A number of studies have shown that colloids can play a major role in the transport of contaminants, including phosphorus (P). In this paper we examine their role in the transport of P in surface runoff from intensively managed dairy pastures in south-east Australia. We characterized the physico-chemistry of P in the ‘dissolved' fraction (<1 mm) of runoff and soil extracts and investigated the effect of soil P status on the mobile forms of P. Data is presented for two soils with contrasting textures and with varying Olsen P concentrations. The soils were typical of those from the main dairying regions in south-east Australia.

The forms of mobile P in the <1 µm fraction varied with soil fertility. In runoff from a sandy-loam of low fertility (i.e. Olsen P of 20 mg/kg), approximately 50% of the <1 µm P is un-reactive P (UP). However, at high fertility levels (i.e. Olsen P of 100 mg/kg) less than 10% of the <1 µm P is UP, the rest being reactive P (RP). Greater than 70% of the <1 µm P also passed through the 1kDa filter. Greater than 50% of the UP in the <1 µm fraction of the runoff also passed through the 1 kDa filter (i.e. this material is sub-colloidal). Colloidal P makes a minor contribution to P transported in runoff. The majority of P in runoff is RP or sub-colloidal UP. We hypothesize that the sub-colloidal UP is a mixture of P containing humic/fulvic acids and low molecular weight organic P compounds. Characterization of this sub-colloidal UP is the subject of on-going research. The dominance of RP in runoff suggests that strategies are needed that reduce rapidly exchangeable P in the immediate topsoil. Such strategies could include chemical amendments or physical strategies such as de-stratification.