The use of Green Manure in Crop Rotation Plans

The purpose of crop rotation is to grow crops in families (roots, brassicas & legumes) by leaving nutrients in the soil for the next crop or using up certain nutrients that the next crop does not like. It also helps reduce the build-up of soil borne pests and diseases that may develop if crops of the same family are grown on the same site.

Usually Brassicas follow Legumes as legumes leave nitrogen in the soil which brassicas e.g. cabbages need to produce healthy leaves. A shortage of nitrogen produces leaves that turn yellow and then drop off. Legumes follow roots as these are generally gross feeders that like a lot of organic matter and legumes prefer a less fertile soil.

Plants cannot use nitrogen directly from air, it can only be converted by bacteria in the soil, legumes are especially good at this as they form nitrogen fixing nodules with the bacteria. When the plant is incorporated into the soil it leaves the nitrogen in the soil which benefits the next crop.

Green manures can fit into these rotations. But it is not good practice to sow a green manure before a vegetable crop of the same family as it may cause the build of soil pests and diseases, e.g. Winter tares is part of the pea/bean family, so do not grow before peas or beans but it can replace these in a crop rotation and it will add nitrogen to the soil which will be beneficial to the next leafy vegetable which will be a brassica.