The use of chemical solubility tests in comparing phosphate fertilisers

The standard solubility tests as used in New Zealand for assessing single superphosphate have been applied to alternative high analysis fertilisers such as triple superphosphate and partially acidulated phosphate rocks using phosphoric acid. Under differing experimental conditions it has been established that the phosphate rock component has a variable solubility in 2% citric acid at all levels, and in neutral ammonium citrate at high levels of PR content, making comparisons between percentages of soluble P derived from different fertilisers inappropriate for chemical or agronomic assessment. Only water, which extracts all the monocalcium phosphate component but none of the phosphate rock, can be used to compare the efficiency of the acidulation process, and neutral ammonium citrate can be used to assess the amount of phosphate rock remaining when the acidulation level is in excess of 50%. 2% formic acid produced almost identical results to 2% citric acid for partially acidulated materials and therefore cannot be recommended as an alternative, improved extractant for fertilisers containing large proportions of residual phosphate rock.