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

The release of SO42--S, K+, Ca2+ and Mg2+ from soil amended with spent mushroom compost (SMC), a byproduct of mushroom production, was measured in leachate from field lysimeters for 30 weeks. Rates of application were 0 and 80 t ha(-1) moist SMC. The SMC contained 1.7% K, 6.5% Ca, 0.4% Mg and 1.2% S (of which 87% is SO42--S), and has a C:S ratio of 26. The break-through curves of ion leaching were polymodal indicating the preservation of soil structure in the lysimeters and its influence on leaching. SO42--S release from SMC was rapid (first-order exponential) and was very similar to the release from a laboratory incubation. The release of K+, Ca2+ and Mg2+ was described using first/zero-order models which were also used to describe their release in the laboratory. The rate and amount of Ca2+ release was similar in the field and laboratory, but the amount of K+ (and to a lesser extent Mg2+) release was less in the field than in the laboratory. Recoveries of SMC applied nutrients in leachate were 80% of S (263 kg ha(-1)), 3% of K (14 kg ha(-1)), 16% of Ca (284 kg ha(-1)) and 37% of Mg (40 kg ha(-1)). Little if any S was mineralised. Using SMC could provide plants with S, K, Ca and Mg but there is potential for SO42--S losses via leaching.