Ability of chemical extraction procedures to assess the agronomic effectiveness of phosphate rock materials

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Abstract
The ability of 7 chemical extraction procedures to assess the agronomic effectiveness of phosphate rock (PR) was evaluated using perennial ryegrass and white clover grown in 6 soils in a glasshouse. Of the conventional, single chemicalextraction procedures evaluated, 2% formic acid appeared to be the most useful. Two 2% citric acid procedures (which produced very similar results) and neutral ammonium citrate (NAC) at the first extraction were poor indicators of agronomic effectiveness. Of the sequential extraction procedures investigated, the 2% formic acid (sum of 2 extractions), both of the 2% citric acid (sum of 3 extractions), and the NAC (sum of 4 extractions) gave the best correlation with agronomic performance. The use of sequential extractions also improved the predictive ability of these extractants in the longerterm, a limitation shown by the conventional, single chemical-extraction procedures. Sequential extraction appears to be the only extraction technique suitable for assessing the agronomic effectiveness of PR containing appreciable amounts of CaCO)3, such as Chatham Rise phosphorite (CRP). for routine analysis, a single extraction procedure is much more suitable. The results of this study indicate that the presently used 2% citric acid procedure is less suitable than extraction with 2% formic acid for assessing agronomic effectiveness of PR.