Vetch-oat mixture is used as a so-called "green manure" because of what and belongs to a group of plants - green manure. Green manure, sow seed or planting between the yield of crops (vegetable farm types, which aims economy as a whole).
Then, after sufficient shoot green manure, it does not collect, mixed with soil tillage (or use ready-made). As a result, the soil is saturated with such essential substances such as phosphorus and nitrogen, and humus (proteins, sugars, starch). Green manure can reduce the amount of manure being injected into the soil.
Peregnivaya, it makes the soil less dense, thereby acting as a leavening agent. Also stopped by the growth of unwanted species of plants - weed crops. It lays the ground for mulching: the soil itself becomes more moist and acquires the ability to retain stored moisture longer, which is why in the mulch develop better earthworms.