Green Manures & Cover Crops Are An Ancient Farming / Gardening Technique That Improves Soil Fertility, Smothers Weeds, Feeds Soil Fauna & Improves Soil Structure. Green Manuring Utilises A Wide Range of Plant Species & Can Be Sown At Any Time of Year.

 

What is Green Manuring?

Green manures include the use of crops such as clover.
Green manuring and cover cropping include the use of crops such as clover.
Firstly the use of green manures and cover crops is a natural process in that in nature soil is rarely bare. Nature “abhors a vacuum” and the natural order f things is that if soil becomes bare, perhaps after a landslide or after erosion, plants will soon grow to fill the bare soil. Traditional farming practices have recognised this.  Even where a fallow period was built into the rotation, weeds soon grew unless repeated cultivation was practised. Recognising this many farmers grew green manure crops in the fallow part of the rotation. For example US prairie farmers, on arriving from Europe, grew buckwheat, oats and rye as green manures.  

The History of Green Manures and Manuring

Green manuring got back long before their use on the prairies of North America (and should the practice have continued then perhaps the dustbowl would never have occurred). The Greeks grew broad beans for ploughing back into the soil. And the Vrikshayurveda, The Hindu Science of Plant Life, recognised the value of green manures many thousands of years ago. And in China there are agricultural texts from centuries ago that discuss the use of grasses and weeds as being of value when ploughed in.  

What is Green Manuring?

A green manure plant is a specific plant grown for a variety of purposes either on areas that would have otherwise been left without crops or between rows of crops. The term is virtually synonymous with a cover crop. Ie a crop grown to cover the ground. Green manures and green manuring has been practised for millennia and was a traditional agricultural practice for thousands of years. Green manures confer many benefits, which we cover later in this article.  

Green Manure Benefits

Many benefits result when green manures and cover crops are grown.

Soil Structure Improvement

This occurs for two different but related reasons. Firstly the green manure plants root into the soil and open it up. This improves drainage, allowing better water penetration (and hence less flooding and/or run-off). When the crop dies the roots are left in the soil and as they decay their nutrients go back into the soil and are available to the next crop. Plus, as they die, they leave channels in the soil which aids the drainage mentioned above.  

Weed Smothering

Most green manure crops are fast growing and act as a smothering blanket, suppressing slower weeds. This encapsulates the whole idea of cover crops where the soil is covered to serve a number of purposes. For example buckwheat is a frost susceptible crop that can be sown with other slower crops. The buckwheat suppresses the weed competition with the crop which then grows away the following year without the competition from weeds. Today, some farmers are embracing this concept with autumn sown oilseed rape (OSR). The OSR is drilled with buckwheat. This suppresses weeds in the early stages. The buckwheat is then killed by the frost and the OSR is left with no buckwheat or weeds to compete with it.  

Erosion Prevention

When soil grows a crop the rain cannot batter it during storms, the crops roots also anchor the soil particles and the combination decreases the risk of soil erosion. In my mind soil should never be left without a crop in it. Especially overwinter when rainfall is at its peak. Ensuring soils are anchored with a growing crop is paramount in my mind. One crop prone to soil erosion in the UK is maize grown for silage. It is often harvested in late autumn, in wet conditions and the land is then prone to erosion. If a cover crop were to be sown asap after harvest art would serve to bund the soil together over winter.  

Soil Fertility Improvement

Soil fertility can be improved in several ways. Deep rooting green manures can bring useful nutrients from the depths that shallow rooted soil can’t reach. The crop came also be incorporated into the soil when it matures or dies and that adds back nutrients that could have  been lost due too rainfall if they hadn’t been taken up into the plant. Nitrogen fixing plants can also be used to fix nitrogen for the following crops.  

Mulching

A cover crop or green manure can be used as a living mulch. For example the addition of a clover cover crop is ideal when growing tall plants such as sweet corn. When the sweet corn is harvested the clover is killed and allowed to re-incorporate in to the soil. The above technique works in gardens as the crop is shorter and less dense that the farm grown maize crop. Though they are the small species the farm crop is a much taller denser cultivar and cuts out all the light from the cover crop.  It therefore dies. In a garden situation the clover crop could of course be left in situ until there following spring and could provide useful flowers and hence nectar for pollinators.  

Nitrogen Fixation

As previously mentioned clover and other leguminous crops can be grown as green manures and cover crops. In growing they provide nitrogen for future crops.  

Disease Suppression

Disease and pest suppression using cover crops and green manures is well researched and shared. For example this paper Management of Verticillium Wilt of Potato with Disease-Suppressive Green Manure and as Affected by Previous Cropping History by Larkin et al  is well cited.  

Increases Soil Organic Matter

The incorporation of green material back into the soil increases organic matter and feeds the soil.  

Feeding the Soil

Arguably the most important benefit of green manuring is the way the plants feed the soil. In doing so it replicates the life cycle of plants that germinate, grow, die and become incorporated back into the soil. In so doing they become part of the wood wide web of fungi and bacteria, feed those creatures and organisms that rely on plants for sustenance at carious parts of their life cycle and, when decomposing feed another group of organisms. Bare soil starves the soil fungi which then impoverishes the mycelia web. And the other soil micro and macro organisms also suffer. A soil without plants soon becomes a dead soil.    

What’s the Difference Between Green Manures and Cover Crops?

Though the words are often used interchangeably there is a difference. Green manures are plants or crops that provide some manuring or nutritional benefit to the soil. Whilst cover crops are used to cover the soil and prevent weeds, prevent erosion, etc.  

Which Plant Species Can Be Used As Green Manures and Cover Crops?

The crop species that can be grown is large. It ranges from grasses to mustard, winter tares, clover, lupins, alfalfa (aka lucerne which is a legume), buckwheat, peas, comfrey, turnips,  broad beans, Phacelia, asparagus peas, soya bean, radish etc. included all the assorted legumes I’ve not mentioned by name. The reality is that just about any herbaceous plant can be used as a green manure, though some are of course preferred. Preferences however change. In bygone days mustard was often grown between fruit trees as a green manure. Though personally I prefer a grass crop as it can be grazed by sheep, cows, goats or whatever and they will recycle the nutrients via their dung.  

How To Utilise Green Manures & Green Manure Crops

Traditionally green manures and cover crops were grown to be ploughed in to the soil after they’d served their purpose or reached maturity. However, that requires effort from a horse or other plough animal or a tractor and fuel is needed. An easier way is possible on a small scale. That is to smother the crop in situ and let it die and decompose. Simply put the idea is to exclude the light until the crop dies. Methods include covering with light excluding materials of some sort. Alternatively the crop could be scorched with a flame off some sort. Flame guns are sometimes used and they kill some crops in seconds.  On a very small scale putting a recycled double glazed unit onto the soil/crop, and leaving it a few hours when the sun is high, can kill a crop very quickly. Where the crop is killed in-situ, and not ploughed or dug in, the next crop is then planted though the decomposing or dead detritus of the dead crop.  

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