The determination of total phosphorus in soils and parent materials

The amounts of total P extracted by Na 2 CO 3 fusion and by HF and HClO 4 digestion procedures from selected New Zealand soils and parent materials were compared. It is suggested that Na 2 CO 3 fusion is the most reliable procedure. Digestion with HClO 4 gave low results with strongly weathered materials and with samples which contained apatite inclusions within constituent primary minerals; whereas digestion with HF gave slightly low results only with samples which contained included apatite. Incomplete extraction of P by HClO 4 and HF digestion procedures from samples which contained apatite inclusions was ascribed to the partial resistance to attack of the host minerals. Because of occlusion within highly crystalline iron and aluminium oxides, secondary inorganic P in strongly weathered soils was less readily extracted by HClO 4 digestion than by Na 2 CO 3 fusion or HF digestion.