Ph Argo and Fisher

Not for publication or reproduction without the authors consent. Pg. 1

Figure 1.1. Relative solubility of nutrients at different pH levels in one peat-based media (graph based on research by Dr. John Peterson, the Ohio State University).

Understanding pH management and plant nutrition Part 1: Introduction Bill Argo, Ph.D. Blackmore Company, Tel: 800-874-8660, Intl 734-483-8661, E-mail: [email protected] Originally printed in 2003 in the Journal of the International Phalaenopsis Alliance, Vol. 12 (4).

Plants are basically water surrounded by a pretty package. If we place 100 lbs. of healthy living plant material into a special oven to remove all the water, we will have only about 10 lbs. of dry plant material left. In general, plants are about 90% water and 10% dry matter.

The 10 lbs. of dry plant material that we have left is made up of carbon (C), hydrogen (H), oxygen (O), and a number of inorganic salts. If we take the 10 lbs. of dry plant material and remove all the carbon, hydrogen, and oxygen, there will be about 1 lb. of ash left. Thus, plant nutrition is the direct management of about 1% of the plant by weight.

The ash that is left is composed of the essential plant nutrient. However, these nutrients are not all taken up at the same rate. The essential plant nutrients can be separated into two groups, macronutrients and micronutrients. Macronutrients are found at relatively high concentrations in the plant tissue and include nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S). Micronutrients

are found at much lower concentrations in the tissue than macronutrients and include iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), and molybdenum (Mo).

These twelve essential plant nutrients are commonly provided by various fertilizer sources, which includes not only the water-soluble fertilizer, but also can include the irrigation water and container substrate. There are several other nutrients that are considered as essential for normal growth including sodium (Na), chloride (Cl), Nickle (Ni), and possibly chromium (Cr). However, these later four essential plant nutrients are not required by plants in large amounts. Because they are often found as contaminants in a number of different fertilizer sources, it has not been demonstrated that that they have to be specifically apply to plants. Substrate pH and plant nutrition

The term pH is a direct measurement of the balance between acidic hydrogen ions (H+) and basic hydroxide ions (OH-), and can be measured with a pH meter. The pH of a solution can range between 0 (very acidic) and 14 (very basic). At a pH of 7.0, the concentrations of H+ and OH- are equal, and the solution is said to be neutral.

When growing plants in containers, the pH range commonly found in the solution extracted from the substrate is much narrower, from about 4.5 to 8.5. The recommended substrate pH range from growing plants in containers is even more specific, around 5.8 to 6.2, depending on the crop.

The reason that the pH of the solution in the substrate is so important is that it affects nutrient solubility. Using Figure 1 as an example, the solubility of micronutrients (iron, manganese, zinc, boron) and phosphorus decrease with increasing substrate pH.

Substrate pH can also be an indication of problems. For example, low pH can be an indication that sufficient lime was not added to the substrate, or that a fertilizer is being used that is too acidic for the water quality. High pH can be an indication that too much lime was added to the substrate or that there is too much alkalinity left in the irrigation water.

Substrate pH can also affect the uptake of nutrients by the plant. Iron (Fe) uptake generally decreases with increasing pH because it precipitates out of the soil


solution at higher pH levels. Phosphorus (P) also will precipitate out of solution at higher pH levels. Phosphorus uptake will be further reduced above a pH of 7.2 because any phosphorus left in solution is converted into a less available form. Nitrogen (N) uptake can be indirectly affected by medium pH because low pH decreases nitrification (conversion of ammoniacal nitrogen to nitrate nitrogen) or the conversion of urea to ammoniacal nitrogen. Plants and nutrient uptake

Plant species differ in their ability to take up nutrients at a given pH level. While there are not good examples with orchids, there are good examples with other plants produced in containers.

For example geraniums and African marigolds are very efficient accumulators of iron (Fe) and manganese (Mn), and are often grown at a relatively high substrate pH (6.0 to 6.8) compared to most container grown crops. The high pH reduces iron and manganese solubility, which limits the uptake, and prevents toxicity problems.

At the other end of the spectrum are plants like rhododendrons, blue berries, and petunias, which are very inefficient at taking iron from the soil solution, and are often grown at a relatively low substrate pH (5.2 to 6.2). The low pH increases iron solubility, which increases the uptake, and prevents deficiency problems.

There is a third group of plants, like poinsettias, chrysanthemums, and impatiens that can be grown over a relatively wide range of pHs (5.5 to 6.5) without showing any deficiency or toxicity problems.

While I dont know it for sure, I would guess that orchids are like all other plants. Some species will perform better when grown at a low pH, some will perform better when grown at a high pH, and for some, it will not matter. However, for each of these groups, the acceptable range where they will grow and perform the best will be relatively narrow and will be similar that of other plant species. If you had to choose a pH range to grow all orchids, then the recommended range would 5.8 to 6.2, again, just like all other crops. pH management and plant nutrition. ` Many growers make the assumption that growing in containers is like growing hydroponically. Unless water is constantly dripping out of the bottom of the container, then it is not like hydroponics. Others consider growing in containers like growing outside in soil. It is not like that either. Research has shown that the pH and nutritional management of container grown crops, including orchids, is affected by the interaction of a number of different factors, including the water quality, water-soluble fertilizer, and the substrate. In the next issue, I will discuss water quality. For more information on pH management Understanding pH management of container grown crops, by William R. Argo and Paul R. Fisher. Available from: Ball Publishing, Tel: 630-208-9080, web site: www.ballpublishing.com. Meister Publishing, Tel: 440-942-2000, web site: www.meisternet.com


Understanding pH management and plant nutrition Part 2: Water quality Bill Argo, Ph.D. Blackmore Company, Tel: 800-874-8660, Intl 734-483-8661, E-mail: [email protected] Originally printed in 2003 in the Journal of the International Phalaenopsis Alliance, Vol. 13 (1). Water quality is a key factor affecting pH and nutritional management in any container-grown crops, including orchids. One challenge is that the water quality in your operation can differ dramatically from that of your neighbor, and certainly from greenhouses in other locations both inside and outside the U.S. For example, the range of water qualities used by commercial greenhouses in the U.S. can be found in Table 1. For those of you using rain water or reverse osmosis purified water exclusively, the pH will range from 4.0 to 5.5 (if measured correctly), the alkalinity will be less than 10 ppm, and the concentration of other ions will be very low to nonexistent. Understanding a few technical details about water quality will help you improve nutrient management appropriate for your own greenhouse. Always remember that the success or failure of any fertilizer will always depend on the water quality because it is the combination of the two that affect your plants. In Part 2 of this series, we will discuss how water quality affects pH and nutritional management of the substrate. pH and Alkalinity are two different aspects of water quality

There is a great deal of confusion when it comes to understanding the definition of water pH and water alkalinity, and why they are important to the health of your plants.

The term pH is a direct measurement of the balance between acidic hydrogen ions (H+) and basic hydroxide ions (OH-), and can be measured with a pH meter. The pH of a solution can range between 0 (very acidic) and 14 (very basic). At a pH of 7.0, the concentrations of H+ and OH- are equal, and the solution is said to be neutral. When the pH is above 7.0, the concentration of OH- is higher than H+, and the solution is said to be basic or alkaline (not to be confused with alkalinity). When the solution is below 7.0, the concentration of H+ is higher than OH-, and the solution is said to be acidic. Alkalinity is a measure of how much acid it takes to lower the pH below a certain level, also called acid-buffering capacity. Alkalinity is usually measured with a test kit where dilute acid is added until a color change occurs at a specific pH. Alkalinity is not a specific ion, but rather includes the concentration of several ions that affect acid-buffering capacity. Under most conditions, the ions that have the greatest effect on alkalinity are bicarbonates like calcium, magnesium, or sodium bicarbonate and, to a lesser extent, carbonates like calcium or sodium. Several other ions including hydroxides, phosphates, ammonium, silicates, sulfides, borates, and arsenate also can contribute to alkalinity, but their concentration is usually so low that they can be ignored. In a water sample, the concentration of all of the ions that makes up the alkalinity term are combined

Table 1. Average and median values for irrigation water pH, EC, and nutrient concentration used by commercial greenhouses in the United States. Research by Bill Argo, John Biernbaum, and Darryl Warncke. (For more information, See HortTechnology 7(1):49-51).

Units Average Median Range pH 7.0 7.1 2.7 to 11.3 EC (mS/cm) 0.6 0.4 0.01 to 9.8

Alkalinity (ppm) 160 130 0 to 1120 Calcium (Ca) (ppm) 52 40 0 to 560

Magnesium (Mg) (ppm) 19 11 0 to 190 Sulfur (S) (ppm) 27 8 0 to 750

Sodium (Na) (ppm) 33 13 0 to 2500 Chloride (Cl) (ppm) 33 14 0 to 1480

Boron (B) (ppm) 0.2 0.02 0 to 11.7 Floride (F) (ppm) 0.1


and reported as equivalents of calcium carbonate (CaCO3, which is the main component of lime). Alkalinity can therefore be thought of as the liming content of the water. The units used to report alkalinity can be parts per million (ppm), mg/liter, or millequivalents (meq.). Water alkalinity has a big effect on substrate-pH. When it comes to managing the pH of a substrate, the alkalinity concentration has a much greater effect than does water pH. Alkalinity (calcium bicarbonate, magnesium bicarbonate, and sodium bicarbonate) and limestone (calcium and magnesium carbonate) react very similarly when added to a substrate. And just like too much limestone, the use of irrigation water containing high levels of alkalinity can cause the pH of the substrate to increase above acceptable levels for healthy plant growth. For example, a limestone incorporation rate of 5 pounds per cubic yard will supply approximately 100 meq of limestone per 6 inch (15-cm) pot. Applying 16 fluid ounces (0.5 liters) of water containing 250 ppm alkalinity to that 6 inch pot will supply about 2.5 meq of lime. That does not sound like much until you consider that after 10 irrigations you have effectively increased the limestone incorporation rate by 25%. Even if you are using a completely inert substrate, the liming effect that high alkalinity water has will cause your substrate pH to increase to unacceptable levels.

