Colonization rates and growth responses of soybean plants infected by vesicular–arbuscular mycorrhizal fungi

The interactions between soybean plants and two species of mycorrhizal fungi were examined. The number of infection units in young seedlings was found to increase in the presence of increasing quantities of Glomus fasciculatus inoculum, until a maximum was reached. Soybean seedlings remained free of mycorrhizal infection during early stages of development. First infection units appeared 10–12 days after planting which was also the approximate time that rhizobium root nodules and tertiary roots were first observed. The response to colonization by vesicular–arbuscular mycorrhizal fungi was evident as early as 6 weeks after planting.