An assessment of phosphorus soil tests for some tropical acid soils in Southern Thailand

Phosphorus deficiency is a major problem for tropical acid soils in southern Thailand, thus P fertilizer and lime are usually applied to soils to improve crop yield. The objective of this study was to assess 4 different P soil tests (Bray 1, Bray 2, Double acid and Mehlich 3) for predicting P status of some tropical acid soils in southern Thailand. Pot experiment was conducted in five acid soils [Bangnara (Ba): Clayey, kaolinitic, Typic Paleaquults; Ao Luk (Ak): Clayey, kaolinitic, Rodic Paleudults; Na Thon (Ntn): Clayey, mixed, Typic Tropudults; Phuket (Pk): Clayey, kaolinitic, Typic Paleudults; Kohong (Kh): Coarse-loamy,siliceous, Typic Paleudults] which have different parent materials using completely randomized design with 3 replications. The treatments included 7 different P application rates of limed and unlimed soils. Corn was grown for 4 weeks, then dry matter weight, P content in corn, P uptake by corn and soil test P were determined. Highly significant linear correlations among soil P tested by Bray 1, Bray 2, Double acid and Mehlich 3 were found in most soils (r = 0.909** - 0.998**), but in Ba soil the correlation between soil P tested by Double acid and other methods was relatively low (r = 0.723** - 0.745**). Soil P tested by Bray 1, Bray 2 and Mehlich 3 were significantly correlated with P application rate (R2 = 0.802** - 0.975**), P content in corn (R2 = 0.797** - 0.944**) and P uptake by corn (R2 = 0.671** - 0.909**) in all soils. However, the relationships between soil P tested by Double acid and P content in corn or P uptake by corn were quite low in limed treatments of Ba, Ak, Ntn and Pk soils. These results indicate that Double acid method was not appropriate for tropical acid limed soils. Bray 1 and Mehlich 3 could be used for predicting P availability as well as Bray 2 for tropical acid soils in southern Thailand which are limed or unlimed with P fertilizer applications. Therefore, it is suggested that Bray 1 and Mehlich 3 are very promising options for predicting P status of tropical acid soils in southern Thailand due to the 4 times reduction of HCl used for Bray 1 compared with Bray 2, whereas Mehlich 3 can be used for multi-element analysis.