To compare the effect of water pH or alkalinity on the ability to raise pH (or neutralize acid) in a medium, 50 ppm alkalinity (which is a low alkalinity) would be similar to having a water with pH 11 (i.e. an extremely high pH). A water with a pH of 8.0 would have the same effect on substrate pH as an alkalinity concentration of only 0.05 ppm (i.e., almost nothing). Dont ignore water pH. Water pH is still important for crop management. Even though it has little impact on the substrate, water-pH does affect the solubility of fertilizers, and the efficacy of insecticides and fungicides before you apply it to the crop. Generally, the higher the water pH, the lower the solubility of these materials. Minimizing the effects of high alkalinity The common problems associated with high alkalinity result from its tendency to increase substrate-pH. High substrate-pH can causes micronutrient deficiency in container grown crops because micronutrient solubility decrease as the substrate pH increases. In commercial greenhouses, the most common method for minimizing the liming effect of high alkalinity is to add a strong mineral acid (usually sulfuric acid or phosphoric acid) directly to the irrigation water. As the pH of the water decreases, some of the alkalinity is neutralized. The ideal alkalinity concentration will depend on the type of fertilizer being used (to be covered in Part 3). All of the alkalinity has been neutralized when the pH of the water reaches 4.5. For more information on injecting strong mineral acids into irrigation water, you can download the acid addition calculator from Purdue University and North Carolina State University at www.ces.ncsu.edu/depts/hort/floriculture/software/alk.html. For small greenhouse operations and hobbyists, strong mineral acids are very difficult and dangerous to use. Difficult because these acids are highly concentrated and therefore are difficult to add to a small volume of water, and dangerous because small greenhouses and hobbyists typically lack the specialize equipment needed to safely add acid to water. Some acids should never be considered, like anhydrous hydrochloric acid or anhydrous acetic acid because they not only are caustic, but are also fuming acids, which make them extremely dangerous to handle. Nitric acid is especially dangerous and should never be considered. There are alternatives to adding mineral acids for alkalinity control. The first is using a weaker, organic acid, like citric acid. Citric acid is available in a

Units of measure for alkainity The concentration of alkalinity (or any other plant nutrient) can be expressed a number of different ways. 1) Parts per million (ppm or mg/liter). The term ppm is a

weight per weight ratio. One part per million is equivalent to 1 unit of something dissolved in a million units of something else. In the case of anything dissolved in water, 1 ppm is equal to 1 mg per 1,000,000 mg (or 1 Kg = 1 liter) of water. So, 1 ppm is equal to 1 mg/liter. A 1% solution (1 unit in 100 units) is equivalent to 10,000 ppm.

2) Milliequivalent (mEq./liter). The term mEq./liter is a chemistry term that is not only dependent on a materials concentration, but also on its molecular weight and charge. In the case of alkalinity, 50 ppm (or mg/liter) CaCO3 equals 1 meq/liter CaCO3. Sometimes the concentration of bicarbonates is also reported on a water test from a commercial laboratory. In most cases, bicarbonate makes up most of the alkalinity. The relationship is 61 ppm bicarbonate equals 1 meq alkalinity.

3) Grains per gallon (gpg): An outdated term for expressing concentration. 1 gpg = 17.1 ppm


pure granular form. A rate would be about 0.2 grams per gallon to remove 50 ppm alkalinity. Pre-mixed citric acid solutions (Seplex, GreenCare Fertilizer (815-936-0096)) are also available for alkalinity control. Other organic acids like vinegar and lemon juice will also work, but because the concentration of acid in these materials is variable, for example, the acetic acid content in vinegar can range from 4% to 8% by weight, that the results that you get will not be consistent. Another option for alkalinity control is to use acidic fertilizers (to be covered in greater depth in Part 3). Fertilizers high in ammoniacal nitrogen produce an acidic reaction when added to the substrate, which can be used to neutralize the affect of water alkalinity. For example, 20-20-20 (69% NH4-N) has enough acidity to be used with water containing around 200 ppm alkalinity water without further acidification. There are several drawbacks to using fertilizer for alkalinity control. Fertilizers high in ammoniacal nitrogen may cause excessive growth and are not effective when the temperature of the substrate is less than 60oF. In addition, you lose flexibility because you can only choose commercial fertilizers based on ammonium content. For example, high ammonium fertilizers available to you may lack calcium or other key nutrients. Another option for alkalinity control is to change water sources. There are a number of sources, such as rain water or reverse osmosis purified water, that contain little if any alkalinity. Drawbacks to using alternative water sources include cost and storage problems. Changing water sources will also change the composition of the fertilizer solution applied to the crop. Low alkalinity Effects Not everybody in the world has irrigation water with high alkalinity. In the United States alone, there are a large number of growers in states like AL, AR, CA, CO, GA, HI, NC, NJ, NY, VA, and New England states that have alkalinity levels below 40 ppm without any acidification. Even in areas were high alkalinity is considered the norm, some growers have switched to low alkalinity sources such as reverse osmosis purified water or rain water. The primary problem associated with low alkalinity water is a tendency for substrate-pH to drop over time, which can cause micronutrient toxicity problems. Usually, low pH problems are a result of fertilizer selection. Fertilizers high in ammoniacal nitrogen are acidic, and without any alkalinity in the water to balance the reaction (resist lowering of pH), acidic fertilizers will tend to drive the substrate-pH down over time.

What about Hardness? Hardness is a measure of a waters ability to form scale in pipes, produce suds from soap, or to leave spots on leaves. Like alkalinity, the units used to report hardness are calcium carbonate equivalents (CaCO3). However, while alkalinity is a measure of all chemical bases in the water (bicarbonates and carbonates), hardness is really a measure of the combined concentration of calcium and magnesium in the water because it is insoluble salts of ions, like calcium carbonate, that form scale. Another difference is that while alkalinity is an important measure in pH and nutritional management, hardness is not, because its combined concentration tells you little about a waters ability to supply nutrients to a plant. A water softener is typically used to remove hardness. What is occurring with hardness removal is that the calcium and magnesium ions are being replaced with an ion that doesnt cause scale, like sodium or potassium. However, with hardness removal, the carbonates and bicarbonates still remain in the water but they have been changed from calcium and magnesium bicarbonate to sodium or potassium bicarbonate. Thus, hardness removal has no effect on pH management. In comparison, with alkalinity control, an acid is used to neutralize the carbonates or bicarbonates, which will affect pH management, but the calcium and magnesium concentration remains unchanged. What else is important in my water? Electrical conductivity (EC, also know as conductivity or soluble salts) is a term used to measure the total concentration of salts in the water. The higher the EC, the more salts that are dissolved in the water. With irrigation water, EC is used to determine the potential risk for salt buildup when water is applied to a substrate. With fertilizer solutions, EC can be directly correlated with the concentration of individual nutrients (typically nitrogen) from a variety of fertilizer salts, or with the total concentration of nutrients supplied by a water-soluble fertilizer.

Electrical conductivity or EC units have changed over the years. Twenty years ago, the units for measuring EC were millimhos (mmhos) or micromhos (mhos). Currently, the units used to measure EC are millisiemens/cm (mS/cm), microsiemens/cm (S/cm), or decisiemens/m (dS/m). The conversion for all these units are 1000 mhos = 1000 S/cm=1 mmhos = 1 mS/cm = 1 dS/m. A term closely related to EC is total dissolved solids or TDS. A TDS meter measures the EC and then converts the measurement into ppm by multiply by a constant, usually 1 mS/cm = 1000 ppm salts. The problem with TDS measurement is that the constant is


based on one salt (potassium chloride) and therefore TDS measurements do a poor job estimating the actual concentration of fertilizer salts under most situations. It is important to remember that TDS measurements are used to determine the acceptability of drinking water, not fertilizer solutions. For these reasons, commercial greenhouses use EC measurements almost exclusively for fertility management Another important consideration is the concentration of individual plant nutrients. In general, irrigation water is not a significant source of the primary macronutrients nitrogen (N), phosphorus (P), or potassium (K), which are the numbers that you see on a bag or bottle of fertilizer. However, irrigation water can contain high levels of the nutrients calcium (Ca), magnesium (Mg), and sulfur (S). And just like alkalinity, the concentration of nutrients contained in the irrigation water can vary dramatically between different locations (Table 1). Since irrigation water can be an important source of calcium, magnesium, or sulfur, water can contribute a significant amount of the total concentration of these nutrients being applied to a crop. In other words, the water-soluble fertilizer that you apply (like 30-10-10) is not the only nutrient source. However, if you are using a very pure water source, like RO or rain water, the only source of these nutrients may be the fertilizer. Waste ions Some ions contained in irrigation water are either not needed by the plant, or the plant requirement is so low that only small amounts are required. Examples of waste ions are sodium (Na) or chloride (Cl). Generally their presence in irrigation water at high concentrations increases the risk of salt build up in the substrate. Even calcium, magnesium, or sulfur can be considered a waste ion if their concentration is too high or it is difficult to balance their concentration in the nutrient solution with water-soluble fertilizer. With most ions (including Na, Cl, Ca, Mg, or S), excessive concentrations can be removed with reverse osmosis purification. High salt concentrations can also be managed by leaching at a heavier rate than the commonly recommended 20% to remove any excess salt build up. However, if you do use higher leaching rates, then you may also have to increase the fertilizer concentration because leaching washes out all salts from the container including essential plant nutrients.

