THE ESSENTIAL MICRONUTRIENTS REQUIRED BY PLANTS - PART 2

IN THE LAST ISSUE OF MAXIMUM YIELD, the micronutrients boron, chlorine, copper, and molybdenum were discussed. The remaining, iron, manganese, and zinc, are addressed here.

Iron (Fe)

CONTENT IN PLANTS. The sufficiency range for Fe in tomato leaf tissue is between 50 and 100 ppm of the dry matter, the so-called critical concentration being 50 ppm. More than 75 per cent of the Fe in plants is located in the chloroplasts. Iron is stored in plants as a ferric phosphoprotein called phytoferritin. Iron accumulates in plants without any apparent deleterious effect; therefore, it is not unusual to find Fe concentrations in excess of many hundreds of ppm. Total Fe in the plant may be of little importance, somewhat similar to Ca in that the “soluble” or “labile” concentration determines sufficiency. Special tests have been developed to measure this form of Fe in plant tissue. Iron plant chemistry is complex and the relationship between tissue content and function is not clearly understood.

FUNCTION. Iron plays a significant role in various energy transfer functions in the plant due to ease of valence change (Fe2+ = Fe3+ - e-). Iron also has an important role in the photosynthesis process and in the formation of the chlorophyll molecule, the reason Fe-deficient plants look chlorotic. The Fe cations (Fe2+ and Fe3+) have the tendency to form chelate complexes. Other exact roles for Fe are not clearly known.

DEFICIENCY SYMPTOMS. One of the symptoms of Fe deficiency is a loss of the plant’s green colour due to loss of chlorophyll, a green pigment compound. Although the appearance of Fe deficiency is not too dissimilar to that of Mg, an Fe deficiency symptom first appears in the younger plant tissue, whereas Mg deficiency symptoms first appear in the older tissue. Iron deficiency symptoms are not always clearly distinct and can be easily confused with other elemental deficiencies, as deficiencies of S, Mn, and Zn frequently produce leaf and plant symptoms that are not easily differentiated visually from those of Fe. It is; therefore, important to confirm an Fe deficiency by means of a plant analysis or tissue test.

Iron deficiency, once developed in the plant, is very difficult to correct. There is evidence that in some instances Fe deficiency may be genetically controlled, with specific individual plants incapable of normal Fe metabolism and, therefore, unresponsive to correction by foliar Fe application. Some plant species, as well as individual plants within a species, can respond to Fe-deficient conditions as their roots release H+ ions to acidify the area immediately surrounding the root and/or release Fe-complexing substances (i.e., siderophores). Plants that are able to modify their immediate root environment have been designated “Fe efficient,” while those that cannot as “Fe-inefficient.”

Although the use of Fe chelates has markedly improved the control of Fe deficiency in soil and organic soilless mixes, deficiency correction can be a major problem for Fe-sensitive cultivars/varieties and growing situations. Iron deficiency may be easier to control hydroponically than in other systems of growing. Soilless culture systems that employ an organic rooting medium are particularly susceptible to Fe deficiency.

In rapidly growing tomato plants under high light conditions, the author has observed Fe deficiency symptoms on new growth, with the symptoms slowly disappearing with maturity. Evidently, the movement of Fe into newly emerging leaf tissue is insufficient to prevent the visual symptom from appearing, but eventually Fe sufficiency is reached in the developing tissue, and the deficiency symptom disappears.

CONCENTRATION IN A NUTRIENT SOLUTION. Normally, the Fe concentration must be maintained at about 2 to 3 ppm in the nutrient solution to prevent deficiency. Iron in solution exists as either the ferric (Fe3+) or ferrous (Fe2+) cation, depending on the characteristics of the nutrient solution. There may be sufficient Fe in a nutrient solution, depending on the water source and contact with Fe-containing piping and other similar materials. Because Fe is a common contaminant found nearly everywhere, it may be naturally present in sufficient concentration to satisfy the plant requirement and prevent a deficiency from occurring.

