Plants are approximately 90% water. The remaining dry matter is mostly composed of the elements listed below. The elements carbon (C), hydrogen (H) and oxygen (O) comprise 89% of the dry matter of most plants. The plant is able to derive these 3 elements from water and the atmosphere. The other essential elements constitute only about 1% of the total plant weight and about 11% of the dry matter. However, these are the elements (referred to as mineral elements or mineral nutrients) that need to be supplied in the substrate components or fertilization program.

These elements can be broken into 3 major categories. The first is referred to as primary macroelements because the plant needs relatively large amounts of these elements. The primary macroelements include nitrogen (N), phosphorus (P), and potassium (K). The second category is the secondary macroelements and includes calcium (Ca), magnesium (Mg) and sulfur (S). The third category is the microelements. These are required in much smaller amounts but are no less important than the primary and secondary macroelements. The microelements include iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), molybdenum (Mo), chlorine (Cl), and nickel (N). There are several other elements that are qualified as beneficial including silicon (Si), cobalt (Co) and sodium (Na).  Although beneficial, these elements are not considered essential since they are not required for all plants to grow and complete their life cycle.

The goal of a greenhouse fertilization program is to provide these mineral elements to the crop at optimal concentrations and ratios to each other. Mineral elements may be provided in various forms including salts, oxides and chelates. The terms oxides and chelates will be discussed more under the micronutrient section of this learning unit. However, before discussing the macroelements it is important to understand the term “fertilizer salt”. A salt is a compound that is generally water-soluble (dissolves in water) and when placed in water dissociates (breaks apart) into an anion (negatively charged particle) and a cation (positively charged particle).  For example, common table salt is sodium chloride or NaCl. When placed in water it dissolves by dissociating into Na⁺ and Cl^-. Many, although not all, of the fertilizers that are used to provide the macroelements (and often many microelements) are salts (thus we call them fertilizer salts). Some examples are shown below with the fertilizer salt and its respective anion and cation. Therefore, when added to water, fertilizer salts usually readily dissolve and dissociate into their respective anions and cations. This is important because this allows for these water-soluble fertilizer salts to be used to formulate water-soluble fertilizers to be used in liquid fertilization programs in the greenhouse.

Section 2: Primary Macroelements

The primary macroelements nitrogen (N), phosphorus (P) and potassium (K) are required in highest amounts of all of the mineral elements. They are provided to a crop through the substrate components, incorporation of fertilizer sources into the substrate, by controlled release fertilizers or through a liquid fertilization program.

Nitrogen
Nitrogen (N) is required in the highest amount of all of the mineral elements, and nitrogen is often used as the benchmark or starting point for determining the fertilizer solution concentration when conducting a liquid fertilization program. For example, a solution containing 250 parts-per-million (ppm) N may be prepared. In this case, it is assumed that enough P and K are being provided to obtain the proper ratio of N:P and N:K. Generally, N and K should be provided in nearly equal amounts (although in reality K is often supplied at levels somewhat lower than N), but there are some exceptions such as azaleas where a 3:1 (N:K) ratio is preferred and cyclamen where a 1:2 (N:K) ratio is preferred. Classic N deficiency symptoms include reduced growth and general chlorosis of either lower or all (if especially severe) leaves.

Constant liquid fertilization programs generally provide N at concentrations of 50 to 300 ppm depending upon irrigation methods, crop and crop stage. Nitrogen may be supplied to a crop using ammonium nitrate (NH₄NO₃), calcium nitrate [Ca(NO₃)₂], potassium nitrate (KNO₃), urea (NH₂CONH₂), ammonium phosphate [NH₃(HPO₃)] or magnesium nitrate [(Mg(NO₃)₂]. These are the most common sources of N used in formulating fertilizers or fertilizer solutions for greenhouse crops. Greenhouse managers may make fertilizer solutions directly using one or more of these fertilizer salts or may use premixed commercial water-soluble fertilizers that use one or more of these fertilizer salts as N sources.

Other sources of N might be the components used to formulate the root substrate. Many organic substrate components such as animal manures may contain varying amounts of N. This N is generally made available during decomposition of the organic material and the rate at which the N becomes available depends upon the rate of decomposition. Other organic sources of N are fish emulsion, blood meal and feather meal. Nitrogen from these organic sources is first released into the substrate solution as either ammonium (NH₄⁺) or urea. Ammonium may be taken up directly by the plant or the ammonium may be converted to nitrate (NO₃^-) by microorganisms before being taken up by the plant. Urea can be directly taken up by the plant but is often rapidly converted to ammonium in the substrate. The NH₄⁺ may then be absorbed by the plant or converted to nitrate by microorganisms before being absorbed.

As eluded to above, plants primarily absorb N as either NH₄⁺ or NO₃^-. Many fertilizer salts provide nitrogen in one (i.e. KNO₃) or both (i.e. NH₄NO₃, urea) of these forms. However, an exception would be organic materials that provide N. Organically-bound N must first be converted to NH₄⁺ (and then possibly to nitrate) by microorganisms before it is available for uptake by the plant.

Nitrogen is unique among the macroelements in that both the N concentration and the form of the N are both important and must be managed through the fertilization program. If using urea as a nitrogen source, the urea is generally considered for practical or management purposes to be the same as NH₄⁺ since a large portion of it will likely be converted to NH₄⁺ before it is absorbed by the plant. In other words, in practice we consider urea to be the same form of N as NH₄⁺. This results in the two forms of nitrogen that must be considered to be NH₄⁺ and NO₃^-. Understanding the appropriate balance between these two N forms (often denoted as NH₄-N and NO₃-N) and understanding how these two N forms affect substrate chemistry is very important and will be discussed in detail throughout this learning unit.

For most greenhouse crops, not more than 50% (and usually not more than about 30%) of the N should be supplied as NH₄⁺. However, the optimal NH₄⁺:NO₃^- ratio depends upon plant species, age of the plant, and time of year, climate and location. Plant growth can be altered by varying the NH₄⁺:NO₃^- ratio. Typically, plants that are fed a higher ratio of NH₄⁺ than NO₃^-tend to be described as having lush or softer growth versus the compact growth often observed when plants are provided a fertilizer with a low NH₄⁺ to NO₃^- ratio. Very high levels of NH₄⁺ can cause root damage and have been reported to make plants more susceptible to some soil-borne diseases. High levels of NH₄⁺ can also inhibit the uptake of calcium from the substrate and thus induce a calcium deficiency in the crop.

In addition to impacts on plant growth, the form (or ratio of the forms) that N is supplied to the crop can also affect substrate chemistry. Both NO₃^- and NH₄⁺ may be absorbed by the plant. However, as mentioned previously, NH₄⁺ may be converted to NO₃^- in the substrate by microorganisms in the following chemical reaction.

2(NH₄⁺) + 3(O₂) → 2(NO₂^2-) + 2(H₂O) + 4(H⁺)

(NO₂^2-) + (O₂) → 2(NO₃^-)

The production of hydrogen ions (H⁺) in the first step of this conversion is one reason why NH₄⁺-based fertilizers are considered to be acidic and can cause the pH of the substrate to go down over time with continued application (increasing the concentration of H⁺ in the substrate causes the pH to decrease or become more acidic). The substrate pH also decreases when NH₄⁺ is taken up by the plant.  This occurs because in order to take up the NH₄⁺, a plant excretes H⁺ into the substrate to maintain the charge balance across the root membrane. This increases the H⁺ concentration in the substrate and subsequently causes the pH of the substrate to decrease (become more acidic).

Conversely, when NO₃^- [such as supplied by KNO₃ and Ca(NO₃)₂] is absorbed by the plant, an OH^- (hydroxyl group) is excreted into the substrate for each NO₃^- absorbed. The release of OH^- causes the substrate pH to increase over time (become more basic) because it neutralizes H⁺ in the substrate (H⁺ + OH^- → H₂O). Thus, the form of nitrogen supplied to the crop can have a profound effect on substrate pH and can change the pH of the substrate during crop production, and greenhouse managers will often consider the form of nitrogen to supply to the crop not only to maintain an acceptable NH₄⁺:NO₃^- ratio for optimal plant growth but to control substrate pH. Because of the effect of nitrogen form on pH, fertilizers such as NH₄NO₃ and urea are referred to as being acidic fertilizers while fertilizers such as KNO₃ and [Ca(NO₃)₂] are referred to as basic fertilizers.