Boron (B) is a special example of a waste ion. Even though it is an essential plant nutrient, the presence of boron in irrigation water at high concentrations can cause significant challenges. Unfortunately, the difference between deficient, adequate, and toxic levels of boron are very small. In general, it is recommended that the maximum concentration of boron in water used for plants be no more than 1.0 ppm.

Unlike most other waste ions, boron can not be effectively removed with reverse osmosis purification. Instead, the only option for managing excessive boron levels is to maintain a substrate pH above 6.0 and use calcium-based fertilizer. The idea is that the high pH and calcium will caused excess boron to precipitate out of the soil solution, making it unavailable to the plant. Another option for controlling high boron in the water is to change water sources.

High concentrations of iron (Fe) in the irrigation dont usually effect plant nutrition or pH management. However, iron can cause staining problems on plant leaves and other surfaces, and the presence of iron in the water can lead to the presence of iron-bacteria growing in the pipes, which can clog mist nozzles, or anything else with small openings. Water treatments that oxidize the water, such as treatments with ozone or potassium permanganate, can effectively remove iron from the water.

Fluoride (F) and chlorine (Cl2) are commonly added to municipal water at concentrations up to 4 ppm and can cause problems growing crops. Generally, high levels (above 1 ppm) of fluoride and chlorine can cause damage to the foliage (especially at the tip) and the flowers. These materials are easily removed from the water source by using an activated charcoal filter. Water testing is only a starting point

Obtaining a water test is an important first step in determining if your fertility program will work, or if you need to reevaluate. Most water sources (with the exception of rain water) are susceptible to change. In commercial greenhouses, it is recommended to do a water analysis at least once a year, either to make sure that the water source is not changing, or if it is changing, to make adjustments in the nutrition program.

Equally important is understanding how your fertilizer affects pH and nutrition by itself, and through its interaction with your water. Next issue: fertilizer.


Where to get a water test? Obtaining a water test is an important first step in determining if your fertility program will work, or if you need to

reevaluate. The type of testing should be to determine if the water is acceptable for plants, i.e. for greenhouses and nurseries, not suitability for drinking water (there is a difference). The test should include, water pH, EC, and the concentration (in ppm or mEq/liter) of alkalinity (and or bicarbonates), nitrogen, phosphorus, potassium, calcium, magnesium, sulfur, iron, manganese, zinc, copper, boron, sodium, chloride, and fluoride.

There are number of testing laboratories in the U.S. that work closely with commercial greenhouse and nurseries, and so are familiar with many of the issue discussed in this article. A number of these laboratories also have international ties. They are:

Name Location Web site or E-mail Phone Number A & L Southern Laboratory Pompano Beach, FL [email protected] 954-972-3255 J.R. Peters Laboratory Allentown, PA www.jrpeterslab.com 800-743-4769 Micro-Macro International Athens, GA www.mmilabs.com 706-548-4557 Quality Analytical Laboratories Panama City, FL www.qal.us 850-872-9535 Soil and Plant Laboratories, Inc. Orange, CA www.soilandplantlaboratory.com 714-282-8777

The cost of a water test will range from $25 to over $100 per sample. Remember that UPS and FedEx will ship anywhere in the US, so it pays to shop around

Many state universities still operate testing laboratories, so you can also have your water tested through the state extension service. Fees vary from state to state, and the time required to get the test back is usually longer than with commercial laboratories.

Drinking water companies will also perform water testing, but they are testing for the suitability for drinking, and whether or not you need some type of water treatment. If you want to grow plants, you need better, and more precise testing than is supplied by these companies.


Understanding pH management and plant nutrition Part 3: Fertilizers Bill Argo, Ph.D. Blackmore Company, Tel: 800-874-8660, Intl 734-483-8661, E-mail: [email protected] Originally printed in 2003 in the Journal of the International Phalaenopsis Alliance, Vol. 13 (2).

When you select a water-soluble fertilizer for your plants, the primary goal should be to supply your plants with a sufficient amount of essential plant nutrients for good growth and flowering. The problem is that there are probably as many misconceptions about fertilizers as there are fertilizers labeled as orchid special.

The best fertilizer to use on your plants is the one that not only supplies nutrients, but also complements the alkalinity and nutrient content of your irrigation water. In this article, we will help you understand how selecting a fertilizer will affect the pH and nutrient levels in the substrate. You will learn why water-soluble fertilizers are classified as acidic, neutral, or basic based on their fertilizer reaction in the substrate. Finally, with the information given in this article, you should be able to decide for yourself which fertilizers will work best for your growing conditions. Solution pH and the effect that fertilizer has on substrate-pH two different aspects of water-soluble fertilizers

There is a great deal of confusion when it comes to understanding the difference between the pH of the fertilizer solution and the effect that fertilizer has on substrate pH, and why they are important to the health of your plants.

Just like with water pH, the pH of the fertilizer solution is a direct measurement of the balance between acidic hydrogen ions (H+) and basic hydroxide ions (OH-), and can be measured with a pH meter. The pH of a solution can range between 0 (very acidic) and 14 (very basic). At a pH of 7.0, the concentrations of H+ and OH- are equal, and the solution is said to be neutral. When the pH is above 7.0, the concentration of OH- is higher than H+, and the solution is said to be basic or alkaline (not to be confused with alkalinity). When the solution is below 7.0, the concentration of H+ is higher than OH-, and the solution is said to be acidic. The effect that a water-soluble fertilizer has on substrate pH is dependent on the reactions that take place once the fertilizer has been applied to the crop and are based on the type of nitrogen contained in the fertilizer. There are three types of nitrogen used in water-soluble fertilizers: ammoniacal nitrogen (NH4-N), nitrate nitrogen (NO3-N) and urea (Figure 1). Uptake of

ammoniacal nitrogen causes the substrate-pH to decrease because H+ (acidic protons) are secreted from roots in order to balance the charges of ions inside the plant with the solution surrounding the outside of the roots. Urea is easily converted into ammoniacal nitrogen in the substrate and therefore can be thought of as another source of ammoniacal nitrogen. In contrast, uptake of nitrate nitrogen increases substrate-pH because OH- or HCO3- (bases) are secreted by plant roots in order to balance nitrate uptake.

Another important fertilizer reaction is a process called nitrification. Several types of bacteria in container substrates (including inert substrates like coir, bark, peat, rockwool, and scoria) convert ammoniacal nitrogen to nitrate nitrogen. Nitrification releases H+ (acidic protons), causing the substrate-pH to decrease.

Consider the difference in the amount of acidity supplied by a solution with a pH of 5.0 verses the amount of acidity supplied by 100 ppm of ammoniacal nitrogen. A solution with a pH of 5.0 would supply about 0.01 mEq/liter of acidic hydrogen ions to the substrate. If all the 100 ppm ammoniacal nitrogen were converted into nitrate nitrogen through nitrification, the maximum amount of acidity produced would be 14.2 mEq/liter of acidic hydrogen, or about 1,400 times more acidity than would be supplied by a solution with a pH of 5.0. Put another way, applying 100 ppm of

Figure 1. The effect of different forms of nitrogen on medium-pH. Nitrate nitrogen (NO3-N) only effects medium-pH through plant uptake [1]. Ammoniacal nitrogen (NH4-N) effects medium-pH through both plant uptake [2] and nitrification [3]). Urea must first be converted into ammoniacal nitrogen before it can be taken up by the plant [2] or go through nitrification [3].


ammoniacal nitrogen has the potential to supply the same amount of acidity as a solution with a pH of 1.8. The acidity produced by a solution with a pH of 5.0 would be equivalent to the nitrification of 0.14 ppm ammoniacal nitrogen (almost undetectable).

While the effect that different nitrogen forms have on the substrate pH is more complicated than this simple example, it does give you an idea why the nitrogen form of the fertilizer has a much greater effect on the substrate-pH than does the solution pH.

The main problem with predicting how the nitrogen form affects substrate pH is that the key reactions are not consistent. For example, the application nitrate nitrogen (NO3-N) can cause the substrate-pH to increase, but only if it is taken up by the plant. If plants are small, or are stressed and not growing, nitrate has little influence on substrate-pH. The application of ammoniacal nitrogen (NH4-N) can cause the substrate-pH to decrease even if the plant is small or is not growing, because in addition to plant uptake, nitrification will occur independently of the plant. However, nitrification is inhibited by low substrate-pH (starting at around 5.5), low substrate temperature (less than 60oF or 15oC), and lack of oxygen through water-logging.

Finally, you never apply either all nitrate nitrogen or all ammoniacal nitrogen to your plants. Most fertilizer is a mixture of salts containing different forms of nitrogen and so the overall reaction produced by the fertilizer will depend on the ratio of the different nitrogen forms. There are also other factors that either magnify or buffer the reaction of the fertilizer including the substrate (cation exchange capacity, residual lime, decomposition to be covered in a later article) and the irrigation water. Water alkalinity also influences the fertilizer reaction. When discussing how water-soluble fertilizer affects substrate-pH, it is important to understand that water-soluble fertilizer cannot be applied without irrigation water. The best guide when selecting an appropriate water-soluble fertilizer is to balance the proportion of nitrogen in the ammoniacal form (acid) against the irrigation water alkalinity (base) (see Table 1). Although other factors affect substrate-pH, research has shown that it is the balance between the ammoniacal nitrogen in the fertilizer and water alkalinity that has the greatest effect on substrate-pH on long-term crops.

Table 1. The nitrogen content of selected commercially-available granular and liquid water-soluble fertilizers. The alkalinity concentration that provides a stable substrate pH should be viewed as an approximate guideline only. Use these values as a starting point. Any changes to the fertilizer program should be based on the actual measured pH of the crop.