Most hydroponic formulas call for the use of a chelated form of Fe to ensure that its presence in the nutrient solution is maintained in an available form. Although FeEDTA has been widely used as an Fe source, EDTA is toxic to plants and, therefore, the DTPA chelate form is that currently being recommended, although its possible toxicity to plants has yet to be determined. Other forms of chelated Fe are FeHEDTA and FeEDDHA, as well as the double salt forms NaFeHFDTA, NaFeEDDHA, and NaFeDTPA. However, these forms of Fe chelates are seldom used in nutrient solution formulations. Iron easily complexes with many substances, which makes Fe concentration difficult to maintain in a nutrient solution or soilless medium. In addition, if the pH of the rooting medium is greater than 6.5, Fe availability decreases sharply. It should be remembered that Fe chelates were developed for use in soils; therefore, their use in a hydroponic solution is questionable. When a chelate is added to a nutrient solution, some of the other cations — mainly Cu, Mn, and Zn — will be chelated, the extent of chelation depending on pH and the concentration of elements in solution.

FORMS OF UTILIZATION. Plants can use either ionic form, though that taken in as ferric Fe (Fe3+) must be reduced to the ferrous (Fe2+) form. Ferric Fe can form complexes and precipitates quite easily in the nutrient solution, thereby reducing its concentration and, therefore, availability to plants. The chemistry of Fe in the nutrient solution and its uptake by plants are complex. In addition, utilization of Fe varies among cultivars and plant species. Some species have the ability to alter the character of the nutrient solution in the immediate vicinity of their roots, thereby influencing Fe availability.

NUTRIENT SOLUTION REAGENTS. Although FeEDTA is still listed in nutrient solution formulations, FeDTPA is the recommended chelated form because EDTA can be toxic to plants. Other Fe sources are in organic compounds that are also suitable for use in nutrient solution formulations and were in common use before the chelated Fe forms became available. These Fe-containing compounds are iron (ferrous) sulfate (FeSO4.7H2O), iron (ferric) sulfate [Fe2(SO4)3], iron (ferric) chloride (FeCl3.6H2O) and iron ammonium sulfate [FeSO4(NH4)2SO4.6H2O]. Two organic Fe compounds suitable for use in nutrient solution formulations are iron citrate and iron tartrate, reagents that are unfortunately not readily available. As a general rule, it takes more Fe when in one of these inorganic/organic forms to provide the same level of “available Fe” as compared to the chelated forms. The author has observed excellent results from just adding Fe filings to the rooting medium when it is either sand, fine gravel, or perlite.

Manganese (Mn)

CONTENT IN PLANTS. The sufficiency range of Mn in tomato plant leaf tissue is between 40 to 200 ppm of the dry matter. Manganese accumulates in the leaf margins at a concentration two to five times that found in the leaf blade. Therefore, the ratio of leaf margin to leaf blade can influence an assay result.

FUNCTION. The functions of Mn in the plant are not too different from that of Fe. Manganese is associated with the oxidation-reduction processes in the photosynthetic electron transport system and is a cofactor in the IAA-oxidase enzyme system. Manganese can substitute for Mg in enzymatic reactions and, therefore, its deficiency (affects chloroplast activity) gives rise to similar visual symptoms as Mg deficiency.

DEFICIENCY SYMPTOMS. Manganese deficiency symptoms first appear on the younger leaves as an interveinal chlorosis, not too dissimilar to symptoms of Mg deficiency, which first appears on the older leaves. In some instances, plants may be Mn-deficient (moderate visual symptoms present) and yet plant growth will be little affected. However, when the deficiency is severe, significant reduction in plant growth occurs. Manganese deficiency can be easily corrected with a foliar application of Mn or by additions of a soluble form of Mn to the rooting media.

