“Green Manure” refers to the rototilling of a forage crop to improve the soil. This is done while the crop is green or soon after flowering. It is often confused with “Cover Crops”, which are grown primarily to prevent soil erosion by wind and water. Both can help suppress weeds, reduce insect pests and diseases, and in the case of legumes, fix nitrogen in the soil. They can both be planted any time of year but are commonly planted in the fall.
A major benefit from Green Manure is the addition of organic matter to the soil. During the breakdown of organic matter by microorganisms, compounds are formed that are resistant to decomposition-such as gums, waxes, and resins. These compounds- and the mycelia, mucus, and slime produced by the microorganisms- help bind together soil particles as granules, or aggregates.
Alfalfa and other deep-rooting green manures scavenge nutrients from the subsoil and move them upwards to the surface rooting zone as well loosening and aerating the soil.
The benefits to growing Green Manure are that well-aggregated soil tills easily, are well-aerated and have a high water infiltration rate. Increased levels of organic matter also influence soil humus. Humus is the substance that results as the end product of the decay of plant and animal materials, good stuff.
As for Cover Crops, in addition to fixing Nitrogen from legumes (such as white, red, or strawberry clover, fava beans, vetch or lupine) they help recycle other nutrients like Potassium, Calcium, Magnesium, Sulfur, to mention a few. Annual Ryegrass, Buckwheat, Lupine and Sweetclover extract Phosphorus from soil.
Annual Ryegrass (Lolium multiflorum, also commonly called ‘Italian Grass’) should not be confused with Rye, which is a cereal rye, a completely different species. This annual cool-season grass was introduced to the USA from Europe. It seeds and establishes quickly under a wide range of soils and climates. It is tolerant of wet soils and temporary flooding, and will germinate in cooler soils than most other cover crops. It will grow in sandy soils but is better adapted to heavy clay or silty soils. Annual ryegrass can be used to keep slopes from eroding or to enrich a veggie bed for spring.