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Gardening Tips - Green Manures
Green manuring is a term not readily understood by newcomers to organic farming because the practice has become so rare in conventional agriculture. Green manures are plants which are grown specifically for incorporation into the soil to improve and increase humus content. In fact, the most common green manure used by most of us is our annual weed growth which takes over at the end of the season but this is far from the best method of improving the soil!
Green manures are advantageous to the soil in the following ways:
Increased biological activity in the soil
Keeping a cover of vegetation encourages earthworms and micro-organisms in the soil. When a green manure is turned in biological activity is greatly increased as millions of bacteria and other organisms go to work breaking it down.
Improved soil structure
When a green manure is dug into the soil a small percentage of it will eventually form humus. Humus improves the texture of all soils - it helps light soils to retain water and improves drainage in heavy clay allowing in more air for healthier root growth. Green manure plants with extensive or deep rooting systems also help soil structure. While one application of green manure will not make a big difference to the soil, it's constant use in a rotation over a period of years will greatly improve structure and humus content.
Reduction in nutrient leaching
In areas of high rainfall nutruents are washed down through the soil and out of reach of of plant roots. The leaking of nitrates into water supplies is a common health hazard. Overwintering green manures are particularly useful for storing these nutrients and releasing them back into the soil when decaying.
Reduction in weed growth
Green manures if properly sown can become established very quickly and smother weeds and other vegetation.
Nitrogen fixation
Leguminous green manures have the ability to fix nitrogen from the air by means of bacteria in the root nodules. This advantage means they can be grown on poor soils and the nitrogen will be available to the following crop.
Feeding the soil is particularly important in the organic system especially in an intensive cropping situation. Constant cropping depletes soil nutrients and organic matter must be added back to maintain fertility. This is generally done by adding compost and farmyard manure but these may not always be available in sufficient quantities on a small holding. In this situation green manuring can be very valuable. In very intensive production it may not always be possible to allow a full seasons' break for a green manure crop so catch cropping or ocassionally undersowing are the most common ways of using a manure. Sowing can be done any time between crops or over winter. If at all possible, when planning your rotation allow for an annual green manure to give each plot in turn a rest from cropping. In polytunnels where space is at a premium a short term, quick growing green manure like mustard can be sown and dug in in a few weeks. Undersowing of taller crops such as sweetcorn is also possible in a tunnel but be sure to get the timing right so that the green manure does not interfere with the main crop and does not mature too quickly.
To get the best results it is important to grow the right manure at the right time of year. Ideally, the green manure should fit into the cropping plan; require minimum ground preparation and be relatively inexpensive to grow. Manures should, if possible, be sown in well drained and not too acid soils. The bacteria which break down plants need air and warmth - a soil temperature of at least 10°C to be most effective. A fine tilth is necessary for sowing small seeds. Broadcasting seeds is the fastest method of sowing but ensuring that seeds are buried to the right depth is not easy when raking over the soil. Sowing in drills is less wasteful. Most manures grow rapidly but if left to flower become woody and tough. If you cannot turn the manure in before this stage it is better to cut and compost the plants. Digging in can be done by hand or by mechanical cultivation. Some people cover a green manure with a mulch to kill it off. Taller manures can be strimmed before digging in to make sure they will not continue to grow, otherwise it may be necessary to dig in a second time. Green manures should be turned into the top four inches of soil and not buried too deeply. The length of time between digging in and sowing subsequent crops can vary depending on the maturity of the crop and the time of year. Generally speaking, the younger and softer the growth the faster it will break down in the soil. Following crops in this case should be planted three to four weeks later before the benefits of nitrogen are lost. For tougher manures, a longer period should be allowed and it would be unwise to plant too soon because bacteria breaking down woody material will rob the soil of nitrogen.
Some commonly used green manures:
Mustard
One of the most popular green manures - it can be used as a catch crop because of its fast growth. It is said to be useful in controlling wireworm which like the de-composing plant. Wireworms can take up to five years to reach maturity but if they gorge themselves on mustard they can develop to the adult beetle stage faster. Mustard is a brassica and care should be taken to include it as such in the rotation.
Buckwheat
Buckwheat is a deep rooting plant and also grows very quickly. It should be turned in before it becomes too woody. If left to flower it is very attractive to hoverflies whose larvae eat aphids.
Clover
Clover is generally grown as a maincrop so if your space is not limited it is a good option. There are several varities - Alsilke can stand extremes of temperature and poor soil conditions. Clover can provide several cuts during the summer.
Fodder Radish
Radish is a very versatile green manure. It grows quickly, has a deep rooting system and provides a good amount of bulk for digging in. It can also be used to feed animals during the winter. Radish is a brassica so be careful about it's use in the rotation.
Rye
Known as Hungarian or grazing rye this is an excellent overwintering crop. It has an extensive root system and provides good weed supression.
Phacelia
Phacelia is a good all round green manure which establishes easily and grows quickly. It's flowers are very attractive to bees and other beneficial insects.
Winter Tares (Vetch)
As the name indicates, this is a good overwintering crop but can also be sown in spring before late crops. It fixes nitrogen and provides good bulk for digging in.
Sunflowers
If there's a small area of spare ground in the garden it's worth growing sunflowers. Not only will they look very attractive in a group, but they provide a good deal of bulk for composting when cut. The stems can be tough and may need to be smashed or shredded before composting.
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