Green manures, also referred to as fertility building crops, are crops used to improve the condition of the soil in some way 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 as green manures with different ones providing different benefits. To get the best from a green manure crop, its cultivation should be taken as seriously as any other crop and integrated thoughtfully into the crop rotation.
Benefits of Green Manures
- Protecting soil – leaving soil bare is a big no-no as valuable nutrients will leach away. A green manure will pick up these nutrients and stabilize the soil with their roots. This prevents erosion. Leaving soil bare is also asking for a weed infestation. Promptly seeding open soil with a fast growing green manure will keep weeds at bay.
- Improving soil – Incorporating the green manure back into the soil adds a lot of organic matter which is vital for feeding the biology of the soil. The byproducts of this biology are what gives good soil its characteristics; for example, qualities like the ability to hold nutrients and make them available to plants, moisture retention and good porosity. Soil can be improved structurally too such as with the opening up of deep or heavy soil with tap rooted species.
- Providing nutrients – The nutrients taking up or synthesised in the green manure will be released back into the soil for the benefit of the following crop. This is why, for example, leguminous plants such as clover, vetch, or lupins are often used as green manures. Their ability to fix atmospheric nitrogen into the soil is key for providing enough of this essential nutrient in organic systems where artificial nitrogen fertilisers are not used.
Green manures can be sown at most times of the year depending on the desired effect. The following is a summary of uses but at this time of year it is the latter we are most interested in.
- Fast growing summer green manures – Cover bare patches and smother weeds in areas not ready for sowing yet.
- Undersowing – Sow at the same time as a widely spaced crop. This technique generally uses a low growing non competitive species in between and/or under the vegetable crop. For instance, trefoil (Birdsfoot clover) can be grown under large brassicas like brussel sprouts.
- Long term – Use a green manure as a ley, or an area which is being rested for a year or more. This is great for building fertility and can be attractive too.
- Grow your own compost and mulch – A crop such as red clover can be cut a number of times and added to your compost or be used as mulching material.
- Winter green manures – Green manures sown in late summer/autumn are probably the most beneficial. This is the time when crops are coming out and the land can be left bare. However, at this time the soil is still warm and biologically active. As such, the soil life is still actively releasing nutrients which can be washed out in winter rains.
Winter Green Manure Options
Despite the name, winter green manures must be sown sufficiently early so there is enough development of the root system before colder and shorter days put a check on growth. A well established root system is especially important for holding on to nitrate which, unlike other nutrient ions, is not strongly attracted to soil particles. Sowing from early August is ideal and by the end of September the window has closed to reap the full benefits of a winter green manure.
Legumes (Clovers and Vetches) can produce good amounts of organic matter and some have impressive root structures. They can fix large amounts of Nitrogen from the air into soluble nitrogenous compounds in the soil which are available to plants. Now we know that this amazing example of natural chemistry is based on a symbiotic relationship between leguminous plants and the soil bacterium known as Rhizobium. Fast growing clovers, such as Crimson and Persian clover, are excellent choices as they grow rapidly producing a lot of bulk and nitrogen fixation in a short time. If later planting can't be helped then it is worth including vetch which can be successfully established later than clovers. Vetch is also excellent at covering the soil and out competing weeds.
Grasses and Grains have great root systems, and will improve the soil structure preventing erosion and capping. Although they don't fix nitrogen like legumes, they are excellent at picking up existing nitrogen and holding on to it. Short lived rye grasses such as Westerwolds or Italian rye are excellent choices as they grow rapidly, grabbing any free nitrogen in the soil and prevent loses through leaching. Black oats are also very good and will do wonders for soil structure.
Grass/Legume Mix Growing a mix of a grass species and a nitrogen fixing legume is an ideal and time tested winter green manure strategy. (Check out Landsberger mix, Wild Atlantic Way mix or Westerwolds and Vetch mix). There are a few advantages to this. First, mixes are generally always good because different species have different root structures, nutrient requirements and optimal temperature ranges. As such they fill different niches and make better use of soil and climate resources. Second, nitrogen fixing plants will work more efficiently at taking nitrogen from the atmosphere if nitrogen is more scarce in the soil - such as when in competition for it with another plant. Finally, when incorporated the release of nitrogen back to the soil will be buffered as the legume releases nitrogen fast in comparison to the slower release from decomposing grasses. This spreads out the availability of nitrogen for the following crop.
Brassicas like Mustard and Fodder Radish produce lots of vegetative material and will help keep deeper layers open with their taproots. Otherwise they do not improve the soil structure much, but they will help increase the soil life. These can be some of the cheapest green manures to grow and especially useful if trying to keep costs down in a large area where nitrogen fixation is not a goal. However, care must be taken with the consideration of other Brassica crops in the rotation.
Phacelia grows rapidly and is often used earlier in the summer to fill any gaps where crops were harvested. It's usually allowed to flower, as its blossoms are renowned for attracting beneficial insects such as bees, wasps and hover flies. It is tolerant of cold temperatures and may over-winter if it's not too cold. Phacelia also has the advantage of being unrelated to any of the main vegetable crops making it neutral in the rotation.
Managing your winter green manure
Once established, there is not much to do with your winter manure. However, if you are well organised and have the seeds in early you will be surprised how much mass will be produced before the winter. This is often considered a crop in itself and can be grazed or made into silage on a larger scale. In the garden, it can be cut and used as mulch, chicken feed or added to the compost heap. It can also be left in situ to add to the fertility of the site where the green manure is growing. Cutting delays the green manure from going to seed which keeps lush vegetative growth happening. Without cutting, or 'topping', your green manure can become woody and more difficult to incorporate when spring comes around. Cutting is also important in preventing a weed infestation in your green manure - Cutting about 15-20cm from the ground can prevent fast growing ephemeral or annual weeds that are present from setting seed. Cutting at this height leaves plenty of growing points on the green manure for it to take off again. Unfortunately, you might be left with the difficult decision of whether to prevent your green manure from flowering by cutting to minimise weed problems or enjoy it in bloom and deal with the consequences later.
Incorporating the green manure
This should be done four weeks before you need the soil or before the Green Manure plants becomes woody, whichever comes sooner. You can -
- Cut down with strimmer or scythe and leave soften on the ground.
- Dig the plants back in using a sharp spade. Aim to bury them between 15-18 cms deep.
- If a frost tender manure is used in late summer you can let the cold weather kill it off. Leave on the soil, no need to dig in.
- Annual green manures such as mustard or buckwheat can be hoed in. Excess foliage can be composted.
- Mulching works well - annuals will take a few weeks to break down while perennials and rye will take a bit longer.
Some green manures such as grazing rye and certain clovers produce and 'allelopathic' effect when incorporated back into the soil. This means the chemistry of the plants breaking down inhibits the germination of seeds in the soil. This can be useful as a way of controlling weeds the following year. However, you must be careful if you are planning on direct sowing a crop after. It is best then to incorporate the green manure 4-6 weeks before sowing to insure this effect has worn off.