Micronutrients required for normal plant development and fruiting include boron, copper, iron, manganese, molybdenum, and zinc. Unlike the macronutrients (N, P, K, Ca, Mg and S), the quantities of micronutrients needed by agronomic crops is very small. Kentucky soils may contain large amounts of micronutrients in their mineral composition but only small amounts may be available for plant uptake. The soil availability and crop uptake of micronutrients are influenced by soil pH, soil temperatures, crop root growth, soil moisture, crop species and soil organic matter content.
Thom, William O., "Micronutrients in Kentucky" (1985). Soil Science News and Views. 113.