Some factors associated with the use of the extractants 2% citric acid and 2% formic acid as estimators of available phosphorus in fertiliser products

The influences of calcium chloride, calcium sulphate, monocalcium phosphate, monosodium phosphate, calcium carbonate, sodium carbonate and iron and aluminium sulphates on the solubilities of two phosphate rocks (both ground and unground) in 2% citric and 2% formic acids have been examined. All of these potential impurity elements can affect the solubility of the phosphate rock through a variety of mechanisms, including common ion effects, complexation, acid-base reactions and reduction. 2% formic acid solubilities were slightly more influenced by these effects than 2% citric acid figures, militating against the use of this extractant for assessing mixed component products at present being manufactured in New Zealand.Solid to solvent and P to solvent ratios were also found to affect the solubilities of raw phosphate rocks and a correction for the differences in total P content of the phosphate rocks to produce a consistent P to solvent ratio in solubility tests is suggested. This study has indicated that alternative methods for assessing phosphate rocks in admixture with other components should be examined in future.