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).
