Abstract
Managing phosphorus (P) supply on sands remains a challenge because of the propensity of P to leach while reducing P rates may lead to P deficiency in crops. Biochar and clay are two amendments with potential to improve the retention and availability of P to crops. This study investigated the effect of biochar (produced from wheat straw (WS) or chicken manure (CM)) and clay (kaolin or clay-rich subsoil) on P in soil solution and leachate and on P uptake by wheat on a grey sand from the south coast of West Australia. We hypothesized that P leaching will decrease with the addition of biochar and clay together to sands. More P was released during desorption by WS biochar than by the CM biochar. After 5 successive desorption steps, solution P concentration was 10 mg/L with CM biochar but declined to only 2 mg/L with WS biochar. This suggested that P in WS & CM biochar is readily soluble with more rapid release from WS biochar. Adding either kaolin or subsoil clay (50 t of clay/ha) with WS biochar or CM biochar (10t/ha) decreased the amount of P leached with or without P fertiliser added, however the decline in P leached and in soil solution P concentration was most pronounced with subsoil clay addition. In summary, subsoil clay had a greater effect than kaolin in reducing soluble P in soil solution and in leachate particularly with CM biochar but the soil solution P concentrations still remained above 0.2 mg P/L, and hence was maintained at concentrations adequate for plant growth.