potassium (K)

Potassium (K) is one of the three primary macronutrients required by plants (alongside nitrogen and phosphorus) and the third element of the standard N-P-K specification. Unlike N and P, potassium is not built into plant tissue molecules — it stays as the K⁺ ion inside cells, where it regulates osmotic potential, stomatal opening and closure, enzyme activation, and the translocation of sugars and starches from leaves to fruits and seeds. Potassium-deficient crops show leaf-edge chlorosis (starting at older leaves), reduced disease resistance, poor stem strength, and low fruit quality.

In commercial fertiliser markets, potassium is specified as K₂O equivalent — a legacy convention preserved in the Fertiliser Control Order, 1985. Conversion is K₂O = K × 1.205. Standard sources are Muriate of Potash (MOP, 60% K₂O) and Sulfate of Potash (SOP, 50% K₂O with 18% sulfur co-benefit). India imports 100% of its mineral potash from Russia, Belarus, Canada, and Jordan, costing $300–500 per tonne FOB plus freight — making potassium the most foreign-exchange-sensitive nutrient in Indian agriculture.

In biogas digestate, potassium typically constitutes 1.0–3.0% of dry matter, varying with feedstock. Cattle dung digestate sits at 1.5–2.5% K₂O; agricultural residue digestate (paddy straw, sugarcane trash) at the higher end (2.5–3.5% K₂O) because crop biomass concentrates K from soil. The critical operational fact about digestate potassium is that it is highly water-soluble — almost 100% sits as the K⁺ ion in solution, with virtually none bound in organic complexes. This means it concentrates in the liquid fraction during mechanical separation: 70–85% of digestate's K mass ends up in the centrate or filtrate, leaving the solid cake relatively K-poor. The implication for product design is clear. Plants targeting balanced N-P-K solid fertiliser must blend mineral potash into the cake to restore K levels. Plants selling liquid fertiliser through fertigation get the K-rich product naturally. Agronomically, applying digestate as a substitute for imported MOP is one of the most economically defensible substitutions in Indian agriculture — every tonne of K₂O delivered through digestate avoids 1.67 tonnes of imported MOP plus its FX exposure. NITI Aayog estimates that India's biogas potential could displace 8–12% of imported potassium fertiliser annually if fully developed.