Ammonium and urea N sources are often preferred by producers where it can be used because they are relatively inexpensive. Calcium [Ca(NO₃)₂] and potassium nitrate (KNO₃) are common nitrate-nitrogen sources but are more expensive sources of N. However, because of the issues discussed, greenhouse managers are not able to use NH₄⁺ and urea sources of N exclusively. The appropriate ratio must be supplied to insure that NH₄⁺ concentrations in the substrate do not exceed recommended levels and reach levels that start to negatively impact substrate chemistry and plant growth. The amount of ammonium (or urea) N that can be supplied (or the appropriate NH₄⁺:NO₃^- ratio) changes depending up climate and time of year. Under warm substrate conditions, NH₄⁺ is more rapidly converted to NO₃^- due to higher microbial activity than under cool substrate conditions. Therefore, under warmer substrate conditions a higher NH₄⁺:NO₃^- ratio can be used than under cool substrate conditions. Under warm substrate conditions the NH₄⁺ is rapidly converted to NO₃^- while under cool substrate conditions, the NH₄⁺ conversion is slower and NH₄⁺ may build up in the substrate to undesirable levels. Because of this situation, growers in warmer climates may use higher NH₄⁺:NO₃^- ratios than northern growers. Additionally, growers often use higher NH₄⁺:NO₃^- ratios during warm seasons and reduce or eliminate NH₄⁺ in the fertilization program in cold weather. Additional information regarding managing N and N form will be discussed later in this learning unit.

Phosphorus
The phosphorus (P) level should be approximately 10% - 30% of that of the N. Where a constant liquid fertilization program is being used, a P concentration of 5 to 30 ppm is common for most greenhouse crops. Classic P deficiency symptoms include reduced plant growth and reddening or purpling of leaves.

Phosphorus may be supplied to the crop in several ways. Although not as commonly used as in the past, superphosphate (0-20-0) and triple superphosphate (0-45-0) may be incorporated into the substrate. These are slowly soluble minerals that slowly release P into the substrate as the mineral weathers and breaks down. More detail on superphosphate and triple superphosphate is provided in the "Substrates" learning unit under "Amendments". Many substrate components contain varying amounts of P that may be available for plant uptake. However, greenhouse managers do not generally rely on the substrate components to meet the P need of the crop.

If phosphoric acid is being used to adjust water alkalinity, the P supplied from the acid may be adequate meet part or the entire P requirement of the crop. More information is provided on phosphoric acid in the "Irrigation" learning unit.

Most commonly greenhouse managers use ammonium phosphate [NH₃(HPO₃)] or diammonium phosphate[2NH₄(HPO₃)]  directly or as a component of a premixed commercial water-soluble or slow-release fertilizers to supply P to a crop. Both of these compounds are salts and are readily water soluble.

Phosphorus is important for proper plant development, but excessive P has been demonstrated to cause undesirable excessive elongation. Therefore, supplying more P than required by the crop should be avoided. In some cases, growers of seedling crops (plugs) often restrict the amount of P supplied to promote more compact growth. Excess P (as well as all applied mineral nutrients) should be avoided for both environmental (potential runoff) and financial reasons (wasting money).

Potassium
Common potassium (K) concentrations supplied in a constant liquid fertilization program range from 50 to 250 ppm depending on crop and crop stage. Potassium is typically supplied to greenhouse crops by using potassium nitrate (KNO₃) or potassium phosphate (KPO₄). Other possible sources of K, although not as commonly used, are potassium chloride (KCl) and potassium sulfate (K₂SO₄). These four compounds are all fertilizer salts and water soluble. Although some root substrate components such as coconut coir, vermiculite and rice hulls may contain significant amounts of K, these substrate components do not typically contain enough K to meet the entire crop need.

Classical K deficiency symptoms include necrotic spotting or marginal necrosis of leaves. However, often K deficiency symptoms may be difficult to observe as they may be masked by nitrogen deficiency symptoms that may occur in conjunction with K deficiency.  Other types of symptoms may also occur where N and K are not properly balanced for a given crop. For example, with cyclamen, if K is too low in comparison to N, the leaf petioles will tend to become excessively elongated and the leaves will drop off to the sides of the plant leaving and open center rather than a compact plant. Excess K rarely causes problems as many plants practice “luxury consumption” of K and successfully store the element within plant tissues.

Section 3: Secondary Macroelements

The secondary macroelements calcium (Ca), magnesium (Mg) and sulfur (S) are required in lower amounts than the primary microelements but in higher amounts than the microelements. They are provided to a crop by incorporation into the substrate, irrigation water, or through the fertilization program.

Calcium
Calcium (Ca) is important in the development of cell walls and other plant cell structures. Calcium may be supplied to a crop in numerous ways. Calcium is generally supplied to most greenhouse crops at about 25% to 50% (usually 50%) of the concentration of the nitrogen. The exact recommended Ca concentration is, however, crop and crop stage dependent.

In most commercial sphagnum peat-based substrates, at least part of the crop’s Ca requirement may be met by the limestone incorporated in the substrate to adjust the pH (see "Substrates" learning unit). However, in most situations, the limestone added for pH adjustment does not meet the complete Ca need of a crop. Irrigation water may also contain significant amounts of Ca and this source of Ca also may partially meet the crop’s Ca need. In fact, if limestone is added to the substrate and the water source has significant Ca, adequate Ca might be supplied from these combined sources if the crop duration is short and appropriate pH is maintained.

Long-term crops or crops that require higher levels of Ca (i.e. poinsettia) typically need to have Ca supplied through the fertilization program during the production cycle. Calcium may be supplied directly by using calcium nitrate [Ca(NO₃)₂] which is a water soluble fertilizer salt in the constant liquid fertilization program. Calcium may also be supplied by using a commercial water soluble or controlled release fertilizer that contains Ca [typically from Ca(NO₃)₂].

Calcium sulfate or gypsum (CaSO₄) is sometimes incorporated into the substrate (it is not very water soluble) as a source of Ca. This is often done when the substrate does not require a pH adjustment but a substrate Ca source is desired. The calcium sulfate will provide Ca (and sulfur) but will not significantly affect substrate pH.

Classic Ca deficiency symptoms are usually downward cupping of younger developing leaves (often exhibiting a “draw string” effect), poor uneven leaf expansion, and marginal necrosis of leaves (i.e. tip burn on lettuce) and bracts. Even when Ca is supplied through substrate incorporation or through the fertilization program, Ca deficiencies can still occur. One reason may be that under low pH, Ca in the substrate can be easily leached and become less available for uptake by the plant. Another reason may be because of high substrate NH₄⁺, Mg⁺⁺ or K⁺ concentrations since these cations compete with Ca for uptake (competitive inhibition). Furthermore, since Ca moves through the plant through the xylem, any environmental condition or cultural factor (i.e. high relative humidity, removal of large amounts of leaves such as when pinching) that inhibits transpiration can inhibit the uptake and translocation of Ca to developing plant tissues.

Because of these factors, Ca may sometimes be applied as a foliar spray using Ca(NO₃)₂, calcium chloride (CaCl₂) or a commercially available chelated calcium. By applying Ca as a foliar spray directly to developing tissues, the Ca is directly available for foliar absorption and is not affected by substrate pH or other environmental or cultural conditions that can affect Ca uptake from the substrate. Foliar sprays of calcium chloride or calcium nitrate supplying approximately 100 ppm Ca (1.5 g/L of the fertilizer salt) once or twice a week are most commonly used for this practice.

Magnesium
Magnesium (Mg) is a critical component of the chlorophyll molecule in plants. Therefore, Mg deficiency generally manifests itself as a generalized, marginal or interveinal chlorosis primarily on the lower leaves depending on plant species and severity.

Magnesium is generally supplied at approximately 50% of the Ca concentration. If dolomitic limestone is used to adjust the substrate pH, part of the Mg requirement of the plant may be met from this source. Magnesium is also found in many well-water sources. As with Ca, for short-term crops, these two sources might be adequate to meet a crop’s needs. However, especially for long-term crops or crops that require high levels of Mg (i.e. poinsettias and gerberas), additional Mg is likely needed during production.

In addition to dolomitic limestone and irrigation water, Mg may be supplied from magnesium sulfate (MgSO₄) which is also known as Epsom salts. This is a water-soluble fertilizer salt that provides both Mg and S. Magnesium may be supplied periodically by applying magnesium sulfate through the liquid fertilization program at a rate of 226 to 452 grams per 379 liters of water (8 to 32 oz per 100 gallons) every 6 to 8 weeks. If using a constant liquid fertilization program, Mg may be supplied by using magnesium sulfate in the fertilizer solution at lower rates (i.e. for a continuous fertilization). Finally, Mg may be supplied using a commercial premixed controlled release or water-soluble fertilizer than contains Mg. Most of these commercial premixed fertilizers use magnesium sulfate as the Mg source and the amount of Mg in the fertilizer is design to be at an appropriate ratio to the other mineral nutrients.

Sulfur
Sulfur (S) is important in the formation of proteins in plants. These proteins may serve as stored energy sources, structural components or enzymes that facilitate important chemical reactions in the plant. Sulfur is generally supplied to crops in concentrations similar to Mg. Sulfur deficiency symptoms usually included generalized chlorosis of foliage primarily on younger leaves.

