Green Manure’ doesn’t sound very pleasant but it does wonders for the health of the soil and subsequent plants. It is a crop that is grown mainly to benefit the soil rather than for food or ornament.
The idea of growing a crop purely to benefit the soil and other plants is not a new idea it has been around for centuries but fell out of vogue with the advent of World War 2. Man-made chemicals were produced that could enhance plant growth, kill pests and diseases and increase productivity. Little was known then about the harmful effects of the chemicals on humans or predatory insects. People are now more aware that there are better, more natural ways of ensuring the soil and thereby the subsequent crop is healthy and green manuring is one of the best methods.
The benefits include:
Improving Soil Structure
Some green manures have deep penetrative roots that as they grow open up the soil. This is an advantage on heavy soils as allows drainage to occur more freely and organic matter to be left in the soil and on lighter soils the particles of soil can bind together better so they can hold water better and leaves organic matter in the soil.
Green manures crops grow quickly and their very leafy growth smothers weeds. It is like a living mulch as it suppresses weeds and retains moisture in the soil. It is good practice to make sure the soil is weed free first. That is why they are very important when areas are left fallow especially good in winter.
Certain varieties bring to the surface minerals they would be unusable to plants and leguminous green manures absorb nitrogen from the air and fix it in root nodules on their roots so that when it is dug in it becomes available to the following crop. Specific soil bacteria are required to be present but they are usually present in healthy soil. Nitrogen is required by plants as it encourages healthy stem and leaf growth.
As a living mulch it helps to protect the soil from compaction due to heavy rainfall, prevents the leaching of nutrients, and helps hold the soil together. In the summer it will protect the soil from the drying effects of the sun and wind.
It provides habitats for frogs, beetles and other natural predators that like the damp cover of green manures and that feed on pests such as snails and slugs etc. It also confuses certain insects if the green manure is planted between food crops eg Carrot fly flies low so if a taller not too invasive green manure is planted in between rows the fly will be confused and not attack the whole crop.
Some soils need a rest to recover from constant cultivation and by planting with a green manure it will help soil fertility and structure with very little effort. Green manures can be left in for a year or more, but in the case of most domestic gardens/allotments it is generally a winter thing.
Green manures can be left to grow and then periodically cut down before flowering so as to prevent seeds growing. The plant material can be composted in a compost bin.
It can be allowed to grow and then dug in and left to decompose – allow 30 days before planting next crop. This can be quite hard work and care must be taken not too plant to soon as some green manures eg. Grazing Forage Rye releases a chemical that inhibits seed germination.
No dig systems can still use green manures the crop is simply cut down, the foliage is left on the ground to decompose, and is treated as a mulch and planted through this layer or just move it to one side to sow seeds. The foliage can also be removed and composted.
Be careful with perennial green manures and grazing Forage Rye as they may re-grow after cutting down. You can kill it off with a light excluding mulch such as black plastic or a crop of potatoes grown under straw.