Naturally occurring soil microbes may be used as inoculants to maintain crop yields despite decreased resource (water and nutrient) inputs. Plant symbiotic relationships with mycorrhizal fungi alter root aquaporin gene expression and greatly increase the surface area over which plant root systems take up water and nutrients. Soil bacteria on the root surface alter root phytohormone status thereby increasing growth, and can make nutrients more available to the plant. Combining different classes of soil organism within one inoculant can potentially take advantage of multiple plant growth-promoting mechanisms, but biological interactions between inoculant constituents and the plant are difficult to predict. Whether the yield benefits of such inocula allow modified nutrient and water management continues to challenge crop biotechnologists.