Soybean [Glycine max (L.) Merr.] makes use of two important symbiotic microbial systems: the rhizobial system, in which Bradyrhizobium japonicum fix atmospheric N2 in root nodules, and the vesicular-arbuscular mycorrhizal system, in which VAM fungi increase uptake of phosphate and/or other nutrients from the soil or improve plant drought tolerance. Research has confirmed the advantage of bradyrhizobial and mycorrhizal associations in N and P deficient soils; however, these associations may be inhibitory, or have little effect on plant growth in well-fertilized soils. In Iowa over the past 30-40 years, farmers have been adding large quantities of fertilizers to get maximum yields. The question is, has increased soil fertilization selected against optimal symbiotic microbial associations? If so, how can we improve the present microbial associations for possible use by legumes in low-input, sustainable agricultural systems. Two studies were conducted to investigate the levels of VAM fungal colonization and sporulation in Iowa soils, and the possible influence of P-fertilization on mycorrhizal fungal efficiency and subsequent interaction with bradyrhizobia. The first part, entitled "VAM Fungal Colonization of Soybean and Spore Populations in Iowa Soils," was a field survey of the soybean rhizosphere that evaluated the percentage VAM fungal colonization in soybean roots and the populations of associated VAM fungal genera. This part contains an Introduction, Materials and Methods, Results and Discussion, Conclusions, and References Cited sections. Tables also are included in the text.