Close up view of plant root (a) the mucigel layer is shown containing some bacteria and clay particles on the outside of the root. Also shown is a mycorrhizal fungus sending out its rootlike hyphae into the soil. (b) Soil aggregates are surrounded by thin films of water. Plant roots take water and nutrients from these films. Also shown is a larger aggregate made up of smaller aggregates pressed together and held in place by the root and hyphae.

Healthy plant roots are essential for good crop yields. Roots are clearly influenced by the soil in which they live and are good indicators of soil quality. If the soil is compact, is low in nutrients or water, includes high populations of root pathogens, or has other problems, plants will not grow well. On the other hand, plants also influence the soil in which they grow. The physical pressure of roots growing through soil helps form aggregates by bringing particles closer together. Small roots also help bind particles together. In addition, many organic compounds are given off, or exuded, by plant roots and provide nourishment for soil organisms living on or near the roots. The zone surrounding roots is one of especially great numbers and activity of organisms that live off root exudates and sloughed-off cells. This increased activity by microorganisms, plus the slight disruption caused as roots grow through the soil, enhances the use of active (“dead”) organic matter by organisms—also enhancing nutrient availability to the plant. A sticky layer surrounding roots, called the mucigel, provides close contact between microorganisms, soil minerals, and the plant (figure 4.3). Plant roots also contribute greatly to organic matter accumulation. They are usually well distributed in the soil and may be slower to decompose than surface residues, even if incorporated by plowing or harrowing. For plants with extensive root systems, such as grasses, the amount of living tissue below ground may actually weigh more than the amount of leaves and stems we see above ground.