Phosphate sorbing sites in acid soils. I. An examination of the use of ammonium fluoride as a selective extractant for aluminium-bound phosphate in phosphated soils

Abstract

Soils, aluminium oxides, and iron oxides were phosphated with 1M ammonium phosphate at pH 4.0. These and crystalline iron and aluminium phosphates were extracted at pH 7 and pH 8.5 with ammonium chloride and ammonium fluoride both at 1:300 and 1:2000 sample/extractant ratios. Crystalline iron phosphate was readily distinguished from crystalline aluminium phosphate due to the much lower solubility of the iron phosphate in ammonium fluoride. On the other hand, phosphated iron oxides, especially phosphated haematites, released much of their phosphate to ammonium fluoride and thus could be confused with crystalline aluminium phosphate and phosphated aluminium oxides. The discrimination between phosphated oxides of iron and aluminium although poor was better at the narrower sample/extractant ratio of 1:300, the pH of the extractant being relatively unimportant. Under these conditions, phosphated soils released a larger proportion of their phosphate than did phosphated hydrated iron oxides. This suggests that hydrated iron oxides do not dominate sorption in these soils. X-ray data were consistent with this view. The relative importance of iron and aluminium in phosphated soils containing appreciable amounts of gibbsite, kaolin, and haematite could not be assessed using ammonium fluoride extractions. Since ammonium fluoride does not readily discriminate between phosphated oxides of iron and aluminium, it is unwise to assume that this reagent dissolves mainly aluminium-bound phosphate from soils fertilized with phosphate.