Plants are approximately 90% water. The remaining dry matter is composed of the elements listed below. The elements C, H and 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 elements constitute only about 1% of the total plant weight and about 11% of the dry matter. However, these are the elements that need to supplied in the fertilization program.

These elements can be broken into 3 major categories. The first are referred to as primary macroelements because the plant needs relatively large amounts of these elements. The primary macroelements include nitrogen, phosphorus, and potassium. The second category is the secondary macroelements and includes calcium, magnesium and sulfur. The third category is the microelements. These are required in smaller amounts but are no less important than the primary and secondary macroelements. The microelements include iron, manganese, zinc, copper, boron, molybdenum, chloride, and nickel. 

Primary Macroelements

The primary macroelements nitrogen (N), phosphorus (P) and potassium (K) are required in the highest amounts; These are the three elements commonly listed on a fertilizer label (see Reading and Understand Fertilizer Labels below). Individual fertilizer salts (i.e. ammonium nitrate, calcium nitrate, potassium nitrate, ammonium phosphate, potassium sulfate, etc.) may be used to provide these mineral elements to a crop. These three elements may also be provided using a commercially prepared complete fertilizer (i.e. 20-10-20) in a liquid or slow-release form.

Nitrogen is required in the highest amount of all of the mineral elements. 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). The P level should be approximately 15% - 30% of that of the N. Where a constant liquid fertilization program is being used, a N concentration of 150 - 300 ppm is common for most greenhouse crops (except with no-leach systems where lower concentrations are used). Some common sources of N include ammonium nitrate, urea, ammonium phosphate, calcium nitrate , and potassium nitrate .

Plants may take up N either as nitrate (NO₃^-) or ammonium (NH₄⁺). However, the ratio of these two forms is important. Not only must the amount of N being supplied be considered, but also the form in which it is supplied. For most greenhouse crops, never more than 50% (and usually not more than about 25%) of the N should be supplied as NH₄⁺. However, the optimal NH₄⁺:NO₃^- ratio depends upon plant species and time of year.

Both NO₃^- and NH₄⁺ are commonly used as N sources for greenhouse crops. Although NH₄⁺ may be taken up directly by the plant, it is also converted to NO₃^- in the soil by microorganisms.

2(NH₄⁺) + 3(O₂) => 2(NO₂⁼) + 2(H₂O) + 4(H⁺)

(NO₂⁼) + (O₂) => 2(NO₃^-)

The production of protons in the first step of this conversion is one reason why NH₄⁺-based fertilizers are acidic and tend to cause the pH of the substrate to go down over time. Ammonium nitrogen is preferred where it can be used because it is relatively inexpensive. However, high levels of ammonium can be toxic to plants. Levels can build up over time due to over application of ammonium or due to low substrate temperatures in winter months since conversion of ammonium to nitrate is slowed under low temperatures. This is why many growers (particularly in northern climates) use more ammonium-based nitrogen during warm months but cut back and shift to nitrate-based nitrogen during winter months. Urea must first be converted to NH₄⁺ before the plant can take it up. Under low temperatures conversion of urea can also be slowed and toxic levels may build up.

Calcium [Ca(NO₃)₂] and potassium nitrate (KNO₃) are common nitrate-nitrogen sources. They are more expensive than urea and ammonium nitrate. These fertilizers are often referred to as basic or alkaline fertilizers since their use tends to cause the substrate pH to increase over time. This occurs because the root excretes an OH^- for each NO₃^- that it absorbs. The release of OH^- (hydroxyl ions) into the the substrate solution causes the pH to increase.

Phosphorus is sometimes supplied by incorporating superphosphate(0-20-0) or triple superphosphate (0-45-0) into the substrate. Depending upon the crop and the rate of incorporation, this may supply the entire phosphorus need of the crop. Phosphorus may also be supplied through the fertilization program. Most commercial fertilizers, whether slow-release or liquid, supply phosphorus using ammonium phosphate or diammonium phosphate. Although the recommended range for substrate P levels is very wide, high P levels can promote elongation in seedings. Since excessive elongation is undesirable, excessive P fertilization should be avoided.

Potassium is supplied through the fertilization program. Most commercial fertilizers (water soluble and slow release) supply potassium from potassium nitrate. Potassium nitrate may also be used as a potassium source when preparing a custom-designed fertilizer solution.

Secondary Macroelements

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

Calcium is at least partly supplied by using some type of limestone in the substrate (such as is added to adjust pH). This can often supply the entire Ca requirement if the crop time is not too long. However, long-term crops or crops that require higher levels of Ca (i.e. poinsettia) may need additional applications during the production cycle. Calcium may be applied during production by including Ca(NO₃)₂ in the fertilizer solution or by using a commercial fertilizer that contains Ca (i.e. Excel 15-5-15 Ca-Mg). Calcium may also be applied as a foliar spray using calcium nitrate, calcium chloride or a commercially available chelated calcium such as T.H.I.S. or dynocal.