N-P2O5-K2O Formula % NH4-N % Urea-N % NO3-N

Fertilizer reaction1

Proportion of the total

nitrogen in the ammoniacal form (NH4-N + urea-N)

Alkalinity Conc. (in ppm CaCO3) that provides a stable substrate pH

Granular fertilizers 21-7-7 GC,SC 9.1% 11.9% - A 1520 100% 9-45-15 GC,SC 9.0% - - A 940 100% 30-10-10 GC,GM, SC 2.1% 24.7% 3.2% A 1039 89% 20-20-20 GC,GM, SC 3.9% 10.5% 5.6% A 680 72%

250 or more

6-30-30 GM 2.7% - 3.3% NA 45% 10-30-20 G, SC 4.4% - 5.6% A 425 43% 20-10-20 GC,GM, SC 8.0% - 12.0% A 430 40% 21-5-20 SC 6.5% 1.9% 12.6% A389 40%

150 to 200

19-4-23-2 Ca GC 5.7% - 13.6% A 140 30% 17-5-17-3 Ca-1 Mg GC 4.2% - 12.8% A 0 25% 15-5-15-5 Ca-2 Mg SC 1.2% 2.1% 11.8% B 141 21%

75 to 150

15-3-20-3 Ca-1 Mg GC 2.4% - 12.6% B 75 16% 14-4-14-5 Ca-2 Mg GC 2.0% - 12.0% B 200 14% 13-2-13-6 Ca-3 Mg GC, SC 0.8% - 12.2% B 380 6% 13-3-15-8 Ca-2 Mg GC 0.7% - 12.5% B 420 5%

50 or less

Liquid fertilizers 10-5-5-2 Ca-0.5 Mg DG 3.7% - 6.3% NA 37% 7-9-5-2 Ca-0.5 Mg DG 2.6% - 4.4% NA 37% 150 to 200

7-7-7-2 Ca-0.5 Mg DG 2.1% - 4.9% NA 30% 3-12-6-2 Ca-0.5 Mg DG 0.7% - 2.3% NA 23% 75 to 150 1 Pounds of acidity (A) or basicity (B) per ton of fertilizer. DG = Dyna Gro, GC = GreenCare, GM = Grow-more, SC = Scotts (Peters) To Calculate the proportion of the total nitrogen in the ammoniacal form

% NH4-N + % Urea-N % Total Nitrogen = Proportion of the total nitrogen in the ammoniacal form

Example: 20-20-20

3.9% NH4-N + 10.5% Urea-N 20% total nitrogen = 72% of the total nitrogen is in the ammoniacal form


To understand how the alkalinity concentration in the water and the percentage of ammoniacal nitrogen in the fertilizer interact to affect substrate-pH, picture a balance with water alkalinity on one side pushing the pH up (i.e. liming effect), and on the other side, with the ammoniacal nitrogen pushing the pH down (i.e. acidic nitrogen).

If either of these factors is out of balance, then the substrate-pH will be affected. For example, using a fertilizer very high in ammoniacal nitrogen (like 30-10-10) with low alkalinity water (like RO or rain water) is very effective at driving the substrate-pH down because there is nothing to neutralize all the acidic hydrogens (H+) being produced through nitrification or plant uptake. Another example would be using a fertilizer low in ammoniacal nitrogen (like 13-3-15) with a high alkalinity water source (like well water commonly found in the Midwest of the United States). In this case, there would be little if any acidic hydrogens (H+) produced to neutralize the liming effect of the water alkalinity, plus the large amount of nitrate nitrogen uptake would also add to the liming effect.

It is important to note that the two things that affect substrate-pH the most (water alkalinity and ammoniacal nitrogen) can not be directly measured with a pH meter. Water alkalinity must be measured with an alkalinity test (see Part 2 of this series for a list of commercial laboratories that do alkalinity testing). The percentage of ammoniacal nitrogen in the fertilizer needs to be calculated based on the information supplied on the fertilizer bag (See Table 1).

What about potential acidity or basicity? Many water-soluble fertilizer labels state the potential acidity or basicity of the fertilizer in units of equivalent pounds of calcium carbonate (CaCO3, or agricultural lime) per ton of fertilizer. Potential acidity or basicity indicates the type of reaction produced, while calcium carbonate equivalency indicates the strength of that reaction. For example, 20-10-20 has a potential acidity of 430 lbs. per ton of fertilizer. If one ton of 20-10-20 were applied to a field soil, we would estimate that 430 pounds of CaCO3 (lime) would be required to neutralize the long-term acidity produced from the fertilizer. There are several problems when trying to relate potential acidity or basicity and calcium carbonate equivalency to growing plants in pots containing an inert substrate. The original values come from a method first presented in 1933 using field soil (pH-independent CEC), rather than inert substrates like peat or bark. The calculated values are based on assumptions related to how much of each nutrient remains in the soil profile, is used by the plant, or is leached from the field soil. The equivalent value of pounds CaCO3 per ton of fertilizer has little meaning in soilless culture where fertilizer applications are typically based on the concentration of nitrogen in parts per million contained in a nutrient solution, not the total weight of the fertilizer applied to a pot. Finally, the alkalinity of the irrigation water is not taken into account when calculating acidity or basicity. At best, the potential acidity or basicity and calcium carbonate equivalency should be interpreted as a general tendency of the fertilizer to raise or lower medium-pH over time. Macronutrients.

The second way a water-soluble fertilizer affects nutrition management is through the direct effect it has on nutrient concentrations is in the root medium. A complete fertilizer program provides several macronutrients (needed in large quantities) including nitrogen (N), phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), and sulfur (S).

Blended water-soluble fertilizers that contain nitrogen, phosphorus, and potassium are formulated by

Table 2. Fertilizer salts used to produce selected commercially-available granular and liquid water-soluble fertilizers.

N-P2O5-K2O

Formula

Derived from Granular fertilizers 21-7-7 GC,SC KCl, NH4H2PO4, (NH4)2SO4, urea, 9-45-15 GC,SC KCl, NH4H2PO4 30-10-10 GC,GM, SC KNO3, NH4H2PO4, Urea 20-20-20 GC,GM, SC KNO3, NH4H2PO4, Urea 6-30-30 GM KNO3, NH4H2PO4, KH2PO4, KCl 10-30-20 G, SC NH4NO3, KNO3, NH4H2PO4, 20-10-20 GC,GM, SC NH4NO3, KNO3, NH4H2PO4, 21-5-20 SC NH4NO3, KNO3, Urea phosphate 19-4-23-2 Ca GC NH4NO3, Ca(NO3)2, KNO3NH4H2PO4 17-5-17-3 Ca-1 Mg GC NH4NO3, Ca(NO3)2, KNO3, Mg(NO3)2, NH4H2PO4 15-5-15-5 Ca-2 Mg SC NH4NO3, Ca(NO3)2, KNO3, Mg(NO3)2, Urea phosphate 15-3-20-3 Ca-1 Mg GC NH4NO3, Ca(NO3)2, KNO3, Mg(NO3)2, NH4H2PO4 14-4-14-5 Ca-2 Mg GC NH4NO3, Ca(NO3)2, KNO3, Mg(NO3)2, NH4H2PO4 13-2-13-6 Ca-3 Mg GC, SC Ca(NO3)2, KNO3, Mg(NO3)2, NH4H2PO4 13-3-15-8 Ca-2 Mg GC Ca(NO3)2, KNO3, Mg(NO3)2, NH4H2PO4 Liquid fertilizers 10-5-5-2 Ca-0.5 Mg DG 7-9-5-2 Ca-0.5 Mg DG 7-7-7-2 Ca-0.5 Mg DG 3-12-6-2 Ca-0.5 Mg DG

NH4NO3, Ca(NO3)2, KNO3, MgSO4, NH4H2PO4, KH2PO4, H3PO4, KCl

1 Actual P and K are the actual expected values obtained in a solution at 100 ppm nitrogen and are how the values would be represented if a laboratory analysis were performed on the solution. To calculate actual P as P2O5, multiply value by 2.3, to calculate actual K as K2O, multiply value by 1.2. DG = Dyna Gro, GC = GreenCare, GM = Grow-more, SC = Scotts (Peters) Ammonium nitrate (NH4HO3), ammonium sulfate ((NH4)2SO4), calcium nitrate (Ca(NO3)2), magnesium nitrate (Mg(NO3)2), monoammonium phosphate (NH4H2PO4), monopotassium phosphate (KH2PO4), phosphoric acid (H3PO4), potassium chloride (KCl), potassium nitrate (KNO3),


combining two or more fertilizer salts (Table 2). Fertilizer salts in this case mean any chemicals that contain plant nutrients in a water-soluble form. Ammonium phosphate is an example of a fertilizer salt, and in water this salt dissolves into separate ammonium and phosphate ions. The ammonium provides the plant with N and phosphate provides P.

There are many water-soluble sources of nitrogen, some of which only supply nitrogen like urea and ammonium nitrate. However, for most other nutrients, the choices are limited. For example, calcium nitrate is the only form of water-soluble calcium. There is also typically only one source of potassium, potassium nitrate. Monoammonium phosphate is the usual source of phosphorus. Magnesium is supplied either as magnesium nitrate or magnesium sulfate. Sulfur is supplied by ammonium sulfate or magnesium sulfate.

Because of limitations in the number of fertilizer salts used to blend fertilizers, the ratio of macronutrients (N-P-K-Ca-Mg) directly affects the percent ammoniacal nitrogen. For example, fertilizers that are high in calcium tend to also be high in nitrate, because calcium nitrate is the only water-soluble source of calcium. Fertilizers that are high in phosphorus are often also high in ammonium because phosphorus is usually supplied as monoammonium phosphate.

