The effect of spent mushroom compost on soil conditions and plant growth

Spent mushroom compost (SMC), a byproduct of the mushroom industry, is used widely by horticulturalists and home gardeners. However, no studies have been reported that evaluate SMC as a soil amendment in New Zealand, and work in other countries has been of limited scope. Therefore the objectives of this study were to: evaluate the effect of SMC on soil chemical and physical properties, and on the yield of three vegetable species; to determine the rate of nutrient release from SMC amended soil, and to describe this release using kinetic models; to compare leaching losses from SMC with those from inorganic fertilizer; and to develop practical guidelines for SMC use. The SMC investigated contained 1.8% N (of which 94% was organic), 0.8% P, 1.6% K, 1.3% S (of which 17% was organic), 5.9% Ca and 0.4% Mg (on a dry weight basis) and was obtained from Meadow Mushrooms Limited, Prebbleton, Christchurch. A field trial with treatments including 0, 20, 40 and 80 t ha⁻¹ moist SMC, both with and without inorganic fertilizer, was conducted for two years (10/91 to 11/93) on a Templeton fine sandy loam (Udic Ustochrept, fine loamy, mixed, mesic) at Lincoln. Field applications of SMC produced an improvement in a wide range of soil chemical properties including increases in soil pH, organic-C, biomass-C, total-N, Olsen-P, total-P, phosphate extractable-SO₄²⁻-S, total-S, exchangeable-K, exchangeable-Ca, exchangeable-Mg, total exchangeable bases, cation exchange capacity and base saturation. The soil electrical conductivity was also increased following the application of SMC. Applications of SMC produced a rapid but brief increase in the soil NO₃⁻-N concentration as a result of nitrification of the NH₄⁺-N added in SMC. Following this initial phase, net N mineralization or immobilization occurred. Net N immobilization was most prevalent in the 20 t ha⁻¹ SMC treatment. The addition of SMC to soil improved the physical environment for plant root growth by reducing soil bulk density, increasing aggregate stability, reducing clod and surface crust formation, increasing the infiltration rate, increasing the moisture content of the soil and by insulating the soil against rapid temperature changes. The soil physical properties that had most influence on plant growth were identified using principal component analysis. These were specific for each crop and included bulk density, soil moisture content, surface crust cover, infiltration rate and aggregate size distribution. Four consecutive vegetable crops were grown during the two years of the field trial. Corn and cabbage yields were increased when SMC was applied without inorganic fertilizer, and potato yield was increased irrespective of fertilizer use (i.e. fresh yield increases of 38%, 82-96% & 26-38% respectively for corn cob, cabbage head & potato tubers). Inorganic fertilizer increased crop yield more than did SMC, except for the potato tuber yield which was increased following SMC applications, but was not increased by applications of inorganic fertilizer. Generally SMC reduced the crop dry matter content and may increase plant concentrations of N, P, K and S. However the plant recovery of all these nutrients was generally less from SMC than from inorganic fertilizer. Three open incubation studies were done at 25 or 30°C using soil from the field trial site with a 1 or 2 week interval between leachings. The N data were variable and little inorganic N was released from SMC. However, net N immobilization appeared to occur in the 20 and 40 t ha⁻¹ SMC treatments initially, but not in the 80 t ha⁻¹ SMC treatment. Following any initial N immobilization, inorganic N was released slowly from the SMC. The net recovery in the leachate of N applied in SMC was 8-18% (in the absence of inorganic fertilizer). Net inorganic N release from SMC was modelled using a negative first order exponential term to describe any initial immobilization, and a zero order exponential term to describe subsequent release. The rate of inorganic N release from mushroom compost in the laboratory was slower than that from glycine or chicken litter. Sterilants used during mushroom production did not appear to be responsible for the slow rate of N mineralization from SMC (however chloride and formaldehyde caused a small increase and decrease respectively in the amount of inorganic N released from mushroom compost amended soil). The slow rate of inorganic N release from SMC was largely the result of the recalcitrant nature of the organic N in SMC. Sulphate-sulphur was released rapidly from SMC amended soil in the laboratory incubation, mainly as a result of SO₄²⁻ dissolution as most of the S in SMC is SO₄²⁻-S (i.e. 87%). The net recovery in the leachate of S applied in SMC was 75-83% (in the absence of inorganic fertilizer). The release of SO₄²⁻-S from SMC was modelled using first order kinetics. The release of K, Ca and Mg from SMC amended soil was initially rapid (first order) and then declined to a constant rate (zero order). The net recoveries in the leachate of K, Ca and Mg applied in SMC were 40-45, 14-20 and 43-66% respectively (in the absence of inorganic fertilizer). A leaching study was done during the first cabbage crop (winter/spring 1992) using undisturbed soil monolith lysimeters (176 mm in diameter & 240 mm deep). Leaching losses were compared when SMC (80 t ha⁻¹ moist) and inorganic fertilizer were applied to the lysimeters at rates typical of those used by Canterbury growers. Although more N was applied in SMC than in inorganic fertilizer (472 cf. 338 kg ha⁻¹), less inorganic N was leached from SMC amended soils than from those receiving inorganic fertilizer (61 cf. 246 kg ha⁻¹). Considerably more Sand K were applied in SMC than in inorganic fertilizer (328 cf. 114 kg S ha⁻¹, & 398 cf. 100 kg K ha⁻¹), however smaller proportions of SMC Sand K were leached compared with inorganic fertilizer (80 cf. 94% SO₄²⁻-S, & 4 cf. 14% K). The leaching of inorganic N, Sand K from SMC was characterised by poly-modal breakthrough curves indicating soil structural effects on leaching processes. Laboratory optimized kinetic models of nutrient release from SMC accurately described the leaching of SO₄²⁻ in the field, and when modified to account for field soil temperatures or drainage respectively, estimated similar amounts of inorganic N and K loss as were observed in the field. Practical guidelines for SMC use include applying more than 40 t ha⁻¹ SMC for the initial application or applying SMC well before growing any plants or supplementing SMC applications with inorganic N fertilizer, to decrease the likelihood of N immobilization. Subsequent SMC applications are less likely to cause N immobilization. Also, to reduce the likelihood of SO₄²⁻ leaching, SMC applications should be made to avoid periods prone to leaching (e.g. late autumn or winter).