Relationships between soil microorganisms and nutrient contents of Vetiveria zizanioides (L.) Nash and Vetiveria nemoralis (A.) Camus in some problem soils from Thailand

Relationships between microbial population and soil properties were determined in three problem soils, viz., acid sulfate, shallow and saline soils, in which were planted two groups of vetiver grass by comparison with soils not planted with vetiver. Results showed that microbial populations in vetiver rhizospheres increased, with a greater increase at the 0-30 cm depth than the 30-60 cm depth. Cellulolytic bacteria and actinomycetes were the most numerous inhabitants, in the range of 106 to 108 cells g−1 soil. Cellulolytic fungi, Azotobacter, phosphate solubilizing bacteria (PSB) and phosphate solubilizing fungi (PSF) had similar microbial populations of 102 to 104 cells g−1 soil. Organic matter (OM), macro- and micronutrients, and moisture content where vetiver grass was grown were higher than control, especially in the shallow soil, where OM, N, P, Ca and S content were increased by a factor of two. Investigation of interactions indicated that both PSB and arbuscular mycorrhizal fungi (AMF) correlated significantly with nutrient availability and increase of OM and moisture content of soil under Vetiveria zizanioides.