Certain fertilizers generally cannot be mixed at high concentrations. Salts containing sulfate, for example magnesium sulfate, are not compatible in the same concentrated stock solution with calcium nitrate because a reaction occurs where insoluble calcium sulfate (gypsum) will form as a precipitate . If a blended fertilizer contains both calcium and magnesium, then the sources have to be calcium nitrate and magnesium nitrate or two stock tanks must be used. Similarly calcium nitrate and monoammonium phosphate cannot be mixed in the same concentrated stock solution at high concentrations because insoluble calcium phosphate will form as a precipitate (solid). However, the amount of calcium and phosphorus that can be mixed in the same stock tank can be increased by lowering the pH of the stock tank. Commercially available fertilizers that contain calcium and phosphorus tend to have low levels of phosphorus (i.e. 13-2-13-6 Ca-3 Mg) and will also contain a weak acid to lower the pH of the concentrated stock solution. The nutrient content of the irrigation water is also important. In some cases, it can supply a large percentage of nutrients (especially calcium and magnesium) to the plants. In other cases, the reason for choosing a specific fertilizer is to resist the effects of unwanted ions like sodium, chloride, or boron. Only when the nutrient content of an irrigation water is

extremely low (like with rain water or reverse osmosis purified water) can it be ignored. Micronutrients

Micronutrients (iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), and molybdenum (Mo)) are also required by plants for acceptable growth. In the past, Field soils were the primary source of micronutrients, and so the additional application was not often necessary. However, since the switch to inert substrates for growing plants in containers, the application of micronutrients has become a necessity. The sources of micronutrients used in water soluble fertilizers typically come in two forms, inorganic salts (all micronutrients) or chelates (only iron, manganese, zinc, and copper). Inorganic salts are material that dissolve in water to form ions that are available to the plant. For example, iron sulfate will dissolve into separate iron (Fe) and sulfate (SO4) ions. Chelates are organic molecules that envelop the ion and protect if from interacting with other ions in the soil solution that may make it unavailable to the plant.

There are many chelating molecules available, but only three that are in common use in horticulture, EDTA, DTPA, and EDDHA. These abbreviations refer to the chemical structure of the organic molecule. The difference in the chelates is how tightly the ion is bound. In general, manganese, zinc, and copper chelates are only found in the EDTA form. In comparison, there are three forms of iron chelate, but the most common also is the EDTA form.

How to read a label from a fertilizer bag or bottle. All fertilizer labels should contain three numbers

representing the percentage (by weight) of nitrogen, phosphorus, and potassium contained in the fertilizer. For nitrogen, the value listed represents the actual percentage of nitrogen contained in the fertilizer. However, for historical reasons, fertilizers sold in the United States (and much of the rest of the world) list the percentage of phosphorus as P2O5 and potassium is listed as K2O. To calculate the actual percentage of phosphorus, multiply the listed value by 0.43, and for potassium, multiply the percentage by 0.83. For example, 20-20-20 really contains 20% nitrogen, 8.6% phosphorus (actual P), and 16.6% potassium (actual K).

Nutrients other than nitrogen, phosphorus, or potassium are voluntarily listed on the label under the guaranteed analysis section and the values listed represent the actual percentage in the fertilizer. To be listed on the label, they either have to reach a minimum level (Ca at 1%, Mg at 0.5%, S at 0.5%, Fe at 0.1%, Mn, Zn, Cu at 0.05%, B at 0.02%), or they can be in the fertilizer but left off the label, or the label can contain For continuous liquid feed programs which exempts the fertilizer from the minimum critical level on micronutrients.


Resin-coated fertilizer Resin-coated fertilizers are water-soluble fertilizers covered by a resin or plastic membrane that limits the solubility of the fertilizer salts. In general, resin coated fertilizer contain high levels (50%) of ammoniacal nitrogen (NH4-N) and no calcium (Ca), and typically little if any magnesium (Mg).

The initial release of nutrients from resin-coated fertilizers occurs because of imperfections in the coating of a percentage of the prills. Mixing equipment that damages the coat on the prills will also cause a high initial release. To test for initial release, put some resin-coated fertilizer in a glass of water and allow to sit overnight. If there the EC of the solution increases, then there is an initial release. This initial release should be thought of as a starter fertilizer. The long term release of nutrients from resin-coated fertilizer is affected by only one thing, temperature. In general, the higher the temperature, the higher the release rate, and the lower the temperature, the lower the release rate. Resin coated fertilizers are typically sold based on release durations. For example, Osmocote 14-14-14 has a release rate of 3-4 months. At an average temperature of 68oF (20oC), 14-14-14 will release 80% of the fertilizer salts contained in the prills over 3-4 months. However, if the average temperature of the substrate is much above 68oF, then 14-14-14 may only last 2-3 months. High greenhouse temperatures have been known to cause excessive release of nutrients from resin-coated fertilizer resulting in salt buildup in the substrate.

Conclusion Understanding how to fertilizer your plants

starts with understanding what is in the bag or bottle of fertilizer and what is in your water. However, this still doesnt guarantee success. Proper fertilization of your plants is more than just selecting the right fertilizer. It also applying the fertilizer correctly. In the next article, we will discuss different factors that affect the concentration of fertilizer that you apply to your plants, and some of the concepts about fertilizers that may or may not be correct.


Understanding pH management and plant nutrition Part 4: Substrates Bill Argo, Ph.D. Blackmore Company, Tel: 800-874-8660, Intl 734-483-8661, E-mail: [email protected] Originally printed in 2004 in the Journal of the International Phalaenopsis Alliance, Vol. 13 (3). A wide range of substrates are available on the market to grow orchids, or many other plants. Some people are using substrates manufactured by large companies for the production of container grown crops other than orchids. Other people are using substrates manufactured primarily for growing orchids. Still others are blending their own substrates from individual components. The choice of substrates will affect the effectiveness of your fertilizer program. Substrates can differ substantially in both their physical properties and chemical properties. In part 4 or this series, we will discuss key aspects of physical and chemical properties, and also leaching, and how these factors affect plant nutrition. Physical properties Physical properties deal with the ratio of air:water:solid in a substrate. Container substrates should be thought of as a sponge. A sponge is made up of the material used to make the sponge (solid space) and holes (pore spaces). If a material has a high bulk density (high weight per unit volume), then a sponge of this material would have a lot of solid space with little pore space. Examples of high bulk density materials are sand, clays, or field soils. In comparison, a sponge made from materials that have a low bulk density (low weight per unit volume) would have little solid space but lots of pore spaces. Examples of low bulk density materials are peat, coir, bark, vermiculite, things commonly found in container substrates. Pore space can be filled with either air or water. The ratio of air to water in a given substrate will depend on size and distribution of the pores. During an irrigation, small pores (called micropores) tend to fill completely with water, while large pores (called macropores) tend to drain, which allows air to get back into the substrate. It has been said that after an irrigation, the ideal container substrate would have 25% of its volume taken up with pores filled with air, 60% of its volume taken up with pores filled with water, and the remaining 15% taken up with solids. To put numbers on these values, an average 6 inch (15-cm) azalea pot has a volume of about 1.6 liters. The volume of air, water, and solid occupied by the ideal substrate in this pot would be 0.4 liters of air

space, 0.96 liters of water, and 0.24 liters of solid. In general, substrates used for propagation tend to be very fine (lots of micropores) and so hold more water (on a relative basis) at the expense of air space when compared to the ideal substrate. Coarse substrates have lots of macropores and so have greater air space at the expense of water. Container height also affects the ratio of air:water held in a substrate after an irrigation. In general, the shorter the height of the container, the greater the percentage of pore space that is filled with water and the lower the air space. For example, after a thorough watering, the average air and water porosity of five different commercially available root media in a 6 inch (15 cm) tall pot was 19% (air) and 64% (water), in a 4 inch (10 cm) tall pot was 13% (air) and 70% (water), in a 3 inch (8 cm) tall cell bedding flat was 7% (air) and 76% (water), and a 1 inch (2.5 cm) tall plug flat was 2% (air) and 82% (water), respectively. The percentage of solid material in the root media remained constant in the different container sizes. It was the ratio of air space to water space that changed with the different container heights. This is one reason why it is easier to overwater a small pot than it is a large pot because the air space in the small pot is lower than that found in the larger pot after an irrigation. Finally, the ability of a substrate to absorb water will affect physical properties. Ideal physical properties are measured in a laboratory by allowing the substrate to remain submersed in water for 24 hours before allowing it to drain (the difference between the saturated weight and drained weight (in grams) is a measure of air porosity). In comparison, a typically irrigation may last for only 30 seconds or less. That means that under a typical irrigation, most substrates will not rewet to maximum saturation, resulting in more air space and less water-holding capacity than is measured in a laboratory test. Another problem with organic substrates like peat and (especially) bark, is that they become water-repellent if allowed to dry too much. Commercial substrates will often contain a wetting agent or surfactant that aids in rewetting (increases water absorption). For long-term greenhouse crops, like hanging baskets, it is often recommended to reapply a surfactant to the substrate every month or two because


