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

Abstract

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), Egnér–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. Very low amounts of P were extracted by the Pdw and Pca tests, making their use for agronomic purposes limited. The abilities of the Pfe and Pol tests to extract P in study soils were similar. The Peg test showed less ability to extract P than the Pb2 and Pm3 tests. The Pox test showed strong ability to dissolve P bound by amorphous iron and aluminum. The high positive correlation between the Pfe test and labile bioavailable P may warrant the use of Pb2, Pm3, Peg, and Pol tests to predict the amounts of labile bioavailable P for Angolan soils. Nevertheless, the little amounts of P extracted by Peg test could make it difficult to establish fertility classes. The main sources of P extracted by studied tests were mostly surfaces of active noncrystalline (amorphous or poorly crystalline inorganic) and organically complexed forms of iron and aluminum. Future work is necessary to assess the agronomic and the environmental meaning of these tests in Angolan soils.