Sulfur may be supplied in several ways.  One of the most common sources is the irrigation water which may contain significant amounts of S. Many substrate components contain S but the amounts vary from component to component and not all of the S in the substrate component is readily available for uptake. Field soils also contain S, and if a significant amount of field soil is included in the substrate, the crop’s S requirement may be met.

Elemental sulfur (S), aluminum sulfate [(Al)₂(SO₄)₃] and iron sulfate [(Fe)₂(SO₄)₃] serve as sources of S. Elemental S reacts and releases S slowly (as a result of microbial activity) into the substrate solution whereas aluminum sulfate and iron sulfate react quickly in the substrate. These three materials will also cause the pH of the substrate to decrease if used in high enough concentrations.  The reactions that occur in the substrate for these three materials are shown below:

Elemental sulfur:
2S + 3O₂+2H₂O → 2H2SO4   (sulfuric acid)

Aluminum sulfate:
Al₂(SO₄)₃ → 2Al³⁺ + 3SO₄^-

Al³⁺ + H₂O → AlOH²⁺ + H⁺

AlOH²⁺ + H₂O → Al(OH)2⁺ + H⁺

Al(OH)₂⁺  + H₂O → Al(OH)₃ + H⁺

Iron sulfate:
Fe₂(SO₄)₃ → 2Fe³⁺ + 3SO₄^-

Fe³⁺ + H₂O → FeOH₂⁺ + H⁺

FeOH²⁺ + H₂O → Fe(OH)₂⁺ + H⁺

Fe(OH)₂⁺ + H₂O → Fe(OH)₃ + H⁺

The protons (H⁺) generated in these reactions cause the pH of the substrate to decrease (become more acidic).

Many fertilizer salts are sulfate (SO₄^2-) salts. Examples of these would include [(Fe)₂(SO₄)₃], ZnSO₄, CuSO₄, and MgSO₄. Therefore, when sulfate salts are used to formulate fertilizer solutions (or when used as components in premixed water-soluble or controlled release fertilizers) S is also provided to the crop. Finally, S is also a common contaminant (even if not listed on the label) in the fertilizer sources such as superphosphates and these contaminates serve as a source of S for crops.

Section 4: Microelements

The microelements iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), molybdenum (Mo), chlorine (Cl) and nickel (Ni) are required by plants in very small amounts. However, they are just as important as the macroelements. Microelements serve as hemes, enzyme cofactors, or as part of photoreceptors and are critical for cell wall synthesis and cell division among a multitude of other functions.

Micronutrients are supplied in very small concentrations as compared to the primary and secondary macronutrients. However, they are critically important and deficiencies can have major impacts on crop growth and quality. Deficiency symptoms of some of the micronutrients are relatively easy to recognize while others are less distinct and may be different for different plant species. Iron deficiency is generally interveinal chlorosis on the younger leaves of the plant. On young plants, boron deficiency is usually expressed as a yellow to white shoot tip and in severe cases the shoot tip may die. On mature plants boron deficiency is usually expressed as thick leaves and “witches brooming”. Molybdenum deficiency is often expressed as marginal foliar chlorosis occurring in the leaves along the middle of the plant axis.

There are numerous sources of micronutrients as well as different approaches for providing micronutrients to greenhouse crops. When field soil and composted manures are used in the substrate, some or all of the microelement requirement may be met by the elements provided by these components. However, many producers still supplement the substrate with additional microelements to insure an adequate supply. In substrates without field soil or compost, nearly all of the microelements must be supplied through the fertilization program. There are numerous micronutrient sulfate salts such as CuSO₄, ZnSO₄ and MnSO₄. These serve as readily soluble sources of micronutrients. There are also nonsulfate-salt micronutrient sources such as borax and boric acid, which serve as sources of boron, and sodium molybdate and ammonium molybdate which are sources of Mo.

Some micronutrients may be supplied as chelated micronutrients. Chelating agents are negatively charged molecules (generally organic) that surround a metal cation (i.e. Fe²⁺, Zn⁺, Ca²⁺, etc.).  The chelating agent has a unique binding characteristic that forms a heterocyclic ring around the micronutrient cation.  This ring or claw-like characteristic of the chelate allows it to stay in this stable associated form with reduced influence from soil and water conditions such as high or low pH and nutrient imbalances. Several micronutrients such as Fe and Mn are commercially available in chelated forms to greenhouse growers.  Calcium, a secondary macronutrient, can also be found in chelated form. Essentially the chelating agent keeps the micronutrient soluble and available in the substrate for uptake by the plant. Types of chelating agents include EDTA, EDHA and EDDHA. Micronutrients may also occur as oxides (i.e. FeO, ZnO, etc.) or as sucrates (bound with sugars).

There are several strategies that may be used to actually supply micronutrients to a crop. Micronutrients may be added to the substrate before use by adding composted manures or field soil to the substrate. They may be supplied by adding a complete microelement fertilizer package to the substrate before use.

Promax® and Micromax® are commercial products that are mixtures of micronutrient salts with different carriers.  These products are slowly soluble and designed to be blended into substrates before planting.

Esmigram® is a micronutrient fertilizer in which the micronutrients are impregnated on clay particles. The particles are blended into the substrate before planting and the micronutrients are slowly released in the substrate.

Water-soluble fertilizer packages (i.e. S.T.E.M., M.O.S.T. or Compound 111) may also be added to the substrate before, during or after planting. The micronutrients in these fertilizers are either water-soluble salts or chelated minerals that are readily available to the crop. This strategy is designed to provide adequate micronutrients for the entire crop cycle prior to planting of the crop.

Another strategy would be to water in the crop after planting with a water-soluble micronutrient fertilizer package. Again, the application rate would be designed to supply the micronutrient requirement for the entire crop cycle at the beginning of the crop.

Another strategy would be to use a water-soluble fertilizer in the constant liquid fertilization program so that low concentrations of micronutrients are supplied at each irrigation.

Some crops require additional levels of specific micronutrients beyond what is typically supplied by complete microelement fertilizer packages or premixed complete fertilizers. Most commonly, plants may require additional Fe (i.e. blueberry), Mo, (i.e. poinsettia), or B (i.e. celery). Iron may be supplied using a chelated iron compound such as Sprint 138® or Sprint 330® or iron sulfate[(Fe)₂(SO₄)₃]. Molybdenum may be supplied using ammonium molybdate (NH₄Mo) or sodium molybdate (NaMo).  Boron may be supplied through Solubor® (Na₂B₈O₁₃), borax [Na₂(B₄O₅)(OH)₄], ammonium borate (NH₄BO₃) or sodium borate (NaBO₃). These materials may be supplied periodically (i.e. as a monthly application) in the fertilization program or as part of a constant liquid fertilization program (continuously at a low rate).

Section 5: Liquid Fertilization, Controlled-release, and Slow-Release Fertilizers

Mineral nutrients may be supplied to greenhouse crops through a liquid fertilization program (water soluble), a controlled release fertilizer, or a slow-release fertilizer. Each of this methods of fertilization has benefits and limitations for greenhouse crops production.

Liquid Fertilization with Water-Soluble Fertilizers
Liquid fertilization is most common with greenhouse crops. It provides the highest degree of flexibility and control of a crop’s nutrition, and formulations may be designed to match various water quality and crop scenarios. With liquid fertilization using a water-soluble fertilizer, the composition, concentration and frequency of nutrient delivery can be readily changed as needed.  Furthermore, liquid fertilization can be used to help correct pH and E.C. problems that may occur during production.

Water-soluble fertilizers are designed to be injected into irrigation lines or systems. The fertilizer salts used to make these fertilizers are higher quality than those commonly used in turf and field agriculture applications. The higher quality fertilizer salts increase the solubility, decrease turbidity and reduce contaminants. The use of chelated micronutrients in liquid fertilization also enhances solubility and availability for uptake.

Premixed commercial water-soluble fertilizers are marketed under different brand names and are available in numerous formulations (i.e. 20-10-20, 15-5-15, 15-0-15, etc.). These fertilizers are actually a mixture of various fertilizer salts (i.e. KNO₃, NH₄NO₃), chelated elements (i.e. Fe-EDTA, etc.) or other water-soluble nutrient sources (i.e. boric acid) blended together to form a fertilizer with the ratio of mineral nutrients listed on the label. Many commercially prepared water-soluble fertilizers contain a dye that allows visual verification that fertilizer is being injected into the fertilization line. However, this dye should not be used to monitor the suitability or strength of the fertilizer solution. This should be done using an E.C. meter (see checking injector function later in this learning unit).

