NOVEMBER-DECEMBER 2001 3

Manganese

Interveinal chlorosis on dry beans, and whitish-gray spots on leaves of cereal crops are Mn-deficiency symptoms. Brown necrotic spots appear on leaves with vSery severe Mn deficiencies, resulting in premature leaf drop.

Deficiencies of Mn mainly occur on organic soils, and sandy soils low in organic matter, and on over-limed soils. Toxicity of Mn can result in some acidic, high-Mn soils. Crops which are susceptible to Mn deficiency are dry beans and some vegetable crops. There have been no reported Mn deficiencies in Colorado crops.

Molybdenum

Molybdenum deficiency symptoms in legumes are mainly exhibited as N-deficiency symptoms because of the primary role of Mo in N₂ fixation. Deficiency symptoms of Mo in some vegetable crops are irregular leaf blade formation (known as whiptail), interveinal mottling and marginal chlorosis of older leaves.

Molybdenum deficiencies are found mainly on acid, sandy soils in humid regions. Plant uptake of Mo increases with increased soil pH, which is opposite that of the other micronutrients. Crops which are sensitive to Mo deficiency are alfalfa, clovers and some vegetable crops. There have been no reported Mo deficiencies in Colorado crops.

Zinc

Some zinc deficiency symptoms are short internodes (rosetting), a decrease in leaf size, chlorotic bands along the midribs of corn, mottled leaves of dry beans, narrow yellow leaves in the new growth of citrus, and delayed maturity.

Zinc deficiencies are mainly found on sandy soils low in organic matter and on organic soils. They occur more often during cold, wet spring weather and are related to reduced root growth and activity. Uptake of Zn decreases with increased soil pH, and is adversely affected by high levels of available P and Fe in soils. Some crops which are susceptible to Zn deficiency are corn, dry beans, lettuce and onions. Zinc application is recommended for corn grown on Colorado soils testing low (<1.5 ppm) in available Zn.

Micronutrient sources

Micronutrient sources vary considerably in their physical state, chemical reactivity, cost, and availability to plants. The main classes are inorganic products, synthetic chelates and organic complexes.

Inorganic sources include oxides and carbonates and metallic salts such as sulfates, chlorides, and nitrates. The sulfates are the most common of the metallic salts and are sold in crystalline or granular form. An ammoniated ZnSO₄ solution usually is applied in polyphosphate starter fertilizers. Oxides of Mn and Zn are also sometimes used as fine powders, but their immediate effectiveness for crops is rather low in granular form. Oxysulfates are oxides, usually industrial by-products, which have been partially acidulated with sulfuric acid, and generally are sold in granular form. The percentage of water-soluble Mn or Zn in oxysulfates is directly related to the degree of acidulation. Research results have shown that at least 35 to 50% of the total Zn in granular Zn-oxysulfates should be in water-soluble form to be immediately effective for crops. Similar results would be expected for Mn-oxysulfate. Inorganic sources usually are the least costly sources per unit of micronutrient, but they may not always be the most effective for crops.

Synthetic chelates are formed by combining a chelating agent with a metal through coordinate bonding. Stability of the metal-chelate bond affects availability of the micronutrient metals to plants. An effective chelate is one in which the rate of substitution of the chelated micronutrient for other cations in the soil is quite low, thus maintaining the applied micronutrient in chelated form.

Relative effectiveness for crops per unit of micronutrient as soil-applied chelates may be from two to five times greater than that of inorganic sources, but chelate costs may be five to 100 times higher. Several chelates are sold, so relative effectiveness values depend on the sources of chelates and inorganic products compared.

Organic complexes are made by reacting metallic salts with some organic by-products of the wood pulp industry or other related industries. The types of chemical bonding of the metals to the organic components are not well understood. While organic complexes are less costly per unit of micronutrient, they usually are less effective than synthetic chelates. They also are more readily decomposed by microorganisms in soil. These sources are more suitable for foliar sprays and mixing with some fluid fertilizers.

Summary

Micronutrients are as important as the primary and secondary nutrients in plant nutrition. However, the amounts of micronutrients required for optimum crop yields are much lower. Soil tests and plant analyses are excellent diagnostic tools to monitor the micronutrient status of soils and crops. Visual deficiency symptoms of these nutrients also are well recognized in most economic crops. Micronutrient recommendations are based on soil and plant tissue analyses, the crop species and expected yield, management level, and research results.

Because recommended rates usually are low, most micronutrients are applied with NPK fertilizers, but foliar sprays also are frequently applied. Choice of micronutrient source depends on the method of application, compatability with the NPK fertilizer, convenience of application, and the relative agronomic effectiveness and cost per unit of micronutrient.

John Mortvedt
Faculty Affiliate
Colorado State University