Evaluation of double extraction techniques as solubility tests for fertiliser products containing phosphate rock components

Alternative extraction techniques for assessing the ‘available P’ content in multi-component fertilisers such as partially acidulated phosphate rocks have been examined. Two types of double extraction method have been investigated, one involving sequential extraction procedures, and a second examining both the initial fertiliser and its residue independently, using either water or cold neutral ammonium citrate as the first extractant, and either 2% citric or 2% formic acid as the second solvent. The latter method, which used cold neutral ammonium citrate as the initial extractant, produced the most consistent results and provided a reproducible assessment of the solubility of the phosphate rock residue; it was not obvious which second solvent was preferable. Comparisons between the original phosphate rocks and their extracted residues indicated that there were many subtle influences which affected the solubility of the phosphate rock components, but a general ‘deactivation’ of phosphate rock residues on acidulation as previously suggested was not apparent. Before any particular method can be recommended as an alternative to existing one step extraction techniques it is necessary to confirm its validity through agronomic trials.