Phosphorus extraction by selected methods in alkaline and calcareous soils after mono-ammonium phosphate application at different rates

Little research has been done on extractable phosphorus (P) in alkaline and calcareous irrigated soils of South Africa. This study aimed to quantify extractable P by selected methods in these soils after application of mono-ammonium phosphate (MAP) at different rates. Six soils with a pH of 7.1 to 8.2 that contained 0.6–12.4% CaCO3 were incubated for two months after application of MAP at levels equivalent to 0, 16.8, 33.6, 50.4, 67.2, 84.0 or 100.8 kg P ha−1. After incubation, soil P was extracted with the Olsen, Bray 1, Mehlich 3 and Ambic 1 methods. The most soil P was extracted with the Mehlich 3 method, and the least soil P was extracted with either the Bray 1 or Olsen methods, regardless of the MAP application rate. The differences in P extracted by the methods were attributed to the chemical composition of the extractants and the soil properties. Olsen and to a lesser extent Ambic 1 were the most reliable methods for establishing the amount of P required to raise the P status of the alkaline and calcareous soils. Future research should focus on establishing specific threshold values for soil and crop combinations with the Olsen method.