Adaptations of tropical plants to acid soils

It is known that the tropical pastures or grasses; Arachis pintoi, Brachiaria ruziziensis, Panicum maximum, Panicum repensi, Pueraria phaseoloides, and Stylosanthes hamata, and the tropical trees; Acacia mangium, Anacardium occidentale, Hevea brasiliensis, and Melaleuca cajuputi adapt well to acid soils which have low phosphorus (P) and high aluminum (Al). In this study, top and sub soils were collected from highly weathered soils in peninsular Thailand for chemical analysis and phosphorus fractionation, and the adaptive mechanisms of these plants were examined from the aspect of phosphorus utilization efficiency and root exudation of organic acids and acid phosphatase by culturing plant seedling in complete, phosphorus deficient (-P), high Aluminum (+Al), and P deficient / high Al (-P/+Al) solutions. Results showed that both available P and total P in soils were low, and organic P was the main fraction. Fe-P and Al-P were the dominant inorganic P sources in these soils. The secretion of acid phosphatase increased distinctly in -P treatment from roots of Stylosanthes hamata only. Tropical trees that adapt well to these soils were able to secrete oxalic and citric acids to acquire P by solubilizing of Fe-P and Al-P, whereas tropical pastures or grasses showed high P utilization efficiency and high root-shoot ratio by P deficiency.