Although premixed commercial water-soluble fertilizers are an easy way to provide mineral nutrients in a liquid fertilization program, some greenhouse managers may choose to prepare their own water-soluble fertilizers using individual mineral salts [i.e. Ca(NO₃)₂, KNO₃]. In this case, the greenhouse manager selects various fertilizer sources and mixes them at ratios designed to provide the mineral nutrients at the desired concentrations and ratios.

Liquid fertilization may be conducted as once per week applications (“periodic feed”) or as frequently as with each irrigation (“constant liquid feed”). However, rates must be adjusted accordingly and the more frequently the mineral nutrients are provided, the lower the concentration that is used. Keep in mind that if using a periodic feed versus a constant liquid feed program, when clear-water irrigation (water without fertilizer) is used in between fertilization cycles, mineral nutrients in the substrate may be leached if too high of a volume of clear water is applied. Thus, the efficiency of the nutrient delivery may be drastically reduced unless no leach or low leach irrigation is practiced.

Also, a constant liquid fertilization program is often preferred to a periodic feed program. This is because when a high concentration of fertilizer is applied, the concentration of nutrients may exceed the optimal concentration (and E.C. level may exceed optimal). Over time, the levels drop into the optimal range. With more time, levels continue to drop and if enough time occurs between fertilizations, the mineral nutrient levels may drop to below optimal before fertilization is repeated. With a constant liquid fertilization program, the nutrients may more precisely be maintained within an optimal range for the crop.

When mixing fertilizer salts, one must consider that some salts are incompatible with one another especially in high concentrations. In liquid fertilization systems it is not uncommon for two separate concentrate stock tanks to be used. In the first tank, the salts containing Ca and K are dissolved. In the second tank phosphate salts, sulfate salts and micronutrients are dissolved (keep concentrated sulfates and phosphates away from calcium). This practice helps to insure that precipitates do not occur in the stock tanks. Precipitates are generally insoluble, can clog injectors and are unavailable for uptake by the plant.

The two fertilizer concentrates are injected separately into the irrigation line where they are diluted and mixed and the concentration is typically not high enough for significant precipitation to occur. Common precipitates that can occur include CaSO₄ and CaPO₄. These precipitates form more readily under high pH, so many greenhouse managers will acidify the concentrate stock tank water to further help prevent precipitates from forming.

Greenhouse managers who are not using a two-stock tank system must still consider the issue of fertilizer salt compatibility. For instance if a grower is using sulfuric acid (H₂SO₄) injection to reduce the amount of alkalinity in the water, the sulfuric acid may provide significant SO₄^- which can combine with Ca in the fertilizer to form CaSO₄ which can precipitate out of solution. A similar problem can occur when using a fertilizer with significant amounts of Ca and Epsom salts are added. Again, insoluble CaSO₄ can be formed. One way to avoid some of these problems is to use chelated forms of cations.  This strategy along with acids are commonly used in premixed water soluble fertilizers.

Fertilization with Controlled-Release Fertilizers
Controlled-release fertilizers are marketed under several brand names (i.e. Florakote®, Multicote®, Nutricote® , and Osmocote®) and are available in numerous formulations. Controlled-release fertilizers are designed to slowly release the nutrients that they contain into the substrate for uptake by the crop. Controlled-release fertilizers are produced by encapsulating water-soluble fertilizer salts in a polymer or resin (forming small round prills) that allows water vapor to enter and the fertilizer salts to slowly diffuse into the surrounding substrate solution (for polymer-coated) or the capsule may absorb water and swell to the point of bursting (for resin-coated). Increasing the thickness of a polymer coating increases longevity (release time) of the fertilizer by slowing the diffusion rate of the nutrients from within the prill.  Increasing the thickness of the resin results in increased resistance to bursting; thus, delaying the release of fertilizer salts and increasing longevity.  

With both types of coatings, varying the thickness of the coatings helps to provide a predictable release pattern of the nutrients from the prills that is dependent upon temperature. Therefore, controlled-release fertilizer labels will indicate the release period (i.e. 60-day, 6 to 8 months, 100-day, or 12 months, etc.) in addition to the guaranteed analysis (i.e. 15-7-15) and the recommended application rates for various crops and container sizes. The key to the rate of release is the temperature. The ratings by most fertilizer manufacturers are based upon 70°F (21°C).  So a typical “5 to 6 month” controlled-release fertilizer would last up to 6 months if the temperature of the substrate is a constant 70°F. If the temperature is increased to 90°F, that same fertilizer might only last for 3 to 4 months. Because of the gradual release of mineral nutrients, controlled-release fertilizers typically do not significantly affect the substrate pH to the same degree that water-soluble fertilizers can.

Controlled-release fertilizers may be top-dressed onto the surface of the substrate or incorporated into the substrate. Placement of the fertilizer prills may have an effect on fertilizer efficiency depending upon the irrigation method used. If using an overhead irrigation system, the prills may be either incorporated or top-dressed. The flow of water through the substrate at irrigation will move mineral nutrients released from the prill through the substrate. However, if subirrigation is being used, prills should be incorporated into the substrate. This is because if prills are top- dressed and subirrigation is used, the mineral nutrients will not be able to readily move down into the substrates where most of the roots will be located.

In some situations, a grower might incorporate a controlled-release fertilizer into the substrate at a low rate (i.e. half the recommended rate) and supply the remaining fertility requirement through a liquid fertilization program. This provides increased flexibility since once a controlled-release fertilizer is incorporated into a substrate it can't be removed and excess nutrients can't be easily leached out. This tactic can also facilitate feeding crops with various nutrient requirements through single liquid fertilizer injection system.

Fertilization with Slow-Release Fertilizers
Slow–release fertilizers are not as common in general fertilization for greenhouse crop production as they are in turf or field agriculture.  Slow-release fertilizers are generally differentiated from controlled-release as they aren’t coated with a resin or polymer.  The mechanism that controls the nutrient release can be microbial degradation, hydrolysis, or a combination. 

Probably the most common slow-release fertilizer used in greenhouse crop production is methylene urea or urea formaldehyde.  These are commonly used in bark-based substrates as a measure to prevent nitrogen drawdown due to the continuing break down of bark from the composting process.  There are several different manufactures of methylene urea and its breakdown and subsequent nitrogen release is controlled by microbial degradation.  However, different manufacturers formulate the products slightly different (more or less carbon in the chain) resulting in quicker or slower nitrogen release. A similar product is isobutyldiene diurea (IBDU).  Breakdown of this product  is controlled by hydrolysis.  Other products that may fall into the category of slow-release fertilizers include metal-oxides (slowly soluble, pH dependent), sulfur-coated fertilizers (microbial degradation release) and sucrates (slowly water soluble).

Section 6: Reading and Understanding Fertilizer Labels

Fertilizer labels contain a great deal of valuable information. In order to prepare fertilizer solutions, properly apply a fertilizer, or to effectively use a fertilizer, the information on the label needs to be understood.

A fertilizer label lists the % by weight of N, P₂O₅ and K₂O. Therefore, a 20-20-20 is 20% N, 20% P2O5 and 20% K₂O. A 100 lb bag would contain 20 lbs N, 20 lbs P₂O₅ and 20 lbs K₂O. However, this does not tell us how much actual P and K are in 100 lbs of 20-20-20. To determine this we need to know that P₂O₅ is 43% P and K₂O is 83% K (determined from molecular weights). Therefore, a 20-20-20 contains:
 
0.20 N x 100 lbs = 20 lbs N

(0.20 P₂O₅ x 0.43) x 100 lbs = 8.6 lbs P

(0.20 K₂O x 0.83) x 100 lbs = 16.6 lbs K

Therefore, a 20-20-20 is actually 20% N - 8.6% P - 16.6% K. All of these calculations are based on molecular/atomic weights, so if you know the atomic weights you can determine the actual amount of any element from any fertilizer or fertilizer compound. For example, 100 lbs of Epsom salts (MgSO₄ - 7 H₂O) will contain: 24 + 32 + 4(16) +7(18) = 246 and (24/246) x 100 = 9.8 lbs of Mg.

This is not the only important information on a fertilizer label. Fertilizer labels provide information regarding the actual components used to formulate the fertilizer, the ratio of NH₄⁺ to NO₃^-, whether the fertilizer is acidic or basic, the electrical conductivity of different concentrations of the fertilizer, and application recommendations. The links below connect to sample fertilizer labels.

Section 7: Fertilizer Calculations for Liquid Fertilization Programs

Many commercial fertilizers will provide application rates on the label. However, in some cases the application rate may need to be adjusted or individual mineral salts may be used to prepare a custom fertilization program. In either case, it is important to understand how to calculate fertilizer concentrations. These calculations need only be performed for a liquid fertilization program. Controlled-release fertilizers or slow-release fertilizers are applied at recommended rates per container or cubic yard (as recommended on the labels).