Calcium Sulfate (gypsum) may be added to the substrate in lieu of limestone when a source of calcium is desired but not a significant pH increase.

If dolomitic limestone is used to adjust the substrate pH, all or part of the Mg requirement of the plant may be met. However, especially for long-term crops or crops that require high levels of Mg, additional Mg may be needed during production. Although many commercial fertilizers now include Mg, magnesium sulfate (Epsom salts) is commonly used to supplement the Mg supplied to the plant. Magnesium sulfate is usually applied at a rate of 8-16 oz per 100 gallons of water (226 - 452 grams / 379 liters). At this rate, magnesium sulfate is applied approximately every 6 - 8 weeks depending upon the crop and growing conditions. Some producers apply low levels of Mg continuously in the fertilization program.
 
Sulfur may be supplied in several ways. Mineral soils contain S, and if a significant amount of mineral soil is included in the substrate, the crop's S requirement may be met. Elemental sulfur, aluminum sulfate and iron sulfate serve as sources of S. Elemental sulfur reacts and releases S slowly 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 large enough concentrations. Many commercial fertilizers also contain S, since sulfate salts (i.e. iron sulfate, zinc sulfate) are often (but not always) used to supply microelements.

Microelements

The microelements iron (Fe), manganese (Mn), zinc (Zn), copper (Cu), boron (B), molybdenum (Mo), chloride (Cl^-) and nickel (Ni) are required by plants in only very small amounts. However, they are just as important as the macroelements. Microelements serve as enzyme cofactors or as part of photoreceptors.

When field soil is used in the substrate, some or the entire microelement requirement may be met by the elements provided by the mineral constituents of the field soil. However, many producers still supplement the substrate with additional microelements to insure an adequate supply. In substrates without soil, nearly all of the microelements must be supplied through the fertilization program.

There are several methods used to provide microelements. A microelement fertilizer package (i.e. Promax, Esmigran, Micromax or other fritted trace element packages) may be incorporated into the substrate, or plants may be "watered in" with a water-soluble microelement (i.e. S.T.E.M.) solution soon after planting. Micronutrients may be included in the liquid fertilization program or may be included in the slow-release fertilizer used. The required amount may be applied as a single application or as several applications during the crop cycle. Caution must be taken not to apply an excess of micronutrients since toxicities can occur particularly under low pH (i.e Fe-Mn toxicity in marigold).

There are different types of complete microelement packages available and they are designed for different purposes. Some are water-soluble (i.e. S.T.E.M.) and are designed to be applied as a liquid drench or to be included in the liquid fertilization program. The microelements in these fertilizers are typically supplied as sulfur salts (i.e. iron sulfate, manganese sulfate, zinc sulfate, etc.). Others are designed to be incorporated into the substrate. These may be granular salts or oxide forms (i.e. zinc oxide, manganese oxide, etc.) of the minerals. Microelements may also be incorporated into glass or clay particles (fritted). This allows for easier mixing and results in a slower release of the microelements into the substrate solution.

Some crops require additional levels of certain micronutrients beyond what is typically supplied by complete microelement 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 Sequestrene 330 or iron sulfate. Molybdenum may be supplied using ammonium molybdate or sodium molybdate. Boron may be supplied through borax, ammonium borate or sodium borate.

Slow-Release and Water-Soluble Fertilizers

Mineral elements may be supplied through a liquid fertilization program or a slow-release fertilizer. Liquid fertilization is most common with greenhouse crops.

Slow-release fertilizers are marketed under several brand names (i.e. Osmocote, Nutricote, Sierra) and are available in numerous formulations. Slow-release fertilizers are produced by encapsulating water-soluble fertilizer elements in a polymer that allows water to enter, and the elements to slowly diffuse out into the surrounding substrate solution. The polymer may be altered to allow a slower or more rapid release of the fertilizer elements. Therefore, slow-release fertilizers will also indicate the release period (i.e. 60-day, 100-day etc.) in addition to the guaranteed analysis (i.e. 20-10-20) and the recommended application rate for various crops and container sizes.

Slow-release fertilizers may be top-dressed onto the surface of the substrate or incorporated into the substrate. When using subirrigation systems, the slow-release fertilizer must be incorporated into the substrate.

In some cases, a grower might incorporate a slow-release fertilizer into the substrate at half the recommended rate and supply the remaining fertility requirement through a liquid fertilization program. This provides increased flexibility since once a slow-release fertilizer is incorporated into a substrate it can't be removed and excess nutrients can't be leached out.

