Degree of phosphorus saturation of an Oxisol amended with biosolids in a long-term field experiment

Abstract

When applied to agricultural soils, phosphate fertilizers and the mineral or organic compounds present in solid and/or liquid waste may raise phosphorus (P) content and increase soil P saturation. The degree of phosphorus saturation (DPS) is a good indicator of potential P loss from agricultural soils. The purpose of this study was to calculate the DPS of samples from an Oxisol amended for 5years with biosolids and mineral fertilizer. DPS was calculated based on P, iron, and aluminum extracted by ammonium oxalate and oxalic acid (DPSox) or by Mehlich-1 solution (DPSM1). Treatments included NPK mineral fertilization (175kgha(-1) of P), B1 = 19.02tha(-1) of biosolids (350kgha(-1) of P), B2 = 38.17tha(-1) of biosolids (703kgha(-1) of P), B3 = 76.26tha(-1) of biosolids (1,405kgha(-1) of P), and a control (no P added). Water-extractable P (WEP) was also measured. Critical levels of DPSox and DPSM1 (21 and 24%, respectively) were only achieved in the topsoil (0-0.1m) at the highest biosolid dose. Concentration of WEP was positively correlated to DPSox and DPSM1. The DPSM1 method may be an alternative to DPSox for assessing the environmental risk of P loss from soil into surface runoff.