The Best Nitrogen Fixing Cover Crops & Green Manures

Why is nitrogen important?

Before we get to all that, it is important to understand why nitrogen is so important. Nitrogen is vital because it is a key component of amino acids which are the building blocks of proteins. This is the same for all lifeforms including plants, animals, and humans.

It is also vital for chlorohyll formation in plants.

Although some plants can use nitrogen directly from the atmosphere, humans, animals and the majority of plants cannot, meaning they must look to other sources for nitrogen to fulfil their nutrient needs.

In plants such as cereals and most vegetables, this comes from the soil. Nitrogen maybe present as a residue from a former crop, as ammonia from animal urine, or as inorganic nitrogen applied as fertiliser, but whichever form it takes, it is absorbed from the soil by the root hairs and used to make amino acids and plant proteins.

Humans and animals also take nitrogen in in dietary form – by eating protein. Whether this is from plant or animal sources, the protein is broken down into its constituent parts by protease enzymes during digestion, then used to rebuild amino acids and proteins wherever they are needed in the body.

A lack of nitrogen in plants leads to stunted growth and pale, withered leaves. In humans and animals, symptoms of nitrogen deficiency range from a dry mouth, fatigue and weakness, to confusion, rapid heart rate and a loss of consciousness.

But if the vast majority of plants and animals cannot metabolise atmospheric nitrogen gas, how is it converted into a form we can use? This is a process called nitrogen fixing and is the first stage of the nitrogen cycle.

The Nitrogen Cycle

The movement of nitrogen through ecosystems is called the Nitrogen Cycle and it consists of five stages:

• Nitrogen fixing
• Nitrogen assimilation
• Ammonification
• Nitrification
• Dentification

As mentioned, nitrogen fixing is the process of turning atmospheric nitrogen gas into inorganic nitrogen that can be used by plants and animals for growth. This is done by nitrogen fixing bacteria and blue-green algae. A small amount of nitrogen is fixed by abiotic means such as lightening and industrial processes, too.

Nitrogen fixing produces nitrates and ammonia and the assimilation stage of the cycle refers to how these nutrients are absorbed by algae and plants and become integrated into their tissue.

Ammonification occurs when plants and animals die and decompose. The microorganisms which cause decomposition convert nitrogen in the decaying tissues into ammonia and ammonium. Then nitrifying bacteria in the soil convert these into nitrates which can be absorbed by growing plants in the nitrification stage of the cycle.

Finally, these nitrates are metabolised by denitrifying bacteria back into nitrogen gas which returns to the atmosphere to complete the cycle.

For the purposes of farming, nitrogen fixing is the most important stage in this cycle as it converts atmospheric nitrogen into a form which can be used by cash crops.

This can be done in the field by sowing a special class of plants – legumes – which have the ability to fix nitrogen gas in the soil, freeing it up for other plants to use.

As a result, legumes are highly valuable both as cover crops and as cash crops, producing a range of environmental benefits while potentially reducing your fertiliser costs.

Legumes

The legumes are a diverse family of plants that includes clover, beans, peas, alfalfa, chickpeas, lentils, and others. A key characteristic of legumes – except for clover – is that they produce a fruit inside a pod.

However, the most important quality they possess is the ability to fix nitrogen gas and produce ammonia that is usable by future crops. They do this via a symbiotic relationship with nitrogen fixing bacteria which live in the root nodules of the plant. The plant produces carbon which the bacteria need, and in exchange, the bacteria supply nitrogen to the plant in a form it can metabolise.

This quality means legumes are extremely effective as green manures and cover crops, and it is here they fit into the SFI scheme, enabling you to earn money in exchange for growing them.

Green manures and cover crops

Green manures and cover crops are very similar. In fact, green manure is actually a type of cover crop. The only real difference between the two is that a cover crop is usually killed off before the next crop is planted, whereas green manure is ploughed into the field while it is still green.

Both green manures and cover crops have similar aims – to improve soil fertility and structure, help to manage pests and weeds, reduce soil erosion by having roots in the ground, reduce nutrient leaching, increase nutrient use efficiency (NUE) for the following crops, and reduce water and soil run off.

Of course, to optimise nitrogen fixing and increase the levels of ammonia and nitrates available in the soil for future crops, green manures and cover crops must contain legumes.

For this, mixes that contain Trefoils, Clover, Vetch, and Lucerne are effective as they are all excellent nitrogen fixers. For the best results, aim to sow clover crops and green manures in early autumn.

Green manures, cover crops and SFI

Under the SFI scheme, there are a number of options to choose from when it comes to planting nitrogen fixing crops. These are covered in the actions under the nutrient management section and the soil health section.