Water-soluble fertilizers are designed to be injected into irrigation lines. Water-soluble fertilizers are marketed under different brand names and are available in numerous formulations (i.e. 20-10-20, 15-5-15, etc.). Many commercially prepared water-soluble fertilizers may contain a dye that allows visual verification that fertilizer is being injected into the fertilization line. Some greenhouse operations may prepare their own water-soluble fertilizers using individual mineral salts [i.e Ca(NO₃)₂, KNO₃]. Liquid fertilization may be conducted as once per week applications or as frequently as with each irrigation. However, rates must be adjusted accordingly.

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 N, 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 contain a great deal of potentially important information. 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.

Fertilizer Calculations

Many commercial fertilizers will provide application rates on the label (especially slow-release fertilizers). 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. Dry 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 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. However, the material is only 20% N.

Therefore, 100 mg / 0.20 = 500 mg

500 mg 20-20-20 per liter provide 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. 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

Therefore, 100 mg / 0.166 = 602 mg

602 mg 20-20-20 per liter provide 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?

P₂O₅ is 43% P

Therefore, 0.20 x 0.43 = 0.086 or 8.6% P

500 mg x 0.086 = 43 mg

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 101 liters of total solution when put through the injector. Therefore, 500 mg^.L^-1 x 101 L = 50,500 mg (50.5 g) in 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.

40 + [14 + (16)3]2 = 164 28/164 = 0.17 or 17%

39 + [14 + (16)3] = 101 14/101 = 0.14 or 14%

Ca(NO₃)₂ = 17% N

KNO₃ = 14% N

Need 100 mg^.L^-1

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

75 mg / 0.17 = 441 mg Ca(NO₃)₂

25 mg / 0.14 = 179 mg KNO₃

Nutritional Monitoring and Management

Fertility management is one on of the most challenging aspects of greenhouse crop production. Many factors impact the nutritional status of a crop and should be considered as when developing a fertilization program, monitoring the mineral nutrition status of a crop and making adjustments to the fertilization program.

Crop Stage

Newly planted cuttings, young seedlings and plugs require a lower mineral nutrition 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.

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, fertility level may need to be increased to meet the crop's increased demand. Under the cooler temperatures and lower light conditions of winter months, fertilization may need to be reduced.

Ammonium is converted to nitrate in the substrate by microorganisms. When soil temperatures are low, microorganisms are less active and the conversion of ammonium to nitrate is slower. Under these conditions, ammonium can build up to toxic levels unless the amount of ammonium supplied to the crop is reduced. Therefore, growers often change the form of nitrogen supplied to the crop during late fall, winter and early spring. During these times, growers may shift from using nitrogen sources with high levels of ammonium to those with little or no ammonium to prevent the build up of ammonium in the substrate.

Amount of Element

Mineral nutrients may be present at concentrations that are deficient (below the level the plant could utilize), concentrations that are in excess (above the level the plant could utilize), concentrations that are phytotoxic (concentrations at which some type of physiological damage is done to the plant) or at concentrations that interfere with the uptake of other mineral elements (high NH₄⁺ 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.

When using a granular fertilizer material such a superphosphate to supply phosphorus or dolomitic limestone to provide calcium and magnesium, the fertilizer material slowly breaks down or dissociates in the substrate and releases the desired mineral element 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 element into the optimal range. The level of the desired mineral element 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, potassium nitrate, magnesium sulfate) result in a more rapid increase in the level of the mineral elements they supply, but the level may also drop more quickly unless additional sources are provided.

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

When using liquid fertilization, periodic applications of fertilizer (typically 300 - 500 ppm N) are made through the irrigation system. After fertilization, the level of the applied mineral nutrients increase rapidly and may even reach levels that are above optimal depending upon fertilizer concentration. In between fertilization cycles, the level of mineral nutrients decreases as the plants take them up. 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 between fertilizations and the rate of crop growth.

Constant liquid fertilization is one in which the concentration of the fertilizer solution is reduced (i.e. 150 - 250 ppm N) but is applied with each irrigation. Constant liquid fertilization allows for a more constant fertility level to be maintained.

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. As a general rule, N and K should be provided in near equivalent amounts and P should be provided at 15% - 30% the rate of N. There are exceptions to this rule since some crops require higher or lower N:K ratios. For example carnation grows and 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. Typically Mg is provided at 50% of the rate as Ca. Microelements are provided at much lower concentrations than the primary and secondary macroelements.

Problems can occur if mineral nutrients are out of balance. Excess P (in relation to N and K) can cause elongation of annual bedding plant species. Excess levels of certain elements can cause deficiencies of other mineral elements because of competitive uptake. For example, excess K can cause N deficiency and high levels of NH₄⁺ can cause Ca deficiency.