the surfactant will degrade over time, resulting in a decrease in water absorption (more air space). If you want to apply a wetting agent to your orchids, choose one that is designed specifically for organic substrates and cut the rates found on the label in half to reduce the potential for phytotoxicity. Chemical properties Chemical properties generally refer to a substrates ability to buffer the water held in the substrate against changes in either pH or nutrition. The term most often used to describe chemical properties is cation exchange capacity or CEC. CEC refers to the ability of substrate particles (such as peat) to absorb and release positively charged cations like potassium, ammonium, calcium, or magnesium, thus buffering the substrate against sudden changes in pH or nutrient levels. An example of how CEC affects pH and nutrient management occurs when a fertilizer solution is applied to a substrate. A fertilizer high in ammoniacal-nitrogen produces acid (H+). The acid is absorbed by the substrate and a different cation, usually calcium, is released. Conversely, a fertilizer high in nitrate-nitrogen (usually calcium nitrate based) produces base (either OH- or HCO3-). The base causes an acid (H+) bound by the substrate to be released, which will then react with the base to produce water (H2O) or CO2. In both cases, the net result is that the pH and calcium concentrations remain stable. Substrates that have high CEC (more buffered) can resist a change in pH for long periods of time, whereas pH can change very rapidly in substrates that have low CEC (less buffered). CEC can play an important role in pH buffering when a field soil is added to a container substrate. CEC-based pH and nutrient buffering does occur with field soils because the substrate has a high bulk density (weight). In contrast to field soils, research has shown that the CEC of peat, coir, or bark-based substrates has little effect on resisting change in pH or in supplying nutrients. This does not mean that the substrate plays no role in pH or nutritional management. Peat tends to be very acidic. Limestone is commonly added to peat-based substrates to neutralize the acidity and bring the pH up to an acceptable level for plant growth. The amount of acidity found in most acidic peats will not be neutralized very quickly by bases found in irrigation water. Using the example given in Part 2 of this series, a limestone incorporation rate of 5 pounds per cubic yard will supply approximately 100 meq of limestone per 6 inch (15-cm) pot. Applying 16 fluid ounces (0.5 liters) of water containing 250 ppm alkalinity to that 6 inch pot will supply about 2.5 meq of lime. That means that 40 irrigations are required to equal the amount of

base found in 5 pounds of limestone. If you are only watering once a week, then it will take 40 weeks to bring the substrate pH up to an acceptable level. If you are using a pure water source without any alkalinity, then you may never get the pH up to an acceptable level. The presence of limestone in the substrate has also been shown to increase the buffering capacity when using acidic fertilizer solutions. Finally, substrate degradation will affect nutrition and pH management. Degradation is the breakdown of the substrate, similar to composting. Of all the materials commonly found in container substrates, bark is the least stable, and therefore the most susceptible to degradation. The problem with degradation is that it not only absorbs all the nitrogen present (causing nitrogen starvation), but the process also tends to be very acidic. Hardwood barks tend to be the most stable. Softwood barks usually require some composting to make them stable. If a bark (any bark) contains any wood, then it is unacceptable for use in container substrates because the wood will cause will cause rapid degradation and nitrogen absorption. Leaching Leaching is the application of water or fertilizer solution beyond what can be held by the substrate. Applying extra water is recommended to thoroughly wet the substrate, and to remove excess salts from the substrate. The leaching fraction is the volume of water that drains from the substrate relative to the volume of water applied. For example, if you apply 15 fluid ounces (0.44 liters) of water, and 3 fluid ounces (0.08 liters) comes out of the bottom of the pot then 3 divided by 15, then times 100 equals a 20% leaching fraction. In other words, 20% of the water applied to the plant came out of the bottom of the pot. It is generally taught that you should have between a 10% and 20% leaching fraction with every watering. However, research has shown that leaching is not necessary for long periods of time if you have a good water source (RO or rain water is ideal) and the fertilizer you use does not contain any harmful salts like sodium or chloride. There are reasons to leach pots, usually because the fertilizer concentration that is applied to the crop is too high for the growth rate, or the water quality is poor, and unused salts (like calcium, magnesium, or sodium) build up in the substrate. In general, whether or not you leach should be based on soil test information showing salt levels actually building up in the substrate, rather than because somebody tells you too. Leaching rates also affect the optimal fertilizer concentration for your crop. Research has shown that the same nutrient levels could be maintained in a peat-


based substrate if a solution containing 400 ppm nitrogen were applied with 50% leaching or a solution containing 100 ppm nitrogen were applied with 0% leaching. This research also showed that applying a solution containing 400 ppm nitrogen with 0% leaching rapidly lead to salts building up in the substrate to unacceptable levels, while applying a solution containing 100 ppm nitrogen with 50% leaching lead to nutrient deficiencies because there wasnt enough of the fertilizer remaining in the pot because of the excess leaching. Applying fertilizer to a substrate When you apply fertilizer to a substrate, which is more important, the concentration of the fertilizer solution, or the volume that you apply? In fact, both are important because as a plant grows, it adds mass, and a portion of this mass is made up of fertilizer nutrients. It has been shown in a number of experiments that it is the amount of fertilizer applied to a crop that affects crop quality, not simply the fertilizer concentration. To calculate the amount of fertilizer applied, you need to know both the fertilizer concentration and the volume applied. For example, applying 1 liter (about 1 quart) of a fertilizer solution containing 100 ppm (100 mg nitrogen/liter) will supply 100 mg of nitrogen. If only 0.5 liters (about 1 pint) were applied of the same fertilizer solution, then only 50 mg of nitrogen would be applied. This can be especially important when you are only applying fertilizer on a weekly basis. If the amount of fertilizer solution being absorbed into the substrate decreases for any reason (decreased water-holding capacity), they you could end up starving your plants. How do commercial growers manage pH and nutrient levels Commercial growers have learned that a single fertilizer concentration may or may not work depending on a number of factors including leaching, growth rates, light levels, irrigation frequency, etc. Instead, many growers will manage the pH or nutrient level within the substrate itself. This requires that the grower tests the pH, electrical conductivity, and perhaps even the specific nutrient levels contained in the substrate on a regular basis (see Sidebar). These measured values can be used to make adjustments to the fertilizer solution. For example, if the substrate pH is too high, then a grower might switch to a fertilizer containing more ammoniacal nitrogen, or they may lower the alkalinity of the water. If the EC of the substrate is too high, the grower may increase the leaching rate, or decrease the concentration of fertilizer

applied to the crop. The point is that by measuring the pH and EC of the substrate, they can make sure that a particular fertilizer solution is doing what they think it is doing, and make changes if things are going wrong, usually long before there are noticeable problems with the plant. Even though there is not a lot of specific knowledge about acceptable ranges for substrate pH and EC with orchids, it is probable that they are similar to almost all other crops and so will grow best in a substrate pH around 6.0. Because they appear to be somewhat salt sensitive, they will also grow best with a substrate EC slightly lower than the optimal level recommended for most crops. If testing with a pour-thru method, then the desired substrate EC would be between 1 and 2 mS/cm.


Monitoring Media pH and Nutrient Levels For successful pH and nutritional management of container grown crops, the goal is to keep the pH and nutritional levels within an acceptable range and to spot problem

trends early on. This is a far better strategy than blindly applying fertilizer and hoping everything is OK, or having to take dramatic steps to rescue stressed plants. Using reliable meters, you can measure pH (which affects the availability of nutrients) and electrical conductivity or EC (the overall concentration of fertilizer salts) in

substrates. Other advantages or in-house testing are that the tests are inexpensive and the results to be obtained quickly, typically in less than 1or 2 hours. How often do you test? Typically commercial growers will test substrate pH and EC every 2 to 3 weeks. That does not mean they test every pot or every crop every two or three weeks. Instead, they may do some random sampling to make sure everything is pH and EC are within acceptable levels, or they may test a few know problem crops and then assume that if their pH and EC are within acceptable levels, then other, less sensitive crops are not having problems.

There are a number of different testing methods commonly used for measuring the pH and EC in container substrates. 1:2 method Saturated media extract method.

For additional information on the saturated media extract method, see Michigan State University extension bulletin E-1736 Greenhouse growth media: Testing and nutrition guidelines by D. Warncke and D. Krauskopf.

Pour-thru method For more information on the Pour-thru method, see the web site www.pourthruinfo.com.

Squeeze Method.

Step 1. Collect a small amount of substrate from the bottom 2/3rd of the pot. For very small plants, like those being grown in plug trays or bedding flats, use the whole cell as a sample. Take samples from 5 to 10 or more plants distributed in the group of plants to be sampled. When a sufficient amount of substrate is collected, thoroughly mix the sample to ensure uniformity.

Step 2. Measure out a known volume of substrate in a

beaker or cup [usually 2-4 oz. (50 to 100 ml)]. The beaker should be firmly filled with the substrate so that it is slightly more compressed than when it was in the pot. Place 2 equal volume of distilled water into cup. Allow the solution to equilibrate (30-60 minutes) before measuring pH and EC.

Step 3. Measure pH and EC directly in the slurry.

Step 1. Collect a small amount of substrate from the bottom 2/3rd of the pot. For very small plants, like those being grown in plug trays or bedding flats, use the whole cell as a sample. Take samples from 5 to 10 or more plants distributed in the group of plants to be sampled. When a sufficient amount of substrate is collected, thoroughly mix the sample to ensure uniformity.

Step 2. About 4 to 8 oz (150 to 300 ml) of fresh

substrate is placed in a cup. Distilled water is slowly added while the sample is constantly stirred with a spatula or knife until it has reached a consistent moisture level. This is determined to be when the sample behaves like a paste, the surface glistens with water, but there is no free water on the surface of the sample. The solution is allowed to equilibrate for 60 minutes

Step 3. Measure pH directly in the slurry. Step 4. Extract the solution from the media by

squeezing slurry through paper towel or a coffee filter. Measure EC in extracted solution.

Step 1. Irrigate the crop one hour before testing, making sure the substrate is thoroughly wet. Allow the pots to drain for 30-60 minutes.

Step 2. Once drainage has stopped, place the pot to be sampled into a plastic saucer and pour onto the surface enough distilled water to get about 2 oz. (50 ml) to come out of the bottom of the pot.

Step 3. Measure pH and EC directly in the leachate

Step 1. Irrigate the crop one hour before testing, making sure the substrate is thoroughly wet. Allow the pots to drain for 30-60 minutes.