EXCESS SYMPTOMS. Initial Mn excess may produce toxicity symptoms not too dissimilar to deficiency symptoms. With time, toxicity symptoms are characterized by brown spots on the older leaves, sometimes seen as black specks on the stems or fruit, a symptom known as “measles.” It is not unusual for typical Fe deficiency symptoms to appear when Mn is in excess. This similarity can result in improper diagnosis that can only be resolved by means of a plant analysis. Phosphorus enhances the uptake of Mn and when high in the rooting medium can contribute to either correcting an insufficiency or creating a possible excess, which could lead to toxicity. Composted milled pine bark is high in Mn that can be sufficient to supply the entire plant requirement.

CONCENTRATION IN A NUTRIENT SOLUTION. Hydroponic formulas call for a Mn concentration of 0.5 ppm in the nutrient solution. Because Mn can be easily taken up by plants, care should be exercised to prevent the addition of excessive quantities of Mn in the nutrient solution. Manganese exists in the nutrient solution as the manganous (Mn2+) cation, though other oxidation states can be present under varying conditions of oxygen supply. Composted pine bark, if used as a rooting medium, contains sufficient available Mn to satisfy the Mn requirement for tomato; therefore, inclusion of Mn in the nutrient solution formulation may not be necessary.

NUTRIENT SOLUTION REAGENTS. The primary reagent source is manganese sulfate (MnSO4.4H2O), though manganese chloride (MnCl2.4H2O) can also be used as a suitable reagent.

Zinc (Zn)

CONTENT IN PLANTS. The sufficiency range of Zn in tomato plant leaf tissue ranges between 20 and 50 ppm of the dry matter. Zinc is unique in that the critical level in many plants, including tomato, is 15 ppm. At around 15 ppm, a difference of 1 to 2 ppm can mean the difference between normal and abnormal growth. Precise measurement of the Zn concentration in the plant when doing a plant analysis determination is, therefore, critical.

FUNCTION. Zinc is an enzyme activator, involved in the same enzymatic functions as Mn and Mg. Only carbonic anhydrase has been found to be specifically activated by Zn. While Zn probably has additional roles, these other roles are not well understood. Considerable research has been done on the relationships between Zn and P and between Zn and Fe. The results suggest that excessive P concentrations (>1.00 per cent) in the plant interfere with normal Zn function, whereas high Zn concentrations interfere with Fe usage, and possibly vice versa.

DEFICIENCY SYMPTOMS. Zinc deficiency symptoms appear as chlorosis in the interveinal areas of new leaves, producing a banding appearance on some plant leaves. Plant and leaf growth become stunted, and when the deficiency is severe, leaves die and fall off. The author has observed P-induced Zn deficiency, the symptoms being a dying of the leaf margins on all leaves, whereas K deficiency results in the marginal dying of lower leaves only. Moderate Zn deficiency symptoms may be confused with symptoms caused by deficiencies of Mg, Fe, or Mn. Therefore, a plant analysis is required to determine which element is deficient.

EXCESS SYMPTOMS. Many plant species are tolerant to fairly high (100 ppm) levels of Zn in their tissues without untoward consequences. However, such high levels of Zn may induce Fe deficiency.

CONCENTRATION IN A NUTRIENT SOLUTION. Hydroponic formulas call for a Zn concentration of 0.05 ppm in the nutrient solution. Zinc exists in the nutrient solution as the divalent Zn2+ cation. There is evidence that if a chelated form of Fe is present in the nutrient solution, Zn uptake and translocation in the plant can be impaired. The author has observed increased instances of low Zn contents in tomato plants that may reflect this chelate effect. One way is to increase the Zn concentration in the nutrient solution to 0.10 ppm. The author has observed that if an inorganic source of Fe, not a chelated form, is used in the nutrient solution formulation, there is no apparent significant reduction in Zn content in plant tissue.

NUTRIENT SOLUTION REAGENTS. Zinc sulfate (ZnSO4.7H2O) is the primary reagent source.

Summary

Small in concentration but mighty in performance characterizes the micronutrients. Their deficiency as well as excess can significantly impact plant growth, the quality of the plant itself, and generated fruit. Care needs to be exercised in the use of these seven essential elements, because their concentration in the rooting media and plant itself can significantly impart the growth and life cycle of all plants.