Before working through a few examples, it is important to understand the term parts-per-million (ppm). When mixing two liquids of the same density and containing 100% of the material of interest (pure), a ppm is simply 1 part (active ingredient of the material of interest) to 999,999 parts water. However, the materials used are usually not pure, and the solutions containing the active ingredients have densities different from that of water. Additionally, with fertilizers, solids are usually being added to water. In these cases the calculations are based on weight of active ingredient where 1 unit of weight of the material of interest to 999,999 weight units of water.
One ml of water weighs 1 gram at 20° C. Therefore, 1 liter (1000 ml) of water weighs 1000 grams or 1,000,000 mg. This gives us the standard definition of a ppm as:
 
1 ppm = 1 mg^.L^-1

It is also helpful to remember that 1% = 10,000 ppm (100 x 10,000 = 1,000,000). Therefore, a 1% solution is equivalent to 10,000 ppm.

Also remember that these calculations are based on the amount of active ingredient or element of interest not the carrier material.

1. Determine how many mg^.L^-1 of a 20-20-20 are required to produce a 100-ppm N solution.
 
100 ppm = 100 mg^.L^-1

If we were dealing with pure N, we would add 100 mg^.L^-1 of the fertilizer. However, the material is only 20% N.

Therefore, 100 mg of N / 0.20 mg N per mg fertilizer = 500 mg of fertilizer

500 mg 20-20-20 per liter of water provides 100 mg^.L^-1 (or ppm) N.

2. Determine how many mg^.L^-1 of a 20-20-20 are required to produce a 100 ppm K solution.
 
100 ppm = 100 mg^.L^-1

If we were dealing with pure K, we would add 100 mg^.L^-1 of fertilizer. However, the material is 20% K₂O, and K₂O is only 83% K.

To determine the % K in this fertilizer, we have 0.20 x 0.83 = 0.166 or 16.6% K in the fertilizer

Therefore, 100 mg K / 0.166 mg K per mg fertilizer = 602 mg of fertilizer

602 mg 20-20-20 per liter of water provides 100 mg^.L^-1 (or ppm) K.

3. In example 1 we determined that 500 mg^.L^-1 of 20-20-20 provided 100 ppm N. How much P is provided by this solution?

The fertilizer is 20% P₂O₅.

P₂O₅ is 43% P

Therefore, 0.20 x 0.43 = 0.086 or 8.6% P

We used 500 mg of the fertilizer (per liter).

500 mg x 0.086 = 43 mg of P

43 mg P/L = 43 ppm P

4. How many mg of 20-20-20 need to be used in a liter of stock solution that when put through a 1:100 injector will provide 100 ppm N?
 
From example #1, we know that 500 mg 20-20-20/L = 100 ppm N.

1 liter of stock will result in 100 liters of total solution ( 1 liter becomes 100 liters when diluted) when put through the injector. Therefore, 500 mg^.L^-1 x 100 L = 50,000 mg (50 g) in 1 L liter of stock.

5. How many mg.L^-1 of Ca(NO₃)₂ and KNO₃ are required to produce a 100 ppm N solution that provides 75% of the N from Ca(NO₃)₂ and 25% of the N from KNO₃. Assume that both fertilizers are pure.
 
The molecular weight of Ca(NO₃)₂ is:

40 + [14 + (16)3]2 = 164

There are 2 N in Ca(NO₃)₂ so the percentage N is:

28/164 = 0.17 or 17%

The molecular weight of KNO₃ is:

39 + [14 + (16)3] = 101

There is 1 N in KNO₃ so the percentage N is:

14/101 = 0.14 or 14%

Therefore:

Ca(NO₃)₂ = 17% N

KNO₃ = 14% N

We need a total of 100 mg N^.L^-1. We can break this into 2 more simple problems by:

We need 0.75 x 100 = 75 mg N from Ca(NO₃)₂ and 0.25 x 100 mg = 25 mg N from KNO₃.

We now have two simply calculation that are similar to the example 1.

75 mg N / 0.17 mg N per mg N in Ca(NO₃)₂ = 441 mg Ca(NO₃)₂

25 mg N / 0.14 mg N per mg N in KNO₃ = 179 mg KNO₃

6. How many mg of 15-5-15 need to be used in 10 liters of stock solution that when put through a 1:100 injector will provide 200 ppm N?
 
200 ppm = 200 mg^.L^-1

If we were dealing with pure N, we would add 200 mg^.L^-1 of the fertilizer. However, the material is only 15% N.

Therefore, 200 mg of N / 0.15 mg N per mg fertilizer = 1,333 mg of fertilizer or 1.33 g of fertilizer per liter.

1 liter of stock will result in 100 liters of total solution (1 liter becomes 100 liters when diluted) when put through the injector and 10 liters of stock solution will become 1000 liters of final fertilizer solution. Thus we need to add enough fertilizer to the stock solution to make 1000 liters.

We needed 1.33 g of 15-5-15 per liter and we are making a total of 1000 liters (final amount of fertilizer after dilution). So, 1.33 g 15-5-15 per liter x 1000 liter = 1,330 g 15-5-15 in the 10 liters of stock.

Section 8: Nutritional Management and Monitoring

Fertility management is one of the most challenging aspects of greenhouse crops production. Many factors impact the nutritional status of a crop and should be considered when developing a fertilization program, monitoring the mineral nutrition status of a crop and making adjustments to the fertilization program. Among the factors that should be understood and considered are crop stage, time of year, amount and ratios of mineral elements, form of mineral elements, substrate and substrate components being used, substrate pH, substrate cation-exchange-capacity, substrate electrical conductivity, water quality and irrigation method.
 
Crop Stage
Newly planted cuttings, young seedlings and plugs require a lower mineral nutrition (fertility) level than actively growing crops. Typically, younger plant materials are grown at lower fertility levels and as the root system develops and the plants start to grow rapidly, fertility is increased. As plants enter into the reproductive stage (flowering), the nutritional requirements decrease compared to the active vegetative growth stage and the concentration of fertilizer provided may be decreased.

Though some greenhouse managers withhold fertilizer from young seedlings and cuttings, it has been demonstrated by researchers that there are benefits to providing mineral nutrients at optimal levels at all stages of plant development.

Time of Year
Under warm temperatures and high light levels, plants grow more rapidly and utilize mineral nutrients at a faster rate. As temperatures and light increase, the fertility level may need to be increased to meet the crop's increased demand for mineral nutrients. Under the cooler temperatures and lower light conditions of winter months, fertilization may need to be reduced.

As discussed earlier, nitrogen can be supplied to the crop in various forms. The NH₄⁺ form can be converted to NO₃^- in the substrate by microorganisms. One caveat of NH₄⁺ is that a small amount readily converts to ammonia (NH₃), which is toxic to plants in very low concentrations. When substrate temperatures are low as they will be in a greenhouse during the winter months (even when heated), microorganisms are less active and the conversion of NH₄⁺ to NO₃^-is slower. Under these conditions, NH₄⁺ can accumulate in the substrate and more NH₄⁺ is converted to NH₃ and it can build up to toxic levels. High levels of NH₄⁺ in the substrate can also inhibit the uptake of Ca⁺⁺ deficiency in the crop. Therefore, changing the form of nitrogen supplied to the crop during late fall, winter and early spring can be beneficial in preventing NH₃ build-up in the substrate and greenhouse managers thus limit the use of NH₄⁺ nitrogen sources (applying more NO₃^- based nitrogen sources instead) during winter months (especially in northern latitudes).

Amount of Mineral Elements
Mineral nutrients may be present at concentrations that are deficient (below the level the plant needs for optimum growth and development), concentrations that are in excess (above the level the plant can use), concentrations that are phytotoxic (concentrations at which some type of physiological damage is done to the plant) or at concentrations that interfere with (antagonize) the uptake of other mineral elements (high Mg⁺⁺ can inhibit the uptake of Ca⁺⁺). The objective in a fertilization program is to maintain each element within an optimal range throughout the crop cycle. The optimal concentration for any given mineral nutrient may be different for different crops and different growth stages of the crop.

When using a granular fertilizer (not the high grade or fast dissolving water soluble fertilizers) such as superphosphate to supply phosphorus or dolomitic limestone to provide calcium and magnesium, these minerals break down or dissociate slowly in the substrate and release the desired mineral nutrients over time. However, very early in the crop cycle the desired mineral nutrient may be below the optimal level until enough is released into the substrate solution to increase the level of the mineral nutrient into the optimal range. The level of the desired mineral nutrients may then be within the optimal range for many weeks. However, over time, the level may drop and fall to concentrations below optimal especially for long-term crops.

