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 method, 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) 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 ammonium(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 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, 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(NO22-) + 2(H2O) + 4(H+)
(NO22-) + (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 they 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 seasons. 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 (not salts) 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 (stretching). 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.
