Phosphorus status of some highly weathered soils in peninsular Thailand and availability in relation to citrate and oxalate application

Chemical properties and phosphorus forms in Kohong (Kh: Typic Paleudults; coarse-loamy siliceous), Hat Yai (Hy: Typic Paleudults; clayey-skeletal, kaolinitic), and Ban Then (Bh: Typic Haplorthods; sandy, non-cemented) soil series collected according to genetic horizons in southern peninsular Thailand were analyzed including fractionation. Phosphorus fertility of these soils was low because the contents of available P (2-3.5 mu g g(-1)) and total P (39-162 mu g g(-1)) of the surface soil were low. The Hy soil, with a finer texture, contained a higher level of total P (145-238 mu g g(-1)) than the Kh soil (35-43 mu g g(-1)). Most of P in these soils consisted of organic P (48-71%) whose content decreased with the soil depth, while the content of occluded P (2-41%), next in proportion, increased according to the soil depth. Calcium phosphate (Ca-P) accounted for only a small proportion (0-5%) and the amounts of aluminum phosphate (Al-P) and iron phosphate (Fe-P) mere larger (1-14%). In the Ph soil, P forms varied considerably in the soil profile and Al-P was dominant (25-51%), followed by organic P (22-45%), In the surface soil of Hy and spodic horizon (90-105 cm) of Bh, the influence of citrate and oxalate application on the availability of soil P was examined. It was found that inorganic and organic P can be extracted by the addition of citrate or oxalate. Phosphorus, as well as Al and Fe, was released into the soil solution in proportion to the increase of the citrate and oxalate application rates (0-50 mu mol g(-1) soil).