THE AGRONOMIC EFFECTIVENESS OF REACTIVE PHOSPHATE ROCKS 2 - EFFECT OFPHOSPHATE ROCK REACTIVITY

The chemical reactivity of apatite phosphate rock (PR) from Sechura (B ayovar), North Carolina, Egypt (Hamrawein), Morocco (Khouribja) and Qu eensland (Duchess), was estimated using the following methods (indices ): percentage of the phosphorus (P) present in the PR that is soluble in one extraction with 2% citric acid and 2% formic acid, and variatio n in crystal chemistry determined using X-ray diffraction to measure t he length of the unit cell a dimension. The indices were related to th e agronomic effectiveness of the PRs for pasture using percentage of t he maximum (relative) yields for PR and superphosphate. At most sites there was a statistically significant trend for the agronomic effectiv eness of the PRs to increase with increasing reactivity of the PRs as assessed by the 3 indices. However, this was not the case for sites wh ere: (i) leaching of water-soluble P from freshly-applied superphospha te occurs in wet years; and (ii) the agronomic effectiveness using SV5 0 values of all the PRs was, uniformly low so that no differences betw een the PRs could be detected. The relationship between the agronomic effectiveness (SV50) Of PR and chemical reactivity of the PR were gene rally different each year and in most cases this variability was relat ed to the variation in total annual rainfall. From this study it is co ncluded that PRs in which greater than 65-70% of their total P content is soluble in 2% formic acid are likely to be highly reactive and the refore agronomically effective P fertilisers for direct application to appropriate soils in suitable environments.