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 and slowly available) 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 that contains Mg. Most of these commercial premixed fertilizers use magnesium sulfate as the Mg source and the amount of Mg in the fertilizer is designed 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 (SO42-) 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.
