Micronutrients in Plant Nutrition?
Fred R. Davis, Ph.D., Kent, Ohio
Micronutrients are chemical elements needed in small quantities for normal plant and animal growth. They generally include iron, boron, manganese, zinc, molybdenum and copper. Today, many of our fertilizer formulations have micronutrient supplements. These supplements are present as soluble salts, chelated compounds or fritted trace elements. The chelated compounds are generally metal salts of ethylene diamine tetra-acetic acid or some polyamine carboxylic acid. They have the advantage of being effective in both acid and alkaline soils, whereas some simple metal salts will not be effective in alkaline soils. The fritted trace elements are manufactured by combining the trace elements, i.e., zinc, iron, manganese, copper, borax, molybdenum, etc., with raw materials necessary to form glass. The mixture is smelted, producing a homogeneous glass containing the trace elements. The glass is then shattered by rapid cooling in water. The shattered particles are called frit, which is ground into the finished product. These fritted micronutrients provide controlled solubility to reduce leaching loss and minimize toxicity hazards.
Some people are skeptical about micronutrient supplements in plant nutrition, saying "their effect is rather unimportant and not measurable", but much evidence from both laboratory and field testing have shown that micronutrients are beneficial to plant growth and reproduction. 1. My own experience in growing rhododendron has borne out the need for micronutrients to correct chlorotic and chronic conditions that sometimes exist, but let me point out here that micronutrient supplements are not the panacea to all plant nutrition problems.
Leaf analysis is used as a standard method of diagnosing plant nutrient disorders and to make fertilizer recommendations for many horticultural crops. In the area of Landscape Horticulture, leaf analysis is not utilized extensively since research has been limited and has related to only a few species.
2. In Lake County, Ohio, 1,400 different types of ornamentals are commercially available, thus indicating the magnitude of the problem.
The objective of this article is the appraisal of the utility of micronutrients in horticulture, particularly for rhododendron, azaleas and evergreens.
THE METAL ION:
FUNCTIONS & DEFICIENCY SYMPTOMS
Iron serves a number of important functions in the overall metabolism of the plant. Iron is usually taken up in the trivalent state (Fe+3), but is generally accepted in the divalent state (Fe+2) as the metabolically active form of iron in the plant. Its chemical role both in synthesis and degradation of chlorophyll is still uncertain. Several authors feel that iron functions in the synthesis of chloroplastic protein and thus may increase with chlorophyll synthesis.
Iron deficiencies manifest themselves as extensive chlorosis in the foliage. The new foliage is generally most affected, although I have seen both old and new foliage affected to about the same extent on rhododendron. Iron-induced chlorosis will show up in the interveinal structure of the leaf and the surface of the leaf usually shows a grid network of green veins between the chlorotic areas. Several have found that there is some correlation between iron deficiency and chlorophyll content, while on the other hand, other investigators found that chlorotic leaves may contain as much or even more iron than their counterparts. Jacobson has proposed that the lack of iron may inhibit the formation of chlorophyll through inhibition of protein synthesis. Iron deficiencies in many plants can be corrected by the use of chelated iron compounds. These compounds are iron salts of some polyamine carboxylic acid, e.g., ethylene diamine tetra-acetic acid. My own experience with these types of compounds for rhododendron has shown a favorable response to correcting chlorosis and producing increased bud set 6 . It is important to realize that these metal chelates can be toxic to the plant if used indiscriminately. Toxic doses produce withered and curled leaves with browning at the edges. The toxicity is probably related to electrolyte disturbances and enzyme inhibition.
Manganese is essential in the respiration and nitrogen metabolism where it functions as an enzyme activator. Manganese is involved in nitrate reduction where it acts as an activator for the enzymes, nitrate and hydroxylamine reductase. The preference of ammonia over nitrate as a nitrogen source by manganese deficient cells supports the above role of manganese.
Manganese deficiency is characterized by chlorotic and necrotic areas in the interveinal portions of the leaf. This symptom appears on the young leaves of rhododendron rather than on the older leaves. I find the manganese deficiency often difficult to distinguish from the iron deficiency. The manganese deficiency is best corrected by using one of the soluble manganese chelates.
Copper acts as a component of certain enzyme systems such as phenolases and ascorbic acid oxidase. It has been suggested that copper may function in photosynthesis. For example, it was found that the chloroplasts of clover contain most of the copper of the plant 7 . Most plants are very sensitive to the concentration of copper ions. Solutions of copper salts are used as herbicides for some of the lower plants.
Copper deficiency usually results in shriveling or malformation of the leaves along with tip burn. The most easily recognized symptoms of copper deficiency are those in a disease of fruit trees called exanthema. A copper deficiency in almonds may result in roughening of bark, gummosis and shriveling of the kernels. Copper deficiencies are corrected by using soluble copper chelates.
