Phosphorus (P) losses from croplands accelerate eutrophication in streams and lakes,
8 consequently decreasing the water quality. Southern Brazil has over 5 million hectares of croplands under no till, of which approximately 70% of the topsoil has high P levels. In addition, supporting conservation practices, such as contour farming and terrace, are not in use anymore. This scenario increases the potential P losses from the soil by surface runoff. The degree of phosphorus saturation (DPS) has been used to evaluate the risk of these losses. This work sought to investigate a DPS index to estimate the risk of P losses from croplands to surface water in 65 highly representative soils from Southern Brazil widely used for crop production. Three labile P forms were measured (Mehlich-1, Mehlich-3 and oxalate), and six P sorption indexes were tested. With little information available regarding P losses by runoff throughout Southern Brazil, we used the water extractable P (WEP) as an index of P susceptible to loss by surface runoff. The DPS obtained using P extracted with Mehlich-1 and the P single point sorption ranged from 1 to 25%, while that using Fe+Al extracted with ammonium oxalate ranged from 1 to 55%. All calculated DPS values were highly correlated with WEP. From the point of view of practical use and easy implementation, the DPS obtained using P and Fe+Al extracted with Mehlich-1 (currently the standard P method) can be used in Southern Brazilian soils to estimate the risk of P losses