Form of Element

The form in which an element is provided can have a significant impact on plant growth. Plants may take up nitrogen as either ammonium or nitrate. Plants however respond differently to the two forms of nitrogen. While some plants thrive when provided nitrogen from sources high in ammonium (i.e. azalea). Other species perform better when most nitrogen is provided as nitrate with only a small proportion of the total nitrogen coming from ammonium.

Ammonium can either be toxic at certain levels or may inhibit the uptake of calcium and thus induce calcium deficiency symptoms.

Substrate Components

The substrate components may contain mineral elements and at least partially provide for the crops requirements. Vermiculite contains significant amounts of potassium and although it cannot meet the entire crop need, it can contribute towards the crop's potassium requirement. Composted manures contain significant levels of micronutrients and if composted manures are added to the substrate they may provide enough micronutrients to meet the crops 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 tire contain toxic levels of zinc).

Substrate pH

The pH of the substrate affects the availability of mineral elements. Under low pH, microelements such as Fe, Cu, and Zn become more available. Under high pH, even if these elements are present in the substrate, they may be unavailable for uptake by the plant and therefore the plant may suffer from pH-induced deficiency. Under high pH, elements such as Ca, Mg and Mo become more available. Under low pH, even if these elements are present in the substrate, they may be unavailable for uptake by the plant.

The fertilization program may in turn be designed to manipulate substrate pH. Different fertilizers can have different effects on substrate pH. Ammonium nitrogen causes a reduction in the substrate pH and nitrate nitrogen causes an increase in substrate pH. Fertilizer salts such as NH₄NO₃ will tend to cause the pH to decrease. Calcium nitrate [Ca(NO₃)₂] and KNO₃ will tend to cause the pH to increase. Premixed fertilizers are composed of various fertilizer salts and may have different effects on substrate pH depending upon the fertilizer salts used to formulate the fertilizer. Therefore, by selecting fertilizers based upon their effect on substrate pH, the pH of the substrate may be at least partially manipulated.

Substrate Cation-Exchange-Capacity

The higher the cation-exchange capacity (C.E.C.) of the substrate, the greater the substrate's ability to retain cations. Since many of the mineral elements are cations, the greater the C.E.C., the greater the substrate's nutrient holding capacity. Mineral elements held by the substrate can be exchanged with substrate solution and be available for uptake by the plant. In most greenhouse situations, all of the macroelements and microelements are provided as a continuous liquid feed so the C.E.C. is of less importance because mineral elements are supplied with each irrigation and don't need to be retained for future uptake by the plant. The C.E.C. can be important in constant liquid feed where microelements are only supplied at the being of the crop production or only periodically since the mineral element cations need to be retained in the substrate rather than being leached out.

Cation-exchange capacity also impacts mineral nutrition through its role in substrate pH. The higher the C.E.C., the more resistant the substrate is to changes in pH. As previously discussed, pH impacts nutrient availability. A substrate with a high C.E.C. will be less subject to pH changes and thus changes in mineral element availability.

Substrate Electrical Conductivity

Electrical conductivity (E.C.) provides an overall measure of the mineral elements in the substrate solution. However, it does not provide a measure of the concentration of any specific mineral element. If a complete liquid fertilization program is being conducting 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. 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 significantly below desired levels might be 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 elements 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.

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 always 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. 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 nutritional 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 elements for uptake by the plant. For example, if a very alkaline water source is used (and no corrective measures are taken), the pH of the substrate will increase over time. As the pH of the substrate increases and becomes alkaline, many of the microelements (i.e. Fe, Cu, Zn, etc.) become less available. As the pH continues to increase over time, microelements may become increasingly unavailable, and a pH-induced microelement deficiency may occur.

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, mineral elements are leached from the substrate. If overhead irrigation is used and conducted in such a way as to minimize leaching, less loss of mineral elements from leaching occurs. If subirrigation is used, leaching, and thus loss of mineral elements does not occur. Most fertility recommendations have been developed assuming overhead irrigation with leaching is to be used. If subirrigation without leaching is conducted, fertilization levels are often reduced by 30% to 50%.

Interpreting Substrate and Tissue Tests

It is important to understand how to use information from substrate and tissue tests to adjust the fertilization program. Acceptable ranges for substrate tests using a saturated media extract (SME) are listed in the table below. On-site pH and electrical conductivity measurements should be conducted regularly to monitor changes in pH and E.C. over time. 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.

Recommended tissue levels of mineral nutrients for some common greenhouse crops are listed below. These are levels for dried and ashed fully expanded leaves. Tissue tests are sometimes required to specifically diagnose a mineral element deficiency.

Read the article on Mineral Nutrition of Carnivorous Plants

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© 2005, M.R. Evans