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

Abstract

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.

Keywords:

• extraction methods
• irrigated soil
• phosphate reactions
• phosphorus requirements
• soil solution