone
PotassiumPotassium tends to be more
availableSmall amounts are soluble It is also an exchangeable cation
Plant analysisMeans of measuring nutrient uptake
by plantsNot viable for new plantings – plants
must be established firstWorks well in conjunction with soil
testingCan help determine existing problems
Factors affecting nutrient uptake
Plant species – growth rate, etcSoil temperature – season, weather,
etc.Soil moistureSoil aeration, compactionRoot damage, pest infestationsInteractions among nutrients
Plant analysisSamples should probably be washed
and air dried for 24 hours prior to shipping
Samples should not be shipped in plastic bags
Ultimately, samples should be taken and processed according to the testing facility
Nutrient deficienciesNutrients (elements) are considered
deficient . . . When they are below a threshold
level
Threshold levelThe maximum level of a substance
(or condition) . . . That can be tolerated without ill
effects
Threshold level changesPlant species, plant parts, age of plantPlant health and root health, vigor,
disease Climate, time of year, season, weather
conditionsSoil pH
Nutrient deficienciesAre often recognized by outward
signsChlorosis and . . .Necrosis
ChlorosisReduction of the formation of green
plant pigment chlorophyllPlants become chlorotic Foliage appears unnaturally yellow
or whiteTypically associated with nutrient
deficiencies
NecrosisFormation of dead (or necrotic) plant
tissue – leaves, stems, roots, etc.Can be caused by deficiencies, salts,
disease, insects, various growth stresses like water
Visual symptomsAbnormal growth and size can be
indicatorsThe problem is narrowing down the
cause or causes
Nutrient movementNutrients travel up to the foliage . . .Through the xylem tissue . . .The water-carrying vessels
Nutrient movementThe phloem are the vessels that
conduct sugars manufactured during photosynthesis
Not all nutrients are able to transfer through the phloem equally
Plant nutrientsNutrients divided into two main
categoriesMobil nutrients
nutrients that can travel freely through the phloem
Immobile nutrientsthose that can’t. . . .
Mobile nutrientsDeficiencies appear in older foliageThe plant sacrifices older foliage to
protect new growth
Mobile nutrientsSymptoms appear on older tissue
Nitrogen (N)Potassium (K)Chlorine (Cl)Phosphorus (P)Magnesium (Mg)Sulfur (S) (in some plants)
Immobile nutrientsDeficiencies of immobile nutrients often
show in new growth – new shoot tips, root tips and phloem-fed fruit
Ex. calcium deficiencies and tomatoes, pears, apples
Calcium is highly immobile – soil applied calcium cannot be moved by the plant
Immobile nutrientsSymptoms appear on younger tissue
Copper (Cu)Manganese (Mn)Zinc (Zn)Iron (Fe)Molybdenum (Mo)Sulfur (S) (in some plants)
Highly immobile nutrientsSymptoms appear growing tips and
fruitBoron (B)Calcium (Ca)
Factors affecting mobilityCharge on the ionIon’s tendency to form insoluble
precipitates Soil textureAdsorption to soil surfacesWater movement in soilConcentration of other ions
Factors affecting mobilityPhosphates precipitate or are
adsorbed to soil solidsPhosphates are the least mobile of
the anionsPhosphates only move 1 to 2 cm
from placementExcept in sandy or other porous soils
Factors affecting mobilityK+ and NH4
+ move slowly because of their attraction to cation exchange sites
Nitrate and sulfate are more mobile moving with water
Phosphorus and potassium should be applied as close to the root zone as possible
Definition of a fertilizerAny material containing one or more
of the essential nutrients that are added to the soil or applied to plant foliage for the purpose of supplementing the plant nutrient supply can be called a fertilizer.
Fertilizer analysis (grade)Fertilizer labels display the fertilizer’s
‘grade’ Minimum guaranteed percentages of
nitrogen (N), phosphorus (P) and potassium (K)
Secondary nutrients and micronutrients
Weight and manufacturer
Fertilizer analysis (grade)Provides information about . . . Percentage of total nitrogen (N)Percentage of phosphate (P2O5)Percentage of potassium oxide (K2O)
Fertilizer analysis (grade)The numbers associated with the N-
P-K (ex. 20-20-20 ) on a fertilizer label are referred to as the fertilizer analysis or grade
The numbers indicate a percentage amount of those macro-nutrients in the product
Fertilizer analysisEx. - For a “triple 20” fertilizer, or a
fertilizer with a 20-20-20 analysis on a fertilizer label
The product contains: 20% nitrogen (N)20% phosphate (P2O5)20% potassium oxide (K2O)
The Label
The Label
Fertilizer effect on pHAcid forming fertilizers include:Most nitrogen fertilizersAll ammonium materials Many organic nitrogen fertilizers
Fertilizer effect on pHNeutral fertilizers include:Most all potassium fertilizersSuperphosphate Triple superphosphate
Fertilizer effect on pHBase forming fertilizers include:Potassium nitrate
Acidification of soilsCan be caused by nitrogen fertilizersAmmonium cations (NH4
+) are oxidized by bacteria to form nitrate anions (NO3
-)
NH4+ + 2O2 NO3
- + 2H+ + H2O
bacteria
ammonium
oxygen nitrate hydrogen water
Basic application methodsStarter fertilizerApplied near the seed at plantingTypically phosphates and potassium
because of low mobility
Basic application methodsBroadcast applicationsApplied in uniform applications to
the soil surfaceFertilizers either left on the surface
or incorporated into the soil
Basic application methodsSide-dressingOften broadcast around the bases of
plantsNitrogen can be lost through
volatilizationFertilizer should be ‘scratched’ into
the soil or covered with mulch
Basic application methodsFertigation Fertilizers applies by injecting into
irrigation waterFertilizers applied in large quantities
of irrigation waterThis is not a ‘foliar’ application
Basic application methodsFoliar applicationsFertilizers applies in solution to
wetted foliageNutrients move into the plant
through the stomata on leaves and through parts of the epidermis
Basic application methodsFoliar applicationsBest for small concentrations of
nutrient applicationsHigh concentrations can cause salt
burnWell suited for micronutrientsLess materials are required that with
soil applications
Basic application methodsFoliar applicationsSuited for iron chelates because of
the tendency to become immobile in soil
Quick uptake when other conditions do not permit nutrient uptake – cold soil temps, root rot, nematode damage . . .
Basic application methodsFoliar applications Quick responseMay be short-livedOften require subsequent applicationsBest applied in calm winds, relative
humidity greater than 75%, temps less than 85°F