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Laboratory investigations of the dissolution of North Carolina phosphate rock (PR) in soil, as affected by soil texture, were conducted using soils created by mixing various proportions of the < 45 pm and > 45 pm fractions of a lateritic soil. The dissolution of PR in soil, and the resulting changes in plant available P, were estimated by measuring increases in exchangeable Ca (ACa) and NaHC0,-extractable P (AP,), respectively. The total fertilizer phosphate dissolved (AP) was calculated from ACa values based on the assumption that dissolution of PR released amounts of Ca and P in a constant ratio determined by the stoichiometry of the PR. AP increased with increasing content of < 45 pm particles. APb also increased with the percentage of < 45 pm fraction, with about 20% of AP being recovered as APb. A smaller proportion of dissolved P was soluble in NaHCO, for the soil comprising only the <45 pm particles. The increase in AP with increased percentage of < 45 pm soil was probably due inter alia to the greater P-sorption, Ca-sorption and the pH buffering capacity of the fine grained soil constituents.

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Chemical measurements of the dissolution of reactive North Carolina phosphate rock (PR) in a lateritic podzolic soil adjusted to different pH values or amended to different P retention capacities were compared with plant response data. Soil pH adjustment consisted of incubating soil mixed with dilute HCl or solid SrCO3. The P retention capacity was adjusted by adding different amounts of synthetic goethite. Clover was grown as a test crop under glasshouse conditions on soils treated with no P and 800 �g Pg-1 soil as PR. The dissolution of PR, measured as the increase in soil exchangeable Ca (�Ca) at harvest time, increased by 46% for a decrease in soil pH from 5.78 to 4.55. Both dry weight and P content of clover tops showed no response to the greater PR dissolution at low soil pH, although twice as much P was bicarbonate-soluble at pH 4 55 compared with pH 5.78. This increase in bicarbonate-soluble P was only partly due to the greater dissolution of PR. Increasing the goethite content of the soil by 9% resulted in a 107% increase in PR dissolution but induced a 54% decrease in the bicarbonate-soluble P level compared with the control soil and consequently reduced both dry weight yield and P content of clover tops by 19% and 34% respectively.