EFFECTIVENESS OF VAM FUNGI IN NONSTERILE SOILS BEFORE AND AFTER OPTIMIZATION OF P IN SOIL SOLUTION

Five tropical soils were either not inoculated or inoculated with the vesicular-arbuscular mycorrhizal (VAM) fungus Glomus aggregatum. The d egree to which VAM effectiveness was expressed in the soils was evalua ted prior and after solution P status was adjusted for optimal VAM act ivity. VAM effectiveness determined by monitoring P concentrations of pinnules of Leucaena leucocephala leaves as a function of time and as dry matter yield determined at the time of harvest, indicated that in three of the soils VAM effectiveness was either very restricted or alt ogether unexpressed irrespective of vesicular-arbuscular mycorrhizal f ungal (VAMF) inoculation if soil solution P was not optimized for VAM effectiveness. After P optimization, effectiveness was significantly i ncreased by VAMF inoculation although in four of the soils, densities of indigenous VAMF propagules greatly exceeded that attained by the in oculum after it was mixed with soil. Mycorrhizal fungal inoculation ef fects varied from soil to soil, depending on the extent to which the e ffectiveness of indigenous and introduced endophytes was enhanced by P optimization and the similarity of inherent soil solution P concentra tions to the range known to be optimum for VAM effectiveness. Of the i ndicator variables monitored, VAMF colonization was least sensitive to treatment effects followed by shoot P concentration measured at the t ime of harvest.