An Environmental Threshold of Soil Test P and Degree of P Saturation of Brazilian Oxisols

To develop P management guidelines for poultry litter application in Oxisols of Brazil, the relationship between water soluble P (WSP), soil test P (STP), and degree of P saturation (DPS) needs to be established. The study examined this relationship in Oxisols of Brazil receiving poultry litter. Soils were analyzed for Mehlich-1 P (M1-P), WSP, P fractions, P sorption index (PSI), and P saturation based on Mehlich-1 extractable P, Al, Fe (DPSM1), and PSI (DPSPSI). Surface water bodies in the catchment area were examined for inorganic and total P. All the surface water bodies in the catchment area of the sampled plots were highly enriched in inorganic and organic P. Enrichment of soil P fractions following poultry litter application followed the order HCl-bound > NaOH-bound > NaHCO3-bound fraction. M1-P was correlated to labile (NaHCO3-P-i), and stable P pools (NaOH-P and HCl-P) and reflected the cumulative P accumulation in these soils. P saturation indices increased with cumulative P addition and increasing M1-P. WSP increased with increasing DPS and a change point was noticed at 23% DPSPSI (equivalent to DPSM1 of 16.5%) corresponding to M1-P of 44.5 mg P kg(-1), which could be regarded as threshold for P loss in these soils.