Green manures are plants grown to improve the soil, rather than for food or landscape decoration. They are grown for their nitrogen fixing characteristics, dense foliage for weed control, or extensive penetrative roots, ideal for opening up heavy clay soils and improving lighter sandy soils.

The benefits of Green manures are as follows:

Add plant foods- Plants such as clovers absorb nitrogen from the air and fix it in nodules on their roots. The nitrogen becomes available to other plants when the green manures are dug in to the soil. Some green manure plants extract mineral from deep in the soil, bringing them up for subsequent shallow-rooted plants.

Protect soil- Green manures protect soil from compaction, particularly heavy clay soils. Green manure plants also absorb plant foods from the soil, so they are not washed out by rain.


 

Improve soil structure- Green manures such as winter rye, with its extensive root system improve heavy soil by opening up the structure. With lighter soils the roots bind with soils particles, helping them to hold water more efficiently.

Smother weeds – Green manures germinate quickly and grow rapidly, smothering weed seedlings.

Control pests- Frogs, beetles and other natural predators thrive in the cool, damp cover provided by green manure. Some insects can be confused by the presence of a green manure crop planted between food crops.

Rest the soil- plant a longer term green manure and leave it to grow for a whole growing season. This will help soil recover from constant cultivation and improve fertility and structure.

Choosing a green manure- depends on what you want to achieve, how long the ground needs to be covered, what was planted before, what will be planted next(crop rotation), the time of the year, and the type of soil you have. See table below.