Evaluation of Different Soil Tests for Phosphate Extractability in Major Soils from Angola

Phosphate (P) extractability was evaluated in twenty-eight superficial and subsuperficial horizons of representative soils occurring in Angola: Typic ferrallitic, weakly ferrallitic, Psammoferrallitic, Eutroparaferrallitic, and Fersilliatic soils. These soils show a wide range of texture, pH, cation exchange capacity, contents of organic C, and active noncrystalline forms of aluminum and iron extracted by acidified ammonium oxalate. Soil P was evaluated using eight different extractants, corresponding to the following tests: distilled water (Pdw), 0.01 M calcium chloride (CaCl2) (Pca), iron oxide–impregnated filter paper (Pfe), Bray 2 (Pb2), Mehlich 3 (Pm3), Egner–Riehm (Peg), Olsen (Pol), and acidified ammonium oxalate (Pox). The overall P extractability by the different tests was in the order Pca < Pdw < Peg < Pfe ≈ Pol < Pm3 < Pb2 < Pox. In the surface and subsurface horizons, the orders were Pca < Pdw < Peg < Pfe ≈ Pol < Pm3 < Pb2 < Pox and Pca < Pdw < Peg < Pfe ≈ Pm3 ≈ Pol < Pb2 < Pox, respectively. ...