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

The forms and strati. cation 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 - 0.01m increment was, on average, 5.7 times greater than in the 0.05 - 0.10m increment. Organic P (% of total P) in the top 0.01m was significantly (P < 0.001) related to soil P content such that low P soils (total P of similar to 600 mg/ kg) had high proportions of P-o (similar to 65%), whereas high P soils (total P of similar to 2000 mg/kg) had low proportions (similar to 25%) of P-o. Runoff P from these soils was predominantly (86%) dissolved (i.e.< 0.45 mu m). There was a significant (P < 0.001) exponential relationship between Olsen P in the top 0.01m and dissolved P concentration in runoff. The form of dissolved P in runoff from soil in repacked trays was also significantly (P < 0.001) related to soil P. Runoff from low P soils (high Po) had high proportions (> 50%) of dissolved unreactive P (DUP), whereas runoff from high P soils (low Po) had low proportions of DUP (< 10%). Ultra filtration of runoff samples revealed that 94 and 65% of the dissolved reactive P and DUP, respectively, was subcolloidal (i.e. < 1 nm). These results highlight the relationship between soil fertility, the forms of soil P, and the concentrations and forms of P mobilised in runoff. Such relationships need to be considered in further studies of P mobilisation and the subsequent development of strategies designed to reduce runoff P concentrations.