Fertilizer salts that are readily soluble [i.e. calcium nitrate [Ca(NO₃)₂], potassium nitrate (KNO₃), or magnesium sulfate (MgSO₄), etc.] result in a more rapid increase in the level of the mineral nutrients they supply, but the level may also drop more quickly unless additional sources are provided. 

Controlled-release fertilizers are designed to allow the fertilizer salts held within a coating to slowly dissolve and diffuse or leach into the substrate. This allows for a more constant supply of the mineral nutrients and allows them to be maintained within an optimal range over a specified period of time. However, even with the use of controlled-release fertilizers, there is a period of time required for the fertilizer salts to enter into the substrate and increase the levels of mineral nutrients to the optimal ranges. Additionally, over time the level of mineral nutrients leaching from the controlled-release fertilizer will decrease and the level of mineral elements in the substrate may drop below optimal levels.

When using periodic liquid fertilization, (typically 300 - 500 ppm N per fertilization with fertilization being once per week or less), the levels of the applied nutrients increase rapidly and may even reach levels that are above optimal depending upon fertilizer concentration and crop. In between fertilization cycles, the level of mineral nutrients decreases as the plants take them up (or they are leached out). Between fertilizations, the level might stay within the optimal range or might drop below optimal levels depending upon the concentration of the fertilizer solution used, the time interval and amount of clear water irrigations used between fertilizations, leaching fractions and the rate of crop growth.

Constant liquid fertilization (CLF) is a practice in which the concentration of the fertilizer solution is reduced (i.e. typically 75 – 250 ppm N) but is applied with each irrigation. Constant liquid fertilization allows for a more consistent fertility level to be maintained.  However, the fertility concentrations may need to be altered depending on plant development stage and other environmental factors.

Often, especially where a liquid fertilization program is to be used, several fertilization cycles may be required to bring the mineral nutrient content of the substrate up to the desired concentrations. During this period, the mineral nutrient levels may be below optimal and may be limiting to plant growth. For this reason, the substrate may be amended with what is referred to as a nutrient starter charge. The nutrient starter charge is typically some type of fertilizer salt that is rapidly available but incorporated at a low concentration and is designed to last only a few irrigations. Essentially, this is a “booster” to provide nutrients to the crop at planting until the liquid fertilization program can begin to effectively provide the required mineral nutrients. Nutrient starter charge fertilizers may be typically Ca(NO₃)₂, KNO₃ or NH₄NO₃ or complete fertilizers that contain all macro- and micronutrients.
  
Proportions/Ratios of Elements
Not only are the absolute amounts of mineral elements provided important, but the ratios of the mineral elements to one another are important. Problems can occur if mineral nutrients are out of balance. Excess P (in relation to N and K) can cause stem elongation of annual bedding plant species. Excess levels of certain elements can cause antagonisms of other mineral elements because of competitive uptake. For example, excess K or excess NH₄⁺ can cause Ca deficiency.

The ratio of K to Ca is important.  A good estimate is to apply nearly twice the amount of K as is provided of Ca.  Also, Mg is typically provided at about 50% of the concentration as Ca resulting in a 4:2:1 ratio of K:Ca:Mg. 

Microelements are provided at much lower concentrations than the primary and secondary macroelements.  The exact chemistries of nutrient uptake and exchange are hard to summarize when evaluating substrate nutrient ranges, but some basic rules are commonly followed for micronutrient application ratios.  For example, iron is often formulated at 0.075 to 0.1 % concentrations for a 20% nitrogen water-soluble fertilizer that is designed to be used in a peat-based substrate.  Manganese is generally supplied at 50% the concentration of Fe and Cu and B at 30% to 40% of the Mn.

These generally recommended ratios of the mineral nutrients to one another do not always apply to all crops. There are many exceptions. For example, carnation develops best when provided K at 1.5 times the amount of N and cyclamen grows and develops best when provided K at twice the concentration of N. Most foliage plants develop best when provided 1.5 times as much N as K.

Form of Element
The form in which an element is provided can have a significant impact on plant growth. Plants primarily take up N as either ammonium (NH₄⁺) or nitrate (NO₃^-). However, plants respond differently to the two forms of N and plants can store NO₃^- much more readily than NH₄⁺.  High NO₃^- fertilizer formulations generally produce stockier and shorter plants compared to high NH₄⁺ sources. This has an impact the ratios of these two forms of N that greenhouse managers may choose to provide to a crop. For example, many plug producers prefer a water-soluble fertilizer which is nearly all NO₃^- nitrogen in order to promote the development of short, stocky and strong plugs.

In addition to the effect that the form of N can have on plant growth, the form of N provided can have a significant impact on substrate pH over time (as discussed previously under "Nitrogen") and this in turn can have a significant impact on plant growth. As mentioned above, the form of nitrogen can alter the substrate pH while residing there as well as a result of plant uptake.

While some plants thrive when provided N from sources high in NH₄⁺ nitrogen (i.e. azalea), most floriculture species grow and develop best when most of the N is provided as NO₃^- with not more than 30% of the total N coming from NH₄⁺ (or urea).

As discussed above, chelated micronutrients offer advantages in many scenarios versus sulfated micronutrients.  In high pH, highly oxidized, or conditions such as very high salt concentrations, sulfated microelements can become relatively unavailable for plant uptake.   Therefore, chelated forms may be more desirable.

There are various chelating agents available. While it is generally accepted that chelated nutrients are more stable than sulfated salts, pH can still have an impact the stability of the cation-chelate bond. For greenhouse crops production, EDTA (ethylenediamine tetraacetic acid) is a common chelate used in water-soluble fertilizers. Iron-EDTA can also be used as a supplemental source of Fe, but EDTA is less effective at maintaining the bond with Fe (or other cations) at moderate to high substrate pH. Even though Fe-EDTA has a higher percentage of Fe compared to other chelated Fe sources, it does not deliver the Fe to the plant as effectively as other chelated Fe sources. DTPA (diethylene triamine penta acid) and EDDHA [ethylenediamenedi (∝-hydoxyphenylacetic acid)] have a lower Fe content but are more stable at higher pH values making them more efficient at delivering Fe to the plant under high pH conditions. In order of effectiveness at delivering iron at high pH are EDDHA, DTPA and EDTA (most effective to least effective).

Substrate Components
Substrate components dramatically affect the chemical properties of the substrate. Some components such as perlite are generally considered chemically inert and do not have a major impact on the pH, E.C. or mineral nutrient content of the substrate. Other substrate components may contribute significant mineral nutrients. For example, coconut coir may have significant levels of K and rice hulls typically contain K, Ca, Si and Mn that can become available for plant uptake. Vermiculite may contain significant amounts of K and Mg and possibly Na. Composted manures can also contain significant levels of K and micronutrients and if added to the substrate they may provide enough micronutrients to meet the crop's need. In some cases, a substrate component may contain toxic levels of one or more mineral elements that limit its use (i.e. shredded rubber tires can contain toxic levels of zinc).

Components such as perlite and sand have a negligible cation-exchange capacity and are thus poorly buffered. Others such as composted manures, composted bark and peat moss have significant cation-exchange-capacities and are more highly buffered. The importance of the substrate cation-exchange capacity is discussed in more detail below.

The substrate pH will be significantly affected by the components used to formulate the substrate. Sphagnum peat moss-based substrates must be amended with calcitic or dolomitic limestone before use to adjust the pH to an acceptable level. The calcitic limestone will not only increase pH but will provide Ca to the crop. Dolomitic limestone will provide Ca and Mg. On the other hand, a coir-based substrate does not need to be amended with limestone for pH adjustment. Therefore, the limestone source of calcium is absent and must be provided though some other means (i.e. by adding calcium sulfate to the mix or providing all the required Ca in the liquid fertilization program). 

The effect of the substrate components on pH, E.C. and cation-exchange-capacity and  the substrate components contribution to mineral nutrients must be considered when designing a fertilization program.

Substrate pH
The pH of the substrate affects the availability of mineral elements. Under low pH, some elements especially cations such as Fe, Mn, Cu, and Zn become more available for uptake by plants. In fact, under prolonged low pH conditions these nutrients are prone to leaching due to their solubility in the acidic conditions which may eventually lead to plant nutrient deficiencies.  If leaching doesn’t occur, then the high degree of solubility of these micronutrients may lead to excessive uptake by the plant and result in phytotoxicity. This phenomenon is common when geraniums and marigolds are grown in a low pH substrate (pH < 5.5), which often results in Fe-Mn toxicity.

Under high pH, even if elements are present in the substrate in the correct concentrations and proportions, they may be unavailable for uptake by the plant and therefore the plant may suffer from pH-induced mineral nutrient deficiencies (this is common for many micronutrients such as Fe, Cu and Zn). Iron deficiency is often a classic example of this situation. The substrate pH may affect the availability of other mineral nutrients differently. For example, Mo generally becomes more available as pH increases and is less available for uptake by the plant at a low pH.

