THE ROLE OF ROCK-PHOSPHATE-SOLUBILIZING FUNGI AND VESICULAR-ARBUSULAR-MYCORRHIZA (VAM) IN GROWTH OF WHEAT PLANTS FERTILIZED WITH ROCK PHOSPHATE

A total of 36 fungal species isolated from soil were tested for their ability to solubilize rock phosphate (RP) in agar plates. Most of thes e fungi were non-rock phosphate solubilizers, but two isolates, Asperg illus niger and Penicillium citrinum, had high activity. Liquid cultur e experiments revealed that both fungi caused a remarkable drop in pH of culture media and solubilized considerable amounts of phosphate. Th e effects on wheat of inoculation with vesicular-arbuscular mycorrhiza l fungi and rock-phosphate-solubilizing fungi and fertilization with r ock phosphate were studied in sterilized pot soils, nonsterilized pot trials and in field plot soils. Rock phosphate fertilization and inocu lation with Glomus constrictum and rock-phosphate-solubilizing fungi ( A. niger and P. citrinum) significantly increased dry matter yield of wheat plants under all experimental conditions. However, the effect wa s more evident in non-sterilized pot soils and in the field than in st erilized pots. Rock phosphate had no significant effect on the total p hosphorus content of plants grown under pot conditions but it was sign ificantly increased in field plots; the effect of inoculation with fun gi (G. constrictum, niger and citrinum) on plant phosphorus was closel y related to this in dry matter production. The greatest positive effe ct on growth and phosphorus contents of wheat plants was recorded in t he treatments that received rock phosphate and were inoculated with a mixed inoculum of the three microorganisms used, followed by dual inoc ulation treatments of G. constrictum plus either A. niger or P. citrin um.