The role of mycorrhizae in soybean growth in P-deficient soil in the humid tropics

Studies of four hundred and sixty-five soybean (Glycine max (L.) Merr.) varieties from different parts of the world were conducted to determine their ability to grow in P-deficient soils and to determine the effect of vesicular-arbuscular (VA) mycorrhizal fungi on the ability of soybeans to grow in such P-deficient soils in the humid tropics.; A field experiment was conducted to investigate the effect of addition of 0 and 60 kg fertilizer P/ha on soybeans. The results showed that soybean varieties differed markedly in their response to P, the time of response to P, grain yield potential, nodulation, mycorrhizal colonization, P absorption, and N{dollar}sb2{dollar} fixation. Phosphorus addition increased the grain yield of some soybean varieties by {dollar}ge{dollar}200%, caused a decline in grain yield of {dollar}ge{dollar}40% in other varieties, and had no effect on grain yield in other soybean varieties. The addition of P fertilizer consistently reduced mycorrhizal infection in the twenty-four soybean varieties tested. The results show that soybean varieties can be obtained that produce reasonably high yields in P-poor soils without P fertilizer.; The symbiosis of twelve different VA mycorrhizal fungi with two soybean varieties was also investigated. The results indicated that VA mycorrhizal fungi differed in their infectivity as well as in their effects on plant growth. Depending on plant variety, the fungi were symbiotic, parasitic or had no effect on plant growth, suggesting the importance of evaluating the compatibility of VA mycorrhizal fungus and plant host.; Studies were conducted of the effect of spore densities of mycorrhizal fungi, interactions between VA mycorrhizal fungi and Bradyrhizobium strains, and fungal inoculation methods on root colonization by the fungi and soybean growth. Significant differences in root colonization or plant growth responses among spore densities were not observed. Three different Bradyrhizobium strains had similar effects on root colonization by two VA mycorrhizal fungi tested. Measurements of plant fresh weight revealed synergistic, antagonistic and no interactions between Bradyrhizobium strains and Glomus species. Placing spores of mycorrhizal fungi under the seeds gave better plant growth, nodulation and root colonization than mixing spores in the soil. The effect of spore-placement methods depended on Bradyrhizobium strain and mycorrhizal species. Phosphorus and N contents of soybean variety V295 were consistently improved by VA mycorrhizal inoculation in P-deficient soils.