Step 2. Collect a small amount of substrate from the

bottom 2/3rd of the pot. For very small plants, like those being grown in plug trays or bedding flats, use the whole cell as a sample. Take samples from 5 to 10 or more plants distributed in the group of plants to be sampled. When a sufficient amount of substrate is collected, thoroughly mix the sample to ensure uniformity.

Step 3. Squeeze the solution from the media. For a

cleaner sample, media can be squeezed through a paper towel or coffee filter. The volume of solution needed will depend on the type of pH or EC meter used for testing.

Step 4. Measure the pH and EC in the extracted

solution

In general, there is no one best method for measuring substrate-pH or EC in the greenhouse. However, with orchids, especially with specimen plants, the pour-thru

method may work best because it will not damage roots. Other reasons for deciding on which method to use in your greenhouse include any experience that you have with a particular method as well as how much help and advice you can get from other people that are close by such as other growers, extension agents, universities, or soil testing laboratories.

Whichever soil testing method you choose, consistency is the key to making that method work. Consistency starts with having a single, trained person taking the test. Other tips include: 1) Choose one soil testing method and stick with it. Different methods can give different results. 2) When removing substrate from the pot, take the sample from the bottom 2/3rd of the pot. The bottom 2/3rd is typically were the roots are in the pot and sampling in this

way avoids fertilizer salts that can accumulate at the substrate surface with all irrigation methods (not just subirrigation). 3) Try to take media samples roughly the same time before or after an irrigation. This is especially important with the squeeze method. 4) Choose a reliable pH and EC meter and calibrate it regularly. Calibrating solution has an expiration day and should be discarded when that date is reached.


Table 1. Interpretation of media pH levels for container grown crops. Values are the same for all testing methods. Adapted from: W. Argo and P. Fisher. 2002. Understanding pH management of container grown crops, Meister Publishing, Willoughby, OH.

Acceptable range Examples Iron-inefficient

or Petunia Group

5.4 to 6.2 Azalea, bacopa, Calibrachoa, dianthus, nemesia, pansy, petunia, rhododendron, snapdragons, verbena, vinca, and any other crop that is prone to micronutrient deficiency (particularly iron) when grown at high media pH.

General Group 5.8 to 6.4

Chrysanthemum, impatiens, ivy geranium, osteospermum, poinsettia, and any other crop that is not generally affected by either micronutrient deficiencies or toxicities.

Iron-efficient or

geranium group 6.0 to 6.6

Lisianthus, marigolds, New Guinea impatiens, seed geraniums, zonal geraniums, and any other crop that is prone to micronutrient toxicity (particularly iron and manganese) when grown at low media pH

Table 2. Interpretation of media electroconductivity (EC) or soluble salt levels. For salt sensitive crops, like

orchids, the low fertility level range would be a good starting point. Values are reported in mS/cm. 2:1

method Saturated media extract method

Pour-thru method

Squeeze method

No fertility 0 0.25 0 to 0.75 0 to 1.0 0 to 1.0 Low fertility 0.30 to 0.75 1.0 to 2.0 1.0 to 2.5 1.0 to 2.5 Acceptable range 0.30 to 1.50 1.0 to 3.5 1.0 to 6.0 1.0 to 5.0 High fertility 0.75 to 1.50 2.5 to 3.5 4.0 to 6.0 2.5 to 5.0 Potential root damage >2.50 > 5.0 > 8.0 > 8.0 The units of measure for EC can be mMho/cm, dS/m, mS/cm, M/cm, or mMho x 10-5/cm. The relationship is 1 mMho/cm=1 dS/m=1 mS/cm=1000 S/cm=100 mMho x 10-5/cm. Special Note: It is important to remember that EC is a measure of the total salt concentration in the extracted solution. It does not give an indication of the concentration of any of the plant nutrients. The only way to determine exactly what ions make up the EC is to use a more extensive commercial laboratory analysis.


Understanding pH management and plant nutrition Part 5: Choosing the best fertilizer Bill Argo, Ph.D. Blackmore Company, Tel: 800-874-8660, Intl 734-483-8661, E-mail: [email protected] Originally printed in 2004 in the Journal of the International Phalaenopsis Alliance, Vol. 13 (4). Over the last year, since the AOS article came out on MSU Magic fertilizer, I have been inundated with calls of people trying to get the fertilizer. Usually, I like to talk with them to get a little information so that I can recommend which of the fertilizers (there now are 4 formulas) will work best for their situation. In this last article of the series, I would like to answer several of the common questions that people are asking.

Q) My water alkalinity is 7.8, which fertilizer will work best? A) Most often, when you hear that the alkalinity is 7.8, the person has actually measured the pH of the solution. Water pH and water alkalinity are not the same thing.

Water pH is a measure of the hydrogen ion concentration in the irrigation water, and will affect the solubility of chemicals and fertilizers in solution. However, in the range of water pH commonly measured in nature (between 5 and 8), there is only a minute amount of acid or base, not nearly enough to influence substrate pH.

In comparison, water alkalinity is a measure of the acid buffering capacity of the water. Because alkalinity is composed of bases (like bicarbonates, carbonates), the effect it has on substrate pH is similar to that of limestone. In addition, the concentration of base supplied by alkalinity commonly found in irrigation water is much higher than that supplied by pH alone. For these reasons, alkalinity (not pH) is the primary factor affecting substrate pH.

However, alkalinity can not be measured with a pH meter, and the pH of the solution will give you no idea how much alkalinity is in the water. In addition, the measurement of total alkalinity is not commonly done by municipal water companies or by water treatment companies. Instead, a water sample should be sent out to a commercial or university laboratory that specializes in testing for greenhouses or nurseries. The cost for these types of tests will range from $25 to over $100 per sample, so it pays to shop around.

The reason that knowing what the alkalinity concentration in the water is important is because it is the balance between the alkalinity of the water and the percent ammoniacal nitrogen in the fertilizer that will determine the ideal fertilizer for your location. See part

3 of this series for more information on fertilizers and how to balance the fertilizer with the alkalinity of the water.

Q) What else should I test for besides alkalinity? A) Besides alkalinity, you want to know the electrical conductivity (EC) or total dissolved solids (TDS) which gives you an idea of the total salt concentration in the water. It is also good to know the exact concentration of two plant nutrients, calcium (Ca), magnesium (Mg), as well as the concentration of ions that may give you problems, boron (B), chloride (Cl), sodium (Na), sulfur (S or SO4-S), and iron (Fe). Any laboratory that will test for alkalinity should also these for these ions. The reason that knowing the concentration of calcium, magnesium, or sulfur is important is that you want to supplement or balance the concentrations of these nutrients in the water with those found in the fertilizer. In addition, you water to check the concentration of waste ions to see if the water is suitable for growing plants, or if it needs additional treatment (for example, RO purification).

Q) How do commercial growers apply fertilizer? A) Commercial greenhouse growers will typically apply fertilizer one of two ways. The first is to apply the fertilizer based on the concentration of a specific nutrient, usually nitrogen. The formulas for calculating how much fertilizer to add to a given volume of water to get a specific nutrient concentration is found in Table 1.

The other way fertilizer is applied to a crop is based on the electrical conductivity (EC) of the fertilizer solution.

Q) What is the relationship between electrical conductivity (EC) and the fertilizer concentration? A) Electrical conductivity is really a measure of how much or how little electrical current can move through water. Electrical current can not move through pure water. When a salt is dissolved in water, it can break apart into positively charged cations and negatively charged anions. For example sodium chloride (NaCl) dissolving in water will break apart into sodium cations (Na+) and chloride anions (Cl-). Because these cations and anions have an electrical


charge, they can allow an electrical current to move through the water. So, the greater the amount of salt dissolved in the water, the higher the electrical conductivity. However, not all salts dissociate (break apart) the same when dissolved in water. Some salts, like sodium chloride will dissociate completely to form ions, while others, like magnesium sulfate (Epson salts or MgSO4) will dissolve, but will not totally dissociate. When equal amounts of sodium chloride and

magnesium sulfate are dissolved in water, the sodium chloride will have the higher EC. Some salts, like urea, will dissolve completely but dont form ions, and so their presence in water doesnt affect EC. Fertilizers are nothing more than combination of salts, but because each formula is different, there is a unique relationship between the concentration you are applying with a specific fertilizer and the EC. For example, 20-10-20 is composed of ammonium nitrate, monoammonium phosphate, and potassium nitrate

Table 1. The amount of fertilizer required to obtain specific concentrations of nitrogen in the fertilizer solution. To convert to grams, multiply the value by 28.

Amount of fertilizer (in ounces) per 100 gallons to get the desired nitrogen concentration

Amount of fertilizer (in ounces) per 5 gallons to get the desired nitrogen concentration

100 ppm N 200 ppm N 300 ppm N 100 ppm N 200 ppm N 300 ppm N 30-10-10 4.4 8.9 13.3 0.2 0.4 0.7 21-7-7 6.4 12.7 19.1 0.3 0.6 1.0 21-5-20 6.4 12.7 19.1 0.3 0.6 1.0 20-20-20 6.7 13.3 20.0 0.3 0.7 1.0 20-10-20 6.7 13.3 20.0 0.3 0.7 1.0 19-4-23-2 Ca 7.0 14.0 21.1 0.3 0.7 1.1 17-5-17-3 Ca-1 Mg 7.8 15.7 23.5 0.4 0.8 1.2 15-5-15-5 Ca-2 Mg 8.9 17.8 26.7 0.4 0.9 1.3 15-3-20-3 Ca-1 Mg 8.9 17.8 26.7 0.4 0.9 1.3 14-4-14-5 Ca-2 Mg 9.5 19.1 28.6 0.5 0.9 1.4 13-2-13-6 Ca-3 Mg 10.3 20.5 30.8 0.5 1.0 1.5 13-3-15-8 Ca-2 Mg 10.3 20.5 30.8 0.5 1.0 1.5 10-30-20 13.3 26.7 40.0 0.7 1.3 2.0 9-45-15 14.8 29.6 44.5 0.7 1.5 2.2 6-30-30 22.2 44.5 66.7 1.1 2.2 3.3 To calculate the amount of fertilizer needed to get a specific nitrogen concentration Step #1 Multiply the desired nitrogen concentration (in ppm N) by the gallons of fertilizer you want. Step #2 Multiply the percent nitrogen in the formula by 75 Step #3 Divide the value from Step #1 by the value from Step #2. Example: How much 20-10-20 do you need to add to 5 gallons to get a fertilizer solution with 100 ppm N Step #1 100 x 5 = 500 Step #2 20 x 75= 1,500 Step #3 500 1,500 = 0.33

You need to add 0.33 ounces (about 9 grams) of 20-10-20 added to 5 gallons of water to get a fertilizer solution with 100 ppm N.

