The Elements of Complete Plant Nutrition: Essential Micronutrients

The following is a brief summary of the role of essential and beneficial mineral nutrients crucial to plant growth. If any one of the essential elements is eliminated from a plantís nutrition, it will display abnormalities of growth, symptoms of deficiency, and may not reproduce normally.

Essential Micronutrients

Iron is a component of many structural and enzymatic proteins. It is essential for electron transport and chlorophyll biosynthesis. It is therefore required for photosynthesis and respiration. Iron is also essential for lipid metabolism. A well-known symptom of iron deficiency is interveinal chlorosis. High soil pH can cause iron deficiency. Toxic levels of iron are associated with waterlogged soils.

Manganese activates many enzymes, but to date, only two are considered manganese-containing enzymes. One of these enzymes is directly involved with the photosynthetic evolution of oxygen. Manganese is required for respiration and carbohydrate and lipid metabolism. Deficiency symptoms in dicots appear as chlorosis between the veins (interveinal) of young leaves. In grasses, greenish gray spots on the more basal leaves is a sign of manganese deficiency. In neutral or alkaline soils, plants often show deficiency symptoms. In highly acidic soils, manganese may be available in toxic levels.

Zinc is a structural component of many enzymes and also acts as a cofactor in others. Zinc is essential to DNA replication, gene expression, protein synthesis, IAA synthesis, membrane integrity, and carbohydrate metabolism. Deficiency symptoms in dicots include shortened internodes and a reduction in leaf size. Chlorosis often accompanies these symptoms. At low soil pH, zinc may accumulate to toxic levels. Raising the pH is the most effective method of reducing zinc availability in soils.

Copper is an integral component of several enzymes and other critical biological proteins. It is required in photosynthesis, respiration, lignin biosynthesis, and in carbohydrate, nitrogen, and lipid metabolism. Copper is also required for pollen grain formation. A copper deficiency can cause die back of the shoot tips, stunted growth, and terminal leaves may develop black necrotic spots. Copper deficiency affects fruit and seed formation much more drastically than vegetative growth.

Molybdenum is a structural component of the enzyme nitrate reductase that reduces nitrates to ammonia. This enzyme is found in all higher plants. Many plants (i.e. legumes) reduce atmospheric nitrogen to ammonia via bacteria located in root nodules. These bacteria use the enzyme nitrogenase which also contains molybdenum. Not surprisingly, other biologically important enzymes beside the two mentioned contain molybdenum. One of the most common signs of molybdenum deficiency is interveinal chlorosis of young leaves. Other symptoms include stunted seedling growth and those symptoms associated with nitrogen deficiency.

Chlorine is involved in osmoregulation, the regulation of movement of water and other solutes into and out of cells. Chlorine is essential for cell division in leaves and in the regulation of opening and closing of stomata. Chlorine is also involved in the photosynthetic evolution of oxygen and nitrogen metabolism. Deficiency symptoms include wilting of leaves, chlorosis, and stunted root growth. High levels of chlorine can be severely detrimental to plant growth.

Nickel has recently been determined to be an essential trace element for plants by the Agricultural Research Service (ARS) in Ithaca, New York. It is required for the enzyme urease, which most plants use to break down urea into usable forms of nitrogen. Nickel is also a necessary component for the function of other enzymes. Nickel is essential for iron absorption. Seeds require nickel in order to germinate. Plants grown without an adequate supply of nickel will gradually reach a deficient level at about the time they mature and begin reproductive growth.

Boron is involved with root elongation, nucleic acid metabolism, cell wall synthesis, and pollen tube formation. Its uptake is closely related to the soil pH. It becomes more readily available as the pH increases. Deficiency symptoms include discoloration or death of young leaves and terminal buds. Plants will also fail to set seed and fruit.

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