ammonium (ammonium)

Ammonium (NH₄⁺) is the positively charged ion formed when ammonia (NH₃) gains a proton in aqueous solution. The equilibrium NH₃ + H₂O ↔ NH₄⁺ + OH⁻ is pH-dependent: at pH 7 and 25°C, more than 99% sits as NH₄⁺; at pH 9.5, the split is roughly 50/50; at pH 11, more than 99% sits as NH₃ (free ammonia). This pH dependence is the central operational fact of ammonium chemistry in biogas plants, fertilisers, and soils.

In biogas digestate, ammonium is the dominant plant-available nitrogen form — typically 50–80% of total nitrogen. It comes from protein and amino-acid deamination during anaerobic digestion and accumulates because no nitrification can occur in the anaerobic environment. Total ammonium-N concentrations in digestate range from 1,000–5,000 mg/L in the liquid fraction. At the digester's normal pH of 7.5–8.0, free ammonia (NH₃) is only 1–5% of total ammonia-N (TAN). Free ammonia is what actually inhibits methanogens — when free NH₃ rises above 100 mg/L it depresses methanogenic activity, and above 250–500 mg/L it causes acute process failure. Feeding high-protein materials (poultry litter, slaughterhouse waste, food waste) drives TAN up and triggers this inhibition.

In soil, ammonium binds to negatively charged clay particles and organic matter via cation-exchange, resisting leaching. Plant roots absorb NH₄⁺ directly through ammonium transporter proteins, particularly in young plants where the energy cost of nitrate reduction would be high. Over 2–4 weeks, soil bacteria (Nitrosomonas, Nitrobacter) oxidise ammonium to nitrate via nitrification, releasing hydrogen ions that lower soil pH — a key reason ammonium-based fertilisers acidify soils over time. From digestate, the practical concern is ammonia volatilisation: surface-applied ammonium-N converts to NH₃ gas and escapes within hours when soil pH is alkaline (above 7.5), temperature is high, and wind is present. Field losses of 20–40% of applied ammonium-N within 24 hours are typical without injection or immediate incorporation. Mitigation requires either trailing-hose/trailing-shoe applicators, immediate irrigation, or fertigation through drip lines that deliver ammonium below the soil surface where volatilisation is suppressed.