Zinc participates in the biosynthesis of the plant auxin indole-3-acetic acid. This was proved by observing that the content of tryptophan, a precursor of of the auxin, parallels the content of auxin in the plant, both when zinc is deficient and when it is supplied to deficient plants. It has been concluded that zinc reduces the auxin content because of its participation in the synthesis of tryptophan.
Zinc plays a role in protein synthesis as evidenced by the accumulation of soluble nitrogen compounds such as amino acids and amides. Zinc is also involved in the plant metabolism as an activator of several enzymes. Carbonic anhydrase was the first zinc containing enzyme to be discovered. It is involved in the catalytic decomposition of carbonic acid into carbon dioxide and water.
Zinc deficiency is sometimes referred to as "rosette" or "little leaf disease." It is most evident in older leaves as chlorosis, necrosis or mottling of the leaves. The interveinal areas turn pale green to yellow; the leaf margins become irregular. The absence of zinc may also have a retarding effect on growth and development of the flower and fruit. The use of soluble zinc chelates in soil applications or foliar applications will correct the deficiency.
Gauch and Dugger 8 have eluded that boron is involved in carbohydrate transport within the plant. They believe that the borate ion complexes with the sugar molecule and this complex is transported across cell membranes more readily than the sugar. They also observed that symptoms of boron deficiencies are associated with symptoms of sugar deficiencies. The first visible sign of boron deficiency in many plants is the death of the shoot tip. The leaves may have a thick coppery texture with curling; generally flowers do not form and root growth is reduced. In fleshy tissue, there is a disintegration of internal tissues resulting in cork formation. Boron deficiencies may be corrected by using sodium tetraborate or boric acid.
Molybdenum is involved in nitrogen fixation and nitrate assimilation. Some investigators have found that a molybdenum deficiency leads to a decrease in the concentration of ascorbic acid in the plant. 9 . There is some evidence that molybdenum is involved in the phosphorous metabolism of the plant, but the mechanism has not been explained. Molybdenum deficiencies may be corrected by using a soluble molybdenum salt such as sodium molybdate.
EFFECT OF SOIL pH
The availability of plant nutrients is highly related to soil pH. Many charts have been published illustrating this relationship such as those prepared by Lucas and Davis for organic soils. It is important to know the soil pH before the micronutrient supplements are applied.
The activity of iron, manganese and aluminum increases as soil acidity increases. In general, the optimum pH for most ornamental plants is 6.0 - 6.8. Nitrogen sources can indirectly affect micronutrient availability because of changes in soil pH. For example, the acidic properties of ammonium sulfate may correct an iron deficiency.
It has been shown many times that a lack of oxygen can curtail the absorption of water and nutrients. Poor soil aeration usually is caused by excess water. Other factors such as microbial activity, temperature and bulk density affect the diffusion and composition of the soil atmosphere. The soil aeration must be carefully considered if one is growing container plants. The growing medium, container type and design must be considered for each type of plant. Azaleas and rhododendron will sometimes develop iron chlorosis if the oxygen supply is inadequate.
The major trend in micronutrient fertilization is for increased usage. Many fertilizers formulated currently have micronutrient supplements; both the metal chelates and fritted trace elements are being used. Due to advances in instrumentation and analytical methods for determining low levels of micronutrients in plants and soils, we can now expect the type of data that will enable us to narrow the range of micronutrient concentrations required for various plants.
- Micronutrients in Agriculture, 1972. Soil Science Society of America, Inc., Madison, Wisconsin.
- Smith, E. M., 1972. "A Survey of the Foliar Mineral Element Content of Nursery Grown Ornamentals." Ohio Agricultural Research and Development center, Wooster, Ohio.
- Wallace, A., O. R. Lunt, 1960. "Iron Chlorosis in Horticultural Plants." Proc. Amer. Soc. Hort. Sci., 75:819-841.
- Price, C. A., 1968. "Iron Compounds and Plant Nutrition." Am. Rev. Plant Physiology, 18:239248.
Jacobson, L., 1945. Plant Physiology, 20:233.
- Davis, F. R., 1972. "The Role of Transition Metal Chelates in Rhododendron Nutrition." American Rhododendron Society Quarterly Bulletin, 21 (3), pp. 190-192.
- Green, L. F., J. F. McCarthy, C. G. King, 1939. Journal of Biological Chemistry, 128:447.
- Gauch, H. G., 1957. Am. Rev. Plant Physiology, 8:31.
- Davidson, H., 1960. Journal Amer. Soc. Hort. Sci., 76:667-672.
- Lucas, R. E., J. F. Davis, 1961. "Relationship Between pH Values of Organic Soils and Availability of 12 Plant Nutrients." Soil Science, 92:177182.