Release of sulphate, potassium, calcium and magnesium from spent mushroom compost under laboratory conditions

The release of sulphate-sulphur (SO42S), potassium (K), calcium (Ca) and magnesium (Mg) from soil amended with spent mushroom compost (SMC), a by-product of mushroom production, was measured for 16 weeks in an open laboratory incubation at 25°C. Rates of application were up to 80 t ha-1 moist SMC (0.84% SMC dry weight) both with and without inorganic fertilizer. The rates of nutrient application in the inorganic fertilizer were: 338 kg ha-1 N, 100 kg ha-1 of both phosphorus and K, and 114 kg ha-1 S. SMC contains 1.7% K, 6.5% Ca, 0.4% Mg and 1.2% S (of which 87% is inorganic), and has a carbon : sulphur ratio of 26. The release of SO42S was rapid, and was described using either a first or mixed order exponential equation, or (underestimated) by the CENTURY model. The release of K, Ca and Mg was initially rapid (first order) and then declined to a constant rate (zero order). Their release was also described using first/first order or first order/parabolic diffusion equations. Model parameters indicated the relative sizes of both readily releasable and recalcitrant nutrient pools. The recovery of SMC-supplied nutrients in the absence of fertilizer was 75-83% of the S, 40-45% of the K, 14-20% of the Ca and 43-66% of the Mg. When fertilizer was applied 33-45% of the S, 22-36% of the K, 12-24% of the Ca and -4 to 20% of the Mg that were supplied by the SMC and fertilizer were recovered in the leachate. The generally lower nutrient recovery when fertilizer was applied could have resulted from the incomplete recovery of fertilizer S and K, from soil fixation of applied nutrients, and from the lower pH following fertilizer application.