Soil organisms interact with plants in diverse ways. Some organisms increase plant growth, while others decrease it. Some alter root structure but others have little effect. Microorganisms can also have important influences on the way that roots branch and grow in soil. These structural modifications affect the uptake of water and nutrients by plants.
Plant roots are of course the main structures through which soil organisms and plants interact. Roots provide a concentrated supply of carbon and energy for many heterotrophic soil microorganisms. In turn, soil animals that feed on fungi and bacteria multiply in response to the higher food supply around roots. Some organisms can invade plant roots.
Living roots are particularly important for many fungi, especially fungal pathogens and mycorrhizal fungi. Most root pathogens form associations with specific plants. Other pathogens colonise roots of a wide range of plants. Arbuscular mycorrhizal fungi generally form non-specific associations with many plant species. Ectomycorrhizal fungi are more specific in the types of roots they colonise.
Before this section considers the effects of soil biota on plant roots, it first examines how plant roots shape the environment of soil organisms.