Dissolution of phosphate rock fertilisers in some soils of Okinawa, Japan

The ability of phosphate rock (PR) to dissolve rapidly in soil is a primary concern in its direct application as P fertiliser. The dissolution of 4 PR materials (Togo, South Africa, Florida, Morocco PRs) in 15 soil samples in Okinawa was investigated in a closed-incubation system for 7 days. The fer tilisers were mixed with the soils at rates of addition of 600-1200 mu g Ca /g soil. The extent and rate of dissolution of the PRs were determined by m easuring the increase in extractable Ca of the fertilised soils compared wi th unfertilised soils, i.e. the delta Ca (Delta Ca) technique. Generally, t he amounts of dissolution of 3 of the PRs were very low (mean 6.7% for Togo PR, 13.6% for South Africa PR, and 20.8% for Florida PR). However, Morocco PR dissolved to an appreciable extent (mean 60.8%), suggesting that it can be an alternative P source, especially in the red and yellow soils of Okin awa. Soil properties identified as affecting dissolution were different for the different PRs. In order to predict the suitability of Morocco PR for O kinawan soils, the extent of its dissolution was related to soil properties in a multiple regression analysis. Results indicated that the best regress ion model for predicting the amount of dissolution was the combination of p H, Ca saturation, and Truog P (r(2) = 0.55**). Measured values of percentag e dissolution of Morocco PR were significantly correlated with calculated p ercentages (r = 0.844***), indicating that the equation obtained could offe r a rapid estimation of amount of dissolution of Morocco PR in Okinawan soi ls.