A TEST FOR PHOSPHATE ROCK REACTIVITY IN WHICH SOLUBILITY AND SIZE ARECOMBINED IN A DISSOLUTION RATE-FUNCTION

A test for phosphate rock (PR) reactivity has been developed based on a representation of certain fundamental properties of the rocks design ated the Dissolution Rate Function. The function and properties have b een shown to be those important in models of the dissolution rate of P Rs in soil. The properties are size or mass distribution with size, so lubility product, P content and particle density. In practice the solu bility product is measured in terms of the P concentration attained in a standard solution, simulating that in an average soil with respect to pH, Ca and ionic strength. The test compares favourably with those using citric and formic acids, using published results for the relativ e agronomic effectiveness of 11 PRs, both unground and ground. The agr onomic data also indicated that particle sizes < 0.15 mm can be regard ed as equivalent to the size range of 0.10 to 0.15 mm.