NEEDS FOR CONTROLLED-AVAILABILITY MICRONUTRIENT FERTILIZERS

Use of micronutrients for agronomic and horticultural crops has increased markedly in recent years. Increased use is related to higher nutrient demands from more intensive cropping practices and also from farming marginal lands. Most of the fertilizers used to correct micronutrient deficiencies are water- soluble inorganic sources or soluble organic products such as synthetic chelates or natural organic complexes. These fertilizers may react with soil to decrease their availability to plants. The rates of such chemical reactions may differ considerably with each micronutrient fertilizer and soil environment. Recommended micronutrient rates have been based on results of numerous experiments, and these rates vary with crop, soil, and other factors. The usual application rates (on an elemental basis) range from 1 to 10 kg ha-1 for Cu, Fe, Mn and Zn; < 1 kg ha-1 for B; and < 100 g ha-1 for Mo. Because the metallic micronutrients (Cu, Fe, Mn, and Zn) generally are sorbed strongly by soil clays, they do not move significantly in soil. Hence, they are not leached readily from the zone of application to lower soil depths or into groundwaters. Mobility of these micronutrient cations is higher in sandy soils, especially with high leaching conditions. Therefore, their movement out of the root zone is possible under some situations. Significant residual effects of soluble Cu and Zn sources greatly reduce the need for controlled-availability Cu and Zn products. Controlled-availability Fe and Mn fertilizers have not been effective because the rapid oxidation of Fe and Mn and reactions with soil reduce their availability upon release. Because soluble B fertilizers form boric acid molecules in soil, they are mobile and subjects to leaching conditions. While mobility of B is less than that of NO3(-)-N in soil, field results have demonstrated loss of applied B from the root zone in sandy soils. Slightly soluble B fertilizers, such as colemanite and ulexite, and fritted B products (powdered glass-like materials whose solubility is controlled by particle size) have been used in sandy soils for some crops. Molybdenum requirements are much lower than those of the other micronutrients. Deficiencies generally are corrected by liming the soil or by seed or foliar applications, so there is little need for controlled-availability Mo fertilizers. Little research has been conducted on controlled availability micronutrient fertilizers. While fritted products are considered in this category, they are difficult to handle and only may be of value in supplying B under specific conditions. Coating soluble granular micronutrient fertilizers also has been attempted, but there are few reported results of their relative effectiveness in comparison with conventional fertilizers and application methods. New micronutrient products may be needed for specific conditions such as greenhouse-culture or container-grown crops, but plant needs also may be met by multiple applications of soluble sources.