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 these fungi were non-rock phosphate solubilizers, but two isolates, Aspergillus niger and Penicillium citrinum, had high activity. Liquid culture experiments revealed that both fungi caused a remarkable drop in pH of culture media and solubilized considerable amounts of phosphate. The effects on wheat of inoculation with vesicular–arbuscular mycorrhizal fungi and rock-phosphate-solubilizing fungi and fertilization with rock phosphate were studied in sterilized pot soils, nonsterilized pot trials and in field plot soils. Rock phosphate fertilization and inoculation 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 was more evident in non- sterilized pot soils and in the field than in sterilized pots. Rock phosphate had no significant effect on the total phosphorus content of plants grown under pot conditions but it was significantly increased in field plots; the effect of inoculation with fungi (G. constrictum, A. niger and P. citrinum) on plant phosphorus was closely related to this in dry matter production. The greatest positive effect on growth and phosphorus contents of wheat plants was recorded in the treatments that received rock phosphate and were inoculated with a mixed inoculum of the three microorganisms used, followed by dual inoculation treatments of G. constrictum plus either A. niger or P. citrinum.