To determine the taxonomic relationship between Thai soybean bradyrhizobia and soybean bradyrhizobia from other regions, a total of 62 Bradyrhizobium strains were isolated in Thailand. The genetic diversity of the strains was examined with reference to 46 Japanese and 15 USDA strains. The degree of sequence divergence in and around common nod gene regions of the 123 strains was estimated by restriction fragment length polymorphism analysis using the Bradyrhizobium japonicum USDA 110 common nodDYABC gene probe. The phylogenetic grouping of the strains resulted in four major clusters. Cluster 1 comprised the Japanese and USDA strains, which originated in temperate regions, whereas clusters 3 and 4 comprised the tropical Thai strains. Cluster 1 strains comprised the DNA homology groups I and Ia, and hence, were classified as B. japonicum. Cluster 2 strains were in the DNA homology group II, and hence, were classified as Bradyrhizobium elkanii. Clusters 3 and 4 strains, however, did not correspond to any known DNA homology groups. These results indicate that Thai soybean bradyrhizobia are distantly related to B. japonicum and B. elkanii.

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The kinetics and heats of phosphatc adsorption were measured o n the <2 pm Na-saturated fractions of thrcc allophanc-rich soils from Japan. Between 5 0 and 2250 pmol P g -' as sodium phosphatc were added to the soil fractions at pH 5 and pH 7. and at initial concentrations of 5 and 25 x M to avoid aluminium phosphate precipitation.An initial 'instantaneous' adsorption associated with exposed sites and, simultaneously. two inverse exponential rates of adsorption on internal and freshly forming external sites were observed. These rates are attributed to changes in the microstructure of allophane and to the desorption of organic matter held on allophanic surfaces. This interpretation is strongly supported by corresponding changes in the heats of adsorption with time. Calorimetry clearly indicates that when very large amounts of phosphate are added, new and very reactive surfaces are progressively exposed.More phosphate was adsorbed when the soil was acid and when the soil contained less organic matter.

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