Stratification, forms, and mobility of phosphorus in the topsoil of a Chromosol used for dairying

The forms and stratification of soil phosphorus (P) and their relationship to mobile forms of P were investigated in soils collected from a subcatchment used for grazing of dairy cattle in the Adelaide Hills, South Australia. Phosphorus in the soils was highly stratified. The concentration of calcium chloride extractable P in the 0–0.01 m increment was, on average, 5.7 times greater than in the 0.05–0.10 m increment. Organic P (% of total P) in the top 0.01 m was significantly ( P < 0.001) related to soil P content such that low P soils (total P of ~600 mg/kg) had high proportions of P o (~65%), whereas high P soils (total P of ~2000 mg/kg) had low proportions (~25%) of P o . Runoff P from these soils was predominantly (86%) dissolved (i.e. 50%) of dissolved unreactive P (DUP), whereas runoff from high P soils (low P o ) had low proportions of DUP (