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

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.