Evaluation of vesicular-arbuscular mycorrhizal fungi in diverse plants and soils

A regional study was made to identify vesicular-arbuscular mycorrhizal (VAM) fungi effective in promoting plant growth in diverse plant and soil systems. Eight cooperators in six states of the eastern United States evaluated six VAM fungal isolates on soybean (Glycine max L. Merr.) and sorghum (Sorghum bicolor L. Moench) in a shared soil and in at least one regional soil from each location. Plants were grown with high VAM inoculum densities (minimum of 20 VAM propagules ml^-1) for 42-57 days in pasturized soils in greenhouses or growth chambers. Shoot and root dry masses, total and colonized root lengths and shoot-P concentrations were determined at harvest. Under the experimental conditions tested, the VAM fungal isolate was more important than the soil or host plant in determining effectiveness. In the shared soil, inoculation with two isolates of Glomus (GE329 and GENPI) resulted in the greatest shoot masses for soybeans, while the same two isolates and GE312 provided maximum response in sorghum. In the regional soils, GE329 and GENPI had the widest range of growth promotion with both soybean and sorghum; however, for both plant species the mycorrhizal response was greatest in soils with less than 10 mg extractable P kg^-1. For soybeans, colonized root length was not related to VAM growth response. For sorghum, there was a positive correlation between colonized root length and plant growth response. We conclude that VAM isolates exist which are effective in promoting plant growth over a range of edaphic and host conditions.