The effect that pH has on the availability of mineral elements in the substrate is one reason why the recommended pH is in the range of about 5.5 to 6.5. At this pH, there is adequate availability of all mineral nutrients but with limited potential for pH-induced toxicities.

Not only does the pH of the substrate impact the fertilization program by affecting the availability of mineral nutrients, but the fertilization program can affect the pH of the substrate. As discussed previously, the use of fertilizers high in NH₄⁺ nitrogen (NH₄NO₃, urea, etc.) tends to cause pH to decrease over time (and could cause pH to decrease to undesirable levels). Continued use of fertilizers high in NO₃^- nitrogen [Ca(NO₃)₂, KNO₃, etc.] sources tends to cause the pH to go up over time with continued use (and the pH may increase to undesirable levels).

Premixed commercial fertilizers are composed of various fertilizer salts, and these salts may provide nitrogen as either NH₄⁺ (or urea), NO₃^- or as a mixture of the forms. The ratio in which the two forms of N are provided will determine if the premixed fertilizer is acidic (causes pH to go down over time) or basic (causes pH to increase over time).  The higher the ratio of NH₄⁺ (and urea) to NO₃^-, the higher the potential acidity of the fertilizer. The higher the ratio of NO₃^- to NH₄⁺, the higher the potential basicity.

The higher the potential acidity of a fertilizer, the greater the potential decrease in pH over time and the higher the potential basicity, the greater the potential increase in pH over time. The table below lists some examples of common fertilizer salts, premixed fertilizers and their potential acidity or potential basicity. It is useful to know the potential acidity or basicity of a fertilizer because it allows the greenhouse manager to understand how the use of the fertilizer will affect substrate pH over time.

The effect that N form has on pH also provides a useful tool for greenhouse managers. As discussed under the "Irrigation" learning unit, carbonates and bicarbonates (and other bases) in the irrigation water will tend to cause the pH of the substrate to increase over time. If the level of carbonate and bicarbonate is very high (has a high alkalinity level), acid injection may be required to neutralize the carbonate and bicarbonate so that the pH does not increase to undesirable levels. However, if the irrigation water alkalinity level is not too high (or the crop time is short), fertilizer selection may be used as a tool for managing pH of the substrate. If the pH of the substrate begins to slowly increase (or if it is known that the pH will slowly increase over time), the greenhouse manager may choose to use a fertilizer that has a significant potential acidity. The use of such a fertilizer will tend to force the pH back down (offsetting the effect of the irrigation water). How much of an effect the fertilizer will have depends on how high is its potential acidity and how high is the irrigation water alkalinity. In contrast, if the pH of the substrate is lower than desired, a basic fertilizer may be used.

There are two primary problems with using fertilizer choice to control pH. The first is that the fertilizer requires time to cause a pH change and typically cannot be used to make major corrections in pH quickly. The second is that the crop being grown and the season may dictate if,  and for how long, a type of fertilizer may be used. For example, a greenhouse manager may have an irrigation water source that has a moderately high alkalinity level. The use of acidic fertilizers may be able to offset some of the increase in pH caused by the water. However, especially during cooler months, continued use of the acidic fertilizers (high in NH₄⁺ or urea) may not be possible since NH₄⁺ could build up to phytotoxic concentrations in the substrate under cold conditions. In such a situation, a greenhouse manager might rotate between using a basic fertilizer for several irrigation cycles and an acidic fertilizer for several irrigation cycles. Therefore, although the selective use of fertilizers to control pH is a tool with some value, it is limited and the crop requirements must be kept in mind.

When substrate pH is either too high or too low and the pH must be adjusted quickly, there are several corrective procedures that may be employed. Two options for increasing pH are the use of a liquid lime product or using potassium bicarbonate.  Liquid lime products are generally dolomitic limestone dissolved into water. Common applications rates are 2 to 4 quarts per 100 gallons of water.  This will typically raise the pH by approximately 0.5 to 1.0 pH unit.  This mixture will need to be kept agitated and can severely wear down injector parts if not diluted prior to use.  This is often best applied by hand watering.  Potassium bicarbonate also raises the substrate pH.  The use rate is 1 to 2 pounds per 100 gallons of water.  This solution is clear and easily passes through injectors, drip tubes, etc.  However, a down side is that if the solution is splashed on foliage and not rinsed quickly, foliage burn is likely to occur.  Also, be sure to follow up the next day with the application of a complete and balanced fertilizer as the system could be overwhelmed with potassium. Some growers with experience using both a liquid lime and potassium bicarbonate in combination will say that the liquid lime provides more residual control or a longer lasting pH increase. 

For a fast acting pH decrease, iron sulfate may be applied to the substrate.  Recommended rates will vary but most experts agree that 2 lbs/ 100 gallons of water is a good starting point.  Similar to the potassium bicarbonate, the foliage should be rinsed after application to avoid burning.  If insufficient pH drop occurs, apply the same iron sulfate solution rate again.  It is better to take this approach and gradually adjust the pH versus using a very high dose initially. 

Acid drenches can also be used to decrease pH quickly, but there is even a higher potential risk of plant damage as well to those handling the solution.  Most common acids can be mixed with water to create a solution pH of about 2.0 which is used to drench substrate in containers.  For sulfuric acid, experiment with adding 8 oz/100 gallons of water to see if that drops the water pH significantly.  When choosing an acid consider that it is likely to supply additional nutrients (such as nitrogen, sulfur, or phosphorus) which may influence the overall nutrient balance.

Substrate Cation-Exchange-Capacity
The higher the cation-exchange capacity (C.E.C.) of the substrate, the greater its ability to retain cations. Since many of the mineral nutrients are cations, the greater the C.E.C., the greater the substrate’s  nutrient-holding capacity. Mineral nutrients held by the substrate can be exchanged with the substrate solution (the liquid phase) and be available for uptake by the plant.

In greenhouse situations, when the mineral nutrients are provided as a continuous liquid feed, the C.E.C. is of less importance because mineral nutrients are supplied with each irrigation and do not need to be retained for future uptake by the plant (consider this dynamic as a hydroponic system). However, the C.E.C. can be important in constant liquid feed where microelements are only supplied at the being of the crop production or when plants are only fertilized periodically since the mineral nutrient cations need to be retained in the substrate rather than being leached out.

Cation-exchange capacity can also impact mineral nutrition through its role in substrate pH. Theoretically, the higher the C.E.C., the more resistant the substrate is to changes in pH. This is because the H⁺ in the substrate solution can be retained on the substrate surface in the same way that other cations are retained. Thus, the exchange sites on the substrate serve as a reservoir of H⁺ and remove or release H⁺ depending on H⁺ concentration in the substrate solution. This acts to help maintain a stable H⁺ concentration in the substrate solution and thus a stable pH.  As previously discussed, the pH of the substrate impacts mineral nutrient availability. A substrate with a high C.E.C. will be less subject to pH changes and thus changes in mineral nutrient availability. 

A common recommendation for increasing the C.E.C. of a peat-based substrate has been to add field soil (i.e. a loam) to the substrate. However, on a weight basis (per gram), peat has a higher C.E.C. than field soil. Therefore, it may not seem logical that adding field soil could increase the C.E.C. of a peat-based substrate. However, the reason is that on a weight basis, peat has a higher C.E.C. but on a volume basis field soil has a much higher C.E.C. This is because of the differences in bulk density between a field soil and peat. Field soil is much heavier. Therefore, a small volume of field soil will weigh much more than an equivalent volume of peat and the addition of a small volume of field soil (which relatively speaking weighs a lot) to a peat-based substrate will add a significant amount of C.E.C. to the substrate. Other components that may be added to a peat-based substrate to increase C.E.C. are vermiculite and calcined clay.

Finally, the more decomposed an organic material, generally the higher the C.E.C. This is one of the advantages of composting bark before using it in a substrate. In addition to removing the potential for nitrogen tie up, the partial decomposition that occurs during composting or aging increases the C.E.C. of the bark (see "Substrates" learning unit).

Electrical Conductivity
Electrical conductivity (E.C.) provides an overall estimate of the ions (i.e. Ca⁺⁺, K⁺, NO₃^-, NH₄⁺, etc.) in the substrate solution. However, it does not provide a measure of the concentration of any specific ion. If a complete liquid fertilization program is being conducted in which all macroelements and microelements are being provided in the appropriate proportions, E.C. can provide a good indication as to whether the overall fertility program is within an acceptable range. The acceptable range for E.C. varies with crop and crop stage (just as does fertility level). However, typically an E.C. of 2.0 - 3.0 mmho/cm is desirable for most actively growing crops based upon the SME (saturated media extract). Lower E.C. levels are desirable for seedlings and plugs and higher E.C. levels may be desired for crops requiring high fertility levels.

