Dissolution characteristics of phosphate rocks in latosols withdifferent parent materials in rubber plantations

Latosols are very acidic, often extremely deficient in phosphorus (P) and have strong P sorption capacity. The state of P in latosols is one of the main limiting factors of growth and yield of rubber trees. Several studies have suggested that phosphate rocks (PR) could be as effective as water soluble phosphate fertilizers in increasing plant yield and improving soil P content in tropical regions. Although water soluble P is the mainly P fertilizer used for improving the state of nitrites in rubber trees, rational use of PR has not been intensively researched. PR application proved promising particularly in tropical plantations and organic agriculture in China. This research simulated and assessed dissolution and availability extents and rates of two PR forms in different latosols from five parent materials. The research also identified major soil factors controlling PR dissolution and availability in tropical soils. The results provided useful information on rational application of PR in tropical crops in China. Thus the dissolution and availability characteristics of two PR forms Kunyang phosphate rocks (KPR) from Yunnan Province and Wucun phosphate rocks (WPR) from Jiangxi Province were investigated in laboratory incubation of ten rubber plantation latosols with different parent materials. PR was added to tropical soils at rates of 0 mg(P).kg^-1 (control) and 500 mg(P).kg^-1. The soil-PR mixtures were placed in plastic containers and incubated at moisture content of ≈60% soil water-holding capacity, 25 ℃ and for 35 days. Results showed that dissolution of two forms of PR increased in all the soils with increasing incubation period. The two PR dissolution kinetics in ten types of latosols were best described by the Elovich equation, followed by Langmuir equation. The Mitscherlich equation did not suitably describe PR dissolution in latosols. Average maximum dissolution potentials of the two PRs in latosols from basalt were 2.16, 1.73, 2.49 and 2.39 times of latosols from sand shale, granite, gneiss and nerite, respectively. Maximum dissolutions of the two PRs in latosols were positive correlated with soil organic matter, soil total hydrolytic acidity, free iron oxide content and cation exchange capacity (CEC). Dissolution rates of the two PRs in latosols were positive correlated with soil total hydrolytic acidity, free iron oxide content and CEC. Stepwise multiple regression indicated that soil free iron oxide content was the soil factor with the most significant control over maximum dissolutions of the two PRs. CEC and soil adsorption parameter (K) were respectively the soil factors with the most effect on KPR and WPR dissolution rates in latosols. Soil available P increased after PR application, but the increment was different for the various soils and sampling periods throughout the incubation process. This study thus indicated that better effect was obtainable when PR was applied first in latosols from basalt. Compared with KPR, it was possible for low grade WPR to have the same effect regarding increasing soil available P in latosols.