Green manures for soil fertility
Green manure trial at The Organic Centre in Rossinver in 2010.
Good fertile soil will produce healthy and nutritious vegetables and soil fertility can be improved through the addition of farmyard manure and compost and also by sowing ‘green manures’.
Green manures are grown primarily to dig into the soil to enrich it and improve soil fertility and structure.
Green manures are crops grown to improve soil during times when there would be no other plants in the ground. They are typically dug back into the soil before the following crop to allow it to benefit from the nutrients released. A wide range of plant species can be used with different ones providing different benefits such as: providing nutrients and organic matter for following crops, breaking up heavy soil and creating habitat for insects.
If even a small piece of ground looks like being vacant for a few weeks it is worth sowing a green manure. They act as a supplementary source of fertility but should not be relied upon as the sole means of fertilizing the land and supplying plant nutrients. Green manures can increase organic matter in the soil and also increase nitrogen levels. They protect the surface over winter, help suppress weeds and increase drought resistance. In some cases the flowers attract beneficial insects.
Green manures fall into 3 main categories:
Fast-growing leafy crops such as mustard, rape seed, fodder radish and Phacelia quickly provide a leafy canopy to suppress weeds and give an almost immediate supply of bulky organic material when dug in.
Leguminous crops including various types of clover, alfalfa, bitter blue lupin, winter field beans, field peas, winter tares, and common vetch have nodules on their roots which fix atmospheric nitrogen in the soil releasing it slowly for the crops that follow them. Winter-hardy legumes such as field beans, tares and crimson clover can be used for over-wintering.
Fibrous-rooted crops develop dense fibrous root systems which improve soil structure. When dug in they increase the organic matter in the soil. Grazing rye and Phacelia fall into this category.
How to choose a green manure?
Green manures can be used either as short-term summer catch crops, for over-wintering or as long-term green manures. It is essential to choose manures that suit your soil. Mustard for example prefers a fertile soil, buckwheat thrives also on poor soils. When planning your rotation you need to consider when they will leave the ground available for the next crop.
Green manures can be broadcast, sown in narrow or wide drills or in the case of field beans sown individually. Generally they are dug in while still young and succulent, before the stems harden, become woody or start to flower. If they are very bulky they can be tricky to dig in. It is easier to cut off the top growth, leave it to wilt and then dig in or put on compost or ground for mulching. Ideally allow 2-3 weeks before sowing the next crop as it takes green manures time to break down in the soil and they may suppress seed germination. In cold weather you may need to wait 4 weeks.
Where to use:
On empty beds, whatever the time of year. Good when potatoes or onions come out as the soil is already prepared.
In the polytunnel after tomatoes, peppers and aubergines.
On areas of low fertility as c over crops.
On areas of poor soil structure.
Between widely spaced plants. For example; undersowing winter brassicas or sweet corn.
Never leave soil uncovered - aim to have 100% groundcover, 100% of the time.
How to sow:
Prepare a level seed bed, reasonably weed free for best results.
Scatter small seeds on top and lightly rake in. Larger seeds have to go deeper.
Water if the weather is dry and definitely water in the polytunnel.
At Neantog we use mostly Phacelia and yellow mustar in the polytunnel during the Winter month.