Estimating Risk of Phosphorus Loss in Southern Brazilian Soils.

Phosphorus (P) losses from croplands contribute to accelerate the eutrophication in streams, lakes and degrading water quality. Southern Brazil has over 3.5 million hectares of soils cultivated under no till with the around 70% of topsoil with high P levels. In addition, the supporting conservation practices such as contour farming and terrace have been removed from the landscape management scheme. This scenario increases the potential of P losses from agriculture by runoff. The degree o f phosphorus saturation (DPS) has been used to evaluating the risk of P loss from soil to runoff. This work sought to investigate a DPS index to estimate the risk of P losses from croplands to surface water in soils from Southern Brazil. We used 65 most representative soils widely used for crop production. Three labile P were measured (Mehlich-1, Mehlich-3 and oxalate) and tested six P sorption indexes. With the little information available regarding P losses by runoff throughout Southern Brazil, we used the P extractable in water as an index of P susceptibility to runoff loss. The DPS obtained using P extracted with Mehlich-1 and the P single point sorption (75 mg L^-1) ranged from 1 to 25% while using Al+Fe extracted with ammonium oxalate ranged from 1 to 55%. All the DPS calculated, those that were used Mehlich-1 and Mehlich-3 as P labile form and that those uses Al+Fe extractable (Mehlich-1, Mehlich-3 and ammonium oxalate) were highly correlated with P extractable in water. From the practical use and easy implementation point of view, the DPS using P and Al+Fe extracted with Mehlich-1 (currently standard P method) could be suitable to use in soils from Southern Brazil to estimate the risk of P losses, and establish the best management P practices.