For people who are only measuring out small quantities of fertilizer, 1 US teaspoon holds about 0.2 ounces (about 6 grams) of fertilizer. Below is the concentration of nitrogen (in ppm total nitrogen) obtained when mixing , , 1, or 3 teaspoons into a gallon of water with different fertilizers. Amount of fertilizer added per gallon of solution teaspoon teaspoon 1 teaspoon 3 teaspoon 30-10-10 120 240 475 1425 21-7-7 85 165 225 1000 21-5-20 85 165 225 1000 20-20-20 80 160 320 950 20-10-20 80 160 320 950 19-4-23-2 Ca 75 150 300 900 17-5-17-3 Ca-1 Mg 70 135 270 810 15-5-15-5 Ca-2 Mg 60 120 240 710 15-3-20-3 Ca-1 Mg 60 120 240 710 14-4-14-5 Ca-2 Mg 55 110 220 660 13-2-13-6 Ca-3 Mg 50 105 210 620 13-3-15-8 Ca-2 Mg 50 105 210 620 10-30-20 40 80 160 475 9-45-15 35 70 145 425 6-30-30 25 50 95 285


(along with a small amount of micronutrients, and dye). Dissolving 1 gram of 20-10-20 in 1 liter of pure water will give you a solution with a concentration of 200 ppm nitrogen and an EC of about 1.3 mS/cm. 20-20-20 is composed of monoammonium phosphate, potassium nitrate, and urea. Dissolving 1 gram of 20-20-20 in 1 liter of pure water will also give you a solution with a concentration of 200 ppm nitrogen but the EC will only be 0.8 mS/cm. 30-10-10 is also composed of monoammonium phosphate, potassium nitrate, and urea. Dissolving 1 gram of 30-10-10 in 1 liter of water will give you a solution with a concentration of 300 ppm nitrogen, but the EC will only be about 0.4 mS/cm.

Dont forget that the irrigation water also has an EC, which needs to be taken into account when determining the relationship between the EC and concentration of a fertilizer solution. For example, dissolving 1 gram per liter of 20-10-20 in pure water (no EC) will give a solution with an EC of 1.3 mS/cm. However, dissolving 1 gram of 20-10-20 in water with an EC of 0.5 mS/cm will give a solution with an EC of 1.8 mS/cm. See Table 2 for more information on the relationship between EC and fertilizer concentrations.

Table 2. The relationship between electrical conductivity (EC) and the fertilizer concentration (in ppm total nitrogen) when dissolved in pure water. Values for EC are given in mS/cm2.

Fertilizer concentration in ppm total nitrogen Formula1 50 100 150 200 300 400 30-10-10 0.07 0.14 0.21 0.28 0.42 0.56 21-7-7 0.28 0.56 0.84 1.12 1.68 2.23

21-5-20 0.29 0.58 0.93 1.16 1.86 2.33 20-20-20 0.20 0.40 0.60 0.80 1.20 1.60 20-10-20 0.33 0.66 0.99 1.32 1.98 2.63

19-4-23-2 Ca 0.34 0.68 1.02 1.36 2.04 2.72 17-5-17-3 Ca-1 Mg 0.32 0.64 0.96 1.28 1.92 2.56 15-5-15-5 Ca-3 Mg 0.39 0.78 1.17 1.56 2.34 3.12 15-3-20-3 Ca-1 Mg 0.35 0.70 1.05 1.40 2.10 2.80 14-4-14-5 Ca-2 Mg 0.35 0.70 1.05 1.40 2.10 2.80 13-3-15-8 Ca-2 Mg 0.40 0.80 1.20 1.60 2.40 3.20 13-2-13-6 Ca-3 Mg 0.34 0.68 1.02 1.36 2.04 2.72

10-30-20 0.48 0.95 1.42 1.90 2.85 3.80 9-45-15 0.60 1.20 1.80 2.41 3.60 4.82

NOTE: There can be some slight differences between the values of the same formulation from different companies. You should always obtain a fertilizer chart from your manufacturer. 1 N-P2O5-K2O formula 2The terms conductivity, soluble salts, or electrical conductivity (EC) are all used to describe the amount of salt contained in a solution. There are also a variety of units used to

measure EC including micromhos (mho), millimhos (mmhos), microsiemens (S), millisiemens (mS), or decisiemens. 1000 mho/cm = 1000S/cm = 1mmho/cm = 1mS/cm = 1dS/m.

Frequently, you are not using a pure water source without any conductivity. Therefore, you need to take the water into account when determining the relationship between EC and fertilizer concentration. Examples are given below. Calculate ppm Nitrogen from a 20-10-20 fertilizer solution with a total EC of 1.8 mS/cm and an using irrigation water with an EC of 0.5 mS/cm.

EC of fertilizer solution - EC of water = EC of only the fertilizer 1.8 mS/cm - 0.5 mS/cm = 1.3 mS/cm

From the chart above, 20-10-20 with an EC of 1.3 mS/cm would give a concentration of about 200 ppm N. To predict the EC of 20-10-20 at 200 ppm N using an irrigation water with an EC of 0.5 mS/cm.

EC of 20-10-20 at 200 ppm N + EC of water = EC of fertilizer solution 1.3 mS/cm + 0.5 mS/cm = 1.8 mS/cm

For growers that use proportioners or injectors, sometimes the EC of the fertilizer solution coming out of the hose is not what you

expect. The problem can be caused by an incorrect dilution rate from the injector (either broken or not properly adjusted) or the fertilizer stock concentration is wrong.

To check the fertilizer concentration, take a small amount from the stock solution, dilute this in water to the target ratio, and check the EC. For example, in you think that your injectors is set at 1:100, then put 10 milliliters into 1 liter of water (this will also give a 1:100 dilution). If the EC of the solution is where is should be, then it is an injector problem. If the EC of the hand-diluted solution is off-target, then the stock concentration is not correct.


Q) What is the difference between electrical conductivity (EC) and total dissolved solids (TDS)?

A) The measurement of EC and TDS are closely related. An EC meter will measure the electrical conductance of the fertilizer solution. A TDS meter will measure the EC of the fertilizer solution and then convert the measurement into parts per million (ppm) by multiplying the EC by a constant. In article 2 of this series, I said that the constant is usually 1 mS/cm = 1000 ppm salt. On further examination, I found five different constants being used by various meters ranging from 420 to 1000. See Table 3 for more information on the relationship between EC and TDS.

Q) How do I know how much of each nutrient I am applying? A) Both EC and TDS measurements are generic measurements, they dont tell you any specifics about the fertilizer solution that you are applying. If you want to know the exact concentration of each of the nutrients that you are applying with the fertilizer, then you need to calculate that from the formula on the bag of fertilizer. See Table 4 for more information on the concentration of individual macronutrient supplied by different fertilizers.

In addition, the irrigation water can supply significant amounts of some nutrients. Unless you are using a pure water source (which contains little if any nutrients), then you should add the concentration of

Table 3. Relationship between electrical conductivity (EC) of selected fertilizer dissolved in pure water at a constant concentration of 100 ppm total nitrogen and total dissolved solids (TDS) measurements. The exact value that you get will depend on the TDS conversion constant used by the meter.

TDS conversion constants (ppm = 1 mS/cm) Formula

EC value at 100 ppm N (mS/cm) 420 ppm 500 ppm 640 ppm 700 ppm 1000 ppm

30-10-10 0.14 59 ppm 70 ppm 90 ppm 98 ppm 140 ppm 20-20-20 0.40 168 ppm 200 ppm 256 ppm 280 ppm 400 ppm 20-10-20 0.66 277 ppm 330 ppm 422 ppm 462 ppm 660 ppm 15-5-15 0.78 327 ppm 390 ppm 500 ppm 546 ppm 780 ppm 13-3-15 0.80 336 ppm 400 ppm 512 ppm 560 ppm 800 ppm At a concentration 100 ppm total nitrogen from 20-10-20, the TDS measurement can range from 277 ppm to 660 ppm, depending on the constant used by the TDS meter. To calculate a TDS for nitrogen concentrations other than those presented above, multiply the corresponding EC from Table 1 by the constant for your meter. Examples are given below. Calculate the expected TDS measurement of 20-10-20 at 100 ppm total nitrogen (in pure water) using a meter with a costant of 1000 ppm = 1 mS/cm.

EC of fertilizer solution at 100 ppm total nitrogen x Constant = TDS of the fertilizer

0.66 mS/cm x 1000 = 660 ppm TDS Predict the nitrogen concentration of 20-10-20 dissolved in pure water with a TDS measurement of 660 ppm

TDS measurement Constant = EC of fertilizer solution

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