How to move towards nitrogen self-sufficiency in a regen system

Understanding how carbon and nitrogen are linked is the key to managing fertility in a regenerative farming system, says French expert Frédéric Thomas.

To move towards self-sufficiency with nitrogen, it takes a different approach, which starts with the soil, requires new thinking and recognises the key role photosynthesis has in getting organic matter into the system, he says.

“The limitations are in our heads, not in our fields,” stresses Mr Thomas, who changed the way he farms in France 25 years ago and is now a world leader in regenerative farming practices.

See also: Farm cuts diesel costs and improves yields in move to regen

Frédéric Thomas

Frédéric Thomas © Frédéric Thomas

For him, no-till was the starting point, but experience with multispecies cover crops and techniques such as relay cropping have developed his understanding of the synergies between plants and soil, and how photosynthesis gets organic matter into the system.

As a result, he is well on his way to exploit this to reduce inputs, get nutrient cycling working well and improve yields – or, as he puts it, letting nature do more of it.

“There is often concern expressed about the cost of cover crops, but they should be viewed as an investment in the farm,” he believes.

“Improved soils bring both better crops and savings. They also manage water better, so the soil biology thrives.”

Harvesting wheat

© Tim Scrivener

Nutrient availability

When soil is disturbed by tillage, so is the soil biology, explains Mr Thomas. “That’s why mineralisation occurs and nitrogen becomes available to the crop. It’s the reason that tillage evolved.” 

The water and carbon cycles are also changed by tillage, as carbon leaves the soil, freeing up some nitrogen.

For this reason, he compares a cultivation-based farming system to mining – with practices that involve soil movement, leaving bare soils between crops, using nitrogen fertilisers and irrigation all helping to make nutrients available, but putting their loss at risk of erosion and leaching.

In contrast, a regenerative system using no-till, cover crops, residues, root exudates and soil life slows the flow of nutrients, but can make them less available.

“With no-till, the fertility stays in the soil.”

However, putting carbon back into the soil has a cost in terms of nitrogen, he acknowledges. “This is why you can be short of nitrogen at the beginning of your regenerative journey and why results can be disappointing in the early years.”

Measuring your farm system

Evaluate where you are by doing a zero-N area in a field, which will show you how much your farming system is supplying, says Frédéric Thomas.

Then, when you make your first nitrogen application to the rest of the field, put on double the amount, to make up for the lack of mineralisation.

“Don’t lose 1t/ha for the sake of 30kg of nitrogen,” he says.

Crop residues

Leaving crop residues on the soil surface has a greater effect on the system than many realise, as it changes the carbon:nitrogen ratio (see “Why is the C:N ratio important?”), lowering it while the residue is broken down by soil microbes, says Mr Thomas.

Why is the C:N ratio important?

The C:N ratio is the mass of carbon to the mass of nitrogen in a particular material or substance. It matters because it has a direct effect on residue decomposition and nitrogen cycling in soils.

As a rule of thumb, the bigger the ratio, the longer it takes for the material to decompose, while a smaller ratio means the plant material will decompose rapidly.

This has a direct relationship to the amount of nitrogen that is tied up in the soil and that will be available to the next growing plant.

“It can put too much carbon into the system, right in the place where there’s already a lack of nitrogen. The soil biology reacts and tries to decay it, using more nitrogen to do so.”   

For this reason, straw management is very important, with chopped straw having a bigger effect, so leaving a longer stubble at harvest is recommended.

“Cut as high as possible,” urges Mr Thomas. “Not only will you have more nitrogen available at the early crop growth stages, you will also get better seed-to-soil contact.”

Cover crops

Cover crops are essential for feeding the soil because they bring sugar, in the form of root exudates, and nitrogen, which balances the C:N ratio and helps the soil life to digest the residue.

Where cover crops are an integral part of the system, any mineralisation is eliminated and nitrogen can be added by having a legume in the mix.

Exploiting soil depth with roots also makes a difference, which he describes as “farming in 3D”.

“In this way, you can organise your farming system to collect nitrogen. The roots of the cover crops, which can go down 2m in the soil, will bring nitrogen and other nutrients to the surface.

“Earthworms help too by taking N down. It becomes a feeding network.”

Growers can get 8-10kg N/t of dry matter from cover crops, he suggests, helping to replace what was provided by tillage.

“Depending on when you destroy your cover crops, you should be able to save 20-30kg/ha N in a following spring barley crop. Getting up to 50kg N/ha in this way is easy, but higher amounts take more effort.”

Organic amendments

Cover crops are a good place to include farmyard manure applications for feeding the soil, but if they contain too much carbon, there will be an effect on nitrogen, and its availability will be affected.

“Apply them in the autumn when the soil biology is looking for food. Don’t worry about soil incorporation – the nutrients will be incorporated into the vegetation.”

Composts are less beneficial, he warns (see “Is there a place for compost?”).

Fertility return takes time, and patience is required, advises Mr Thomas. “Remember that the efficiency of photosynthesis is important for linking the nitrogen and carbon cycles.

“If your soil is alive, you will see results – but you can’t run fertility as you did in a conventional system.”

Frederic Thomas was speaking at the Base-UK members conference, held in Nottingham in February 2023.

As well as farming in Sologne in central France, he started Base France in 2000 and has a website ( dedicated to this fast-growing, farmer-led movement.

Is there a place for compost?

The composting process involves the loss of half of the nitrogen and much of the carbon contained in the component materials, warns Frédéric Thomas.

As a result, the use of compost will still have a positive effect on soil organic matter, but any benefit in terms of feeding soil biology will be lost, he explains.

“Compost is a stable material because the energy has been taken out of it,” he says.

“You spend money to make it and then lose nutrients and lots of carbon in the process. That matters because no-till systems are short of nitrogen.”

As with most things in a regenerative farming system, whether or not to use compost isn’t a simple decision, he adds.

From a feeding the soil perspective, you need both quantity and quality of food, he explains.

“Composting takes organic matter and, as the pile of material shrinks, you lose potassium in the liquid that seeps away, as well as some nitrogen from leaching and volatilisation.

“But the biggest losses are carbon, with two-thirds of what you had at the beginning disappearing. And this is in the form of fermentable energy, or the best carbon for soil microbes.”

Garden waste

© Tim Scrivener

A better and cheaper approach for feeding the soil is to plant a mixed-species cover crop and then apply manure to it in the autumn, so that the food is being provided to the right place at the right time for the soil biology, he advises.

Having said that, Mr Thomas does use compost on his own farm, but adds that his system compensates for its high carbon content with cover crops, which provide the sugars and energy that soil biology requires.

“It’s controversial,” he admits. “But it allows me to bring in fertiliser from outside the farm, and it’s cheap.”

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