They include:

• CNUM2 – legumes on improved grassland
• CNUM3 – legume fallow
• CSAM2 – multi-species winter cover crop
• CSAM3 – herbal leys
• SOH2 – multi-species, spring-sown cover crop
• SOH3 – multi-species, summer-sown cover crop
• SOH4 – winter cover following maize
  
  

CNUM2

The aim of CNUM2 is to have legumes growing from spring to early autumn on improved permanent grassland or temporary grassland to manage nutrient efficiency, protect the soil surface, provide root growth that maintains soil structure, supports soil biology, and minimises nutrient leaching, soil erosion and runoff, and support an integrated pest management approach if located close to cropped areas.

The action pays £102 per hectare per year for a three year duration.

You must include one or more of the following in your mix to qualify for the funding:

• Red clover
• White clover
• Alsike clover
• Sainfoin
• Lucerne
• Bird’s Foot Trefoil

You can establish the legumes by adding them to an existing grass sward or sowing a mix of grass and legumes.

The action is rotational and static meaning you can do it in the same place for the three years of the SFI agreement or you can move it each successive year.

CNUM3

CNUM3 is also a three-year SFI action but it pays £593 per hectare per year.

This aim of this action is to establish a legume fallow with plants that grow and flower from late spring and during the summer months and provide green cover over the winter months in an attempt to manage nutrient efficiency and improved soil health, provide food for farmland wildlife, and support an integrated pest management approach.

Eligible land types include arable land used to grow crops, temporary grassland, arable land lying fallow, and permanent horticultural crops.

You must use a seed mix which contains at least six flowering species, including legumes such as Alsike Clover, Bird’s Foot Trefoil, Black Medick, Common Vetch , and Lucerne.

As per CNUM2, CNUM3 is both a rotational and static action.

CSAM2

CSAM2 is listed under the soil health SFI actions and consists of growing a multi-species winter cover crop. The action attracts a payment of £129 per hectare and lasts for a duration of three years.

To qualify, the crop must be established for the winter months (December, January and February), must be sown and must include a minimum of two plant species from two of the plant families brassicas, legumes, cereals and grasses, and herbs.

This action is both rotational and static.

CSAM3

The aim of CSAM3 is to establish a herbal ley including a mixture of grasses, legumes and herbs or wildflowers to provide varied root structures and to have areas of flowering plants from late spring and during summer months.

The purpose of this is to produce a high volume of forage with minimal use of inorganic fertiliser, improve resilience to drought, and help improve and maintain the soil’s structure, carbon, biology and fertility.

CSAM3 is a three-year action that pays £382 per hectare per year.

As with CSAM2, it can be done on arable land used to grow crops, temporary grassland, arable land lying fallow, and permanent grassland, and it is rotational and static.

To qualify for the funding, you must use a seed mix which includes all of the following (as a minimum):

• One grass species
• Two legume species
• Two herb or wildflower species
  
  

SOH2

SOH2 is the spring cover crop version of CSAM2, although it attracts a higher payment of £163 per hectare and has different mixture guidance.

As per CSAM2, it lasts for three years, is rotational and static, and must include a minimum of two species from plants from the four plant families.

However, in the case of SOH2, grasses can only make up a maximum of 25% of the seed mix by weight.

Cover crops grown under SOH2 must be established in the spring months (March, April and May).

SOH3

SOH3 is the action of growing a multi-species summer cover crop, established in June, July, or August.

Unlike CSAM2 and SOH2, SOH3 requires you to use a rapid-growing seed mix with at least four different species of plant from two or more of the four plant families.

As with SOH2, under SOH3 grass varieties must make up a maximum of 25% of the seed mix by weight.

SOH3 attracts a payment of £163 per hectare and has a duration of three years.

SOH4

Due to the short window available in which to sow a cover crop after maize has been harvested, SOH4 is perhaps not the most effective SFI action to undertake if you are wanting to increase nitrogen fixing.

However, including Vetch in the mix – a quick growing legume – will certainly have this effect.

The principal aim of SOH4 is to reduce soil erosion and surface run off following the harvesting of maize, and to be eligible, the land must have been used to grow maize and must be below the moor line. That said, under-sowing your maize with grass is an effective way to reduce soil erosion.

It pays £203 per hectare per year for a three year period and is rotational and static.

Summary

The SFI scheme has a number of different options available if you want to increase nitrogen fixing on your farm by growing legumes. These mainly include sowing legumes as a part of a cover cropping strategy, which has a range of other soil and environmental benefits, or sowing herbal leys.