Micronutrient

Each micronutrient has a specific biochemical role. Iron and manganese participate in chlorophyll synthesis and photosynthesis. Zinc activates 300+ plant enzymes, including those for auxin synthesis and protein formation. Boron is essential for cell-wall structure, pollen tube growth, and seed set. Copper supports lignin synthesis and stress tolerance. Molybdenum enables nitrogen fixation and nitrate reduction. Deficiencies appear as characteristic visual symptoms: iron and manganese as interveinal chlorosis on new leaves, zinc as stunted internodes and small leaves, boron as hollow stems and poor flowering.

Micronutrient availability depends heavily on soil pH (see soil-ph). Iron, manganese, zinc, and copper availability all drop sharply above pH 7.5 because these cations form insoluble precipitates. Boron and molybdenum, in contrast, become more available at higher pH. Sandy soils, high-organic-matter peat soils, and intensively cropped fields are most at risk. Calcareous soils common in arid regions (Egypt, North Africa, parts of the US West) frequently show iron, zinc, and manganese deficiency even when total soil content is adequate.

Management options include soil-applied micronutrient fertilizers (zinc sulfate at 5–20 kg/ha is the most common application), foliar sprays for rapid correction (see foliar-feeding), chelated micronutrients (EDTA, DTPA) that stay plant-available across pH ranges, and pH amendment to unlock native soil reserves. Tissue testing during the growing season is the most accurate diagnostic — soil tests can miss pH-induced lockouts. IPNI estimates that correcting widespread zinc deficiency alone could add 50–200 kg/ha yield on affected cereal crops, representing one of the highest-ROI interventions in fertility management.