The 7 micronutrients are sufficient in most soils to meet crop needs. However, some sandy soils and other low-organic matter soils are naturally deficient in micronutrients, and high pH soils may make some micronutrients less available and therefore deficient. In the major crops and production areas of North America, the micronutrients most often supplied by fertilization include zinc, manganese, boron and iron. Basic chemical properties of micronutrients help determine their availability in soils (Table 1).
Because of complex chemical reactions within the soil, micronutrient availability is ultimately controlled by the equilibrium between the soil solution, soil organic matter, cation exchange sites, and insoluble compounds of micronutrients. Soil acidity or alkalinity has a large effect on the tie-up of micronutrients or their availability to plants. Micronutrients are most available in acid soils and often unavailable at high pH (Figure 1).