Potential of native phosphate solubilizing bacteria isolated from the rhizosphere of economic crops and vermicast in Northeast Thailand to solubilize insoluble phosphates under in vitro conditions

Although soils generally contain a large amount of total P, only a small proportion is immediately available for plant uptake making it a major constraint on crop production in many tropical countries. Free - living bacteria and fungi can mobilize o rthophosphate from either organic or inorganic P sources such as Phosphate Rock (PR). These phosphate - solubilizing microorganisms (PSM) are characterized by their capacity to solubilize precipitated forms of P, the main P ingredient in PR and could be good bio - fertilizers for improving phosphorus plant nutrition. The present study examined phosphate solubilising bacteria (PSB) isolated from cassava, groundnut , rubber tree, sunchoke , rice, rice - soybean, rice - soybean - corn and rice - chili fields and vermicast o f earthworm varieties in Northeast of Thailand, where soils are mainly sandy and P - deficient. PSB isolates were tested by using different P sources [Tri - calcium Phosphate (Ca 3 (PO 4 ) 2 ), Ferric Phosphate (FePO 4 ) and Aluminium Phosphate (AlPO 4 )] on specific cu lture media (National Botanical Research Institute Phosphate Growth Medium, NBRIP). Our results showed that five of the PSB isolates from economic crops and vermicast of earthworm varieties solubilised a significantly (P 0.01) higher amount of AlPO 4 and FePO 4 over the uninoculated control. The highest activity of solubilization was achieved for AlPO 4 followed by FePO 4 which are the main forms of insoluble phosphates in acidic sandy soils. We found that PSB isolated from vermicast of earthworm varieties Ph eretima posthuma and Eudrilus eugeniae were able to solubilize both AlPO 4 and FePO 4 at relatively high rates (up to 1,918.49 mgP/l ), in contrast, PSB isolated from cassava, rice - soybean and groundnut field soil tended to have lower solubilisation rates for FePO 4 . None of the isolates tested were able to solubilize CaPO 4 . Finally, IAA production was observed only in PSB isolated of E. eugeniae vermicast and rice - soybean field soil. These results highlight variability of specific PSB isolates from different r hizospheres and vermicast of earthworm varieties and provide essential information for the management of soil fertility. (Résumé d'auteur