Cover crops

Cover crops are often called "living mulch". Cover crops are plants which are not intended to be harvested for human food, but rather are used to "feed" the soil and surrounding biota. Different cover crops have varying benefits for crops and soils including, but not limited to: weed or pest suppression, attracting pollinators, addition of biomass, relief of compaction and addition of nitrogen.

Weed and pest suppression
Allelopathy

Reducing plant-eating nematode populations

Attracting pollinators and beneficial insects
Some cover crops are attractive nectar sources for pollinators. Clover, buckwheat, vetch, mustards...

Biomass addition
Some plants produce copious amounts of biomass during the growing season which can create a mulch at the surface to suppress weeds and/or large amounts of roots belowground. Both above and belowground plant parts increases the organic carbon content of the soil when residues are incorporated. Sorghum, rye, oats

Relief of compaction
High traffic areas, such as inter-row spaces and pathways for equipment and people, may become compacted. Certain plants are very good at penetrating compacted soil. Sorghum sudangrass, forage radish

Addition of nitrogen
Nitrogen (N) is typically the most-limiting plant nutrient in soils. Paradoxically, dinitrogen gas (N2) makes up 78% of Earth's atmosphere. Fortunately or unfortunately, this form of nitrogen is essentially inert -- it is not water-soluble and the two nitrogen atoms are linked by three high-energy bonds. Thus, there is a large activation energy which needs to be overcome to create a water-soluble form plants can take up.

Certain soil dwelling bacteria are capable of "fixing" atmospheric N into a plant-available form, namely: ammonium (NH4+). Ammonium can be converted to Nitrate (NO3-) which is also a plant available form. These bacteria may be free-living or in a mutualistic association with specific types of plants.

Some plants (namely the legumes) have the ability to form a symbiotic association with N-fixing bacteria where they provide a place for the bacteria to live (called a root nodule). The root nodule is an interesting structure, because it is engineered to exclude oxygen. Oxygen is highly toxic to the enzymatic process which fixes the nitrogen. Essentially, these plants exchange signals with the bacteria in soil, encapsulate them in a nodule and "farm" them to produce nitrogen for them. In exchange, the plants provide the microbes with carbon compounds they make using photosynthesis.