Micronutrients (Zn, Cu, Fe, Mn) (zinc)

Zinc (Zn), copper (Cu), iron (Fe), and manganese (Mn) are the four most agronomically important micronutrients delivered through biogas digestate. They are required by plants in small quantities — milligrams per kilogram of dry matter rather than grams — but their roles in enzyme function and plant metabolism are non-substitutable, and their deficiency causes specific, recognisable yield-limiting disorders.

Each element has distinct biological functions. Zinc is a cofactor in over 300 enzymes and is essential for auxin (growth hormone) synthesis — deficiency causes interveinal chlorosis, stunted growth, and rosette-like leaves in rice, wheat, and maize. The Indian Council of Agricultural Research estimates that 49% of Indian soils are zinc-deficient. Iron is central to chlorophyll synthesis and to electron transport in photosynthesis — deficiency causes interveinal chlorosis particularly in alkaline soils where iron is locked into insoluble forms. Manganese drives photosystem II in chloroplasts and several enzyme reactions — deficiency causes grey-speck disease in oats and stripe disease in barley. Copper is required for chlorophyll formation and seed production — deficiency is rare but causes wilting and reproductive failure in wheat on sandy soils.

Digestate from mixed Indian feedstocks typically contains Zn at 100–400 mg/kg DM, Cu at 30–100 mg/kg DM, Fe at 1,000–5,000 mg/kg DM, and Mn at 200–800 mg/kg DM. At a typical field application rate of 5–10 tonnes DM per hectare, this delivers a meaningful share of crop requirement for all four elements without the cost of separate micronutrient sprays. However, the same heavy metals are also regulated as contaminants — when feedstock includes industrial effluent, food waste with high packaging contamination, or sewage sludge, levels can rise into toxic territory. The FCO 1985 amendment specifies maximum permissible limits for digestate sold as organic fertiliser: Zn under 1,000 mg/kg, Cu under 300 mg/kg, with parallel limits for Cd, Pb, Ni, and Cr. Routine batch testing every 1,000 tonnes is mandatory under FCO registration. The trade-off in feedstock selection is significant: cattle dung feedstock produces low-micronutrient, FCO-compliant digestate but lower biogas yield; food waste produces higher biogas yield but tighter compliance margins on heavy-metal limits.