Electrical conductivities below desired levels is an indicator that the fertility level is below optimal and the fertilizer concentration being applied needs to be increased (or the frequency of fertilization increased). Electrical conductivities significantly above the desired level may be an indicator that fertility levels are too high in general or that one or more mineral nutrients are too high. If the E.C. is too high, a reduction in fertilization might be required, or if the E.C. is very high, clear-water leaching may be required. Keep in mind that the high E.C. values could be a result of a specific ion coming from the irrigation water such as sodium and may not be indicative of the “fertilizer” content of the substrate.

Electrical conductivity can also be used to monitor the fertilizer solution being applied to a crop. A given concentration of a fertilizer (or fertilizer salt) will have a specific E.C. This information is printed on most fertilizer labels. A sample of the fertilizer solution can be collected as it comes out of the hose, drip tube or emitter. The E.C. of the fertilizer-containing sample is determined. An E.C. of the water source is determined (without fertilizer). The E.C. of the water without fertilizer is subtracted from the E.C. of the fertilizer-containing solution. The resulting E.C. should be similar to the expected E.C. for the concentration of the fertilizer being used (see "Reading and Understanding Fertilizer Labels"). If the E.C. is not correct, either the amount of fertilizer used was incorrect or the injector was not working correctly.

Water Quality
Water quality impacts the nutritional program in two primary ways. First, the water may contain minerals (i.e. calcium, carbonates, borates, iron, sulfur, etc.) that may provide a portion of the mineral nutrient requirement of the crop. Water quality can also impact the pH of the substrate (see water quality under "Irrigation" learning unit). Through this mechanism, the water can affect the availability of mineral nutrients for uptake by the plant. For example, if a water with high alkalinity (high carbonate and bicarbonate content) is used (and no corrective measures such as acid injection are taken), the pH of the substrate will increase over time. As the pH of the substrate increases, many of the microelements (i.e. Fe, Cu, Zn) become less soluble and unavailable. As the pH continues to increase over time, microelements may become increasingly unavailable, and pH-induced microelement deficiencies may occur.

In turn a “pure” or water source with little to no alkalinity can also have an the adverse effect on the nutrition of a crop.  When a water source has little or no alkalinity, the substrate pH tends to drop, especially if acidic fertilizers are being used.  In this situation, the substrate components, limestone and fertilizer choice become more prominent in their effects on the substrate pH. As previously mentioned, where a very low alkalinity water source is being used and the pH tends to decrease over time, basic fertilizers may be used to manage pH.

Irrigation Method
Irrigation method affects fertility primarily by affecting the degree of leaching of mineral elements from the substrate. When overhead irrigating, the water flows through the substrate and drains from the container. As the water drains from the container, nutrients are leached from the substrate. If overhead irrigation is used and conducted in such a way as to minimize leaching, less loss of mineral nutrients occurs.

When trickle irrigation or drip tubes are used similar effects can be observed.  But, since the water is delivered in a specific location channeling is often observed and the uniformity of the leachate may be less than that of traditional overhead water where the entire top of the substrate is irrigated.

If subirrigation is used, leaching, and thus loss of mineral elements does not occur to a significant extent. Additionally, salts tend to accumulate more readily in the top of the substrate due to evaporation. Most fertility recommendations have been developed assuming overhead irrigation with leaching. If subirrigation without leaching is conducted, recommended fertilization levels are often reduced by 30% to 50%.

When collecting substrate samples the irrigation method should be considered.  As discussed, leaching and salt accumulation will vary based upon irrigation method.  Therefore, where and how the substrate sample is collected will affect the testing results.  For example, if a sample is collected by scraping the side of the entire substrate profile then a subirrigated plant will likely have a relative higher E.C. since salts naturally accumulate in high concentrations with this method.  However, if the sample is collected by just selecting substrate from the middle or lower 2/3^rd of the substrate profile then those high E.C. values might not be realized.

If collecting leachate samples by PourThru, the same considerations should be made.  A key element of PourThru is that this method is simply displacing the substrate solution and that substrate solution primarily tends to reside in the lower 1/3^rd of the container.

Section 9: Substrate and Tissue Tests for Managing Mineral Nutrition

Substrate testing and tissue testing offer greenhouse managers a way to monitor the mineral nutrient status of the substrate and the crop. Many greenhouse managers maintain pH and E.C. meters so that they can frequently monitor these two variables on-site. Often, especially for longer-term crops or to diagnose a problem, a complete (i.e. pH, E.C. and all mineral nutrients) substrate test needs to be conducted. For a complete test, greenhouse managers typically send substrate samples to a commercial laboratory although sometimes the greenhouse manager may extract liquid solution from the substrate and send the solution to a lab for testing.

When conducting an on-site pH and E.C. test, the greenhouse manager must first extract the liquid phase of the substrate. It is actually the liquid in the substrate that is generally tested (not the solid particles). There are several ways to extract and collect the liquid for testing. These methods include the saturated media extract (SME), the 1:2, the PourThru, and the Press Extraction Method.

Saturated Medium Extract (SME)
The Saturated Medium Extract (SME) technique is based on the procedure typically used to analyze field soils. The SME uses ½ to one cup of substrate that is saturated with distilled water to the point that the surface glistens (making a paste out of the substrate). Using distilled or deionized water is important in avoiding incidental contamination with minerals in the water.  This results in a solution that is about 1 part substrate to 1 part water.  After equilibrating (allowing sample to sit and incubate at room temperature) for 30-60 minutes, the SME can be measured for pH and EC.  Commercial laboratories generally filter the slurry, or extract the liquid from the substrate using a vacuum, in some way prior to testing to protect the equipment from debris in the slurry. One of the negative aspects of using the SME method is that the term “glistening” is subjective and therefore the amount of water added may vary among people conducting the test as well as for a given individual across times.

PourThru
The PourThru method (also known as the Virginia Tech Extraction Method) is a non-destructive method that uses distilled water poured over the surface of the substrate to push (displace) the substrate solution out of the production container.  It is often collected in trays placed under an individual container.  The extract is then immediately ready to be analyzed for pH and EC or to be sent to a commercial laboratory for a complete analysis.

Considerations for the PourThru method include ensuring that there is sufficient substrate solution to be extracted.  To do so, the PourThru is conducted not more than 1 hour after irrigation. Pouring excessive distilled or deionized water over the potting mix will blend with and dilute the potting substrate solution While the volume needed varies depending on substrate components and several other factors, the table  below provides some guidelines for appropriate water volumes for a given container size.

Because the PourThru extract is undiluted, E.C. and specific nutrient values are typically about 1.5 times higher than values obtained with the SME. The table below provides conversion factors for E.C. and values from three extraction methods. 

1:2 Dilutions
The 1:2 dilution is similar to the SME with the exception that rather than adding distilled water to the point of glistening, 2 parts of distilled or deionized water is added to 1 part substrate. This results in a more diluted solution versus the SME and PourThru. The highly diluted 1:2 sample can make micronutrient analysis difficult for commercial labs (micronutrients are often below detection levels). However  the 1:2 method  is a good tool for greenhouse managers to us in-house to monitor pH and E.C.

After the solution is extracted, it may be tested on site for pH and E.C. or sent to a commercial lab. However, usually if a complete test is being conducted, the greenhouse manager sends the actual substrate sample to the lab and the lab technicians use one of the above methods (SME is most common) to collect an extract.

It is important to understand how to use information from substrate tests to adjust the fertilization program. Acceptable ranges for substrate tests using a saturated media extract (SME) are listed in the table below. Properly monitoring the fertility status of a crop can allow for corrections of potential problems before there is significant damage or crop loss and when corrections are more easily made.

Press Extraction Method
The Press Extraction (PE) method is used for plugs or other small substrate volume containers.  Other substrate extraction methods are not well suited for plug and seedling trays because it is hard to collect sufficient leachate or a lot of samples must be taken. Similar to PourThru recommendations, about one hour after a plug tray is fertigated, while the substrate is fairly wet, “press” several individual cells for a single sample.  Collect the pressed solution for measurement.  Results correlate well to SME standards, but pH readings from PE are typically 0.1 to 0.4 units lower and E.C. readings from PE are 0.1 to 0.6 mS/cm higher than for an SME test.

Sometimes a plant tissue test needs to be conducted. Tissues tests are most commonly conducted to diagnose problems that do not readily appear in the substrate test or to confirm results from substrate tests. This type of test is conducted on dried and ashed plant samples (typically leaves) and the actual mineral nutrient levels in the plant tissue are determined. Recommended tissue levels of mineral nutrients for some common greenhouse crops are listed below. These are levels for dried and ashed recently expanded leaves.

© M.R. Evans and Todd Cavins, 2009, 2011, 2014