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29 Jun 2021

Ditching the plough for the good of our soils

As featured in Arable Farming Magazine

Ditching the plough for the good of our soils

by Arable Farming

In the third article in Arable Farmings series on regenerative agriculture, Alice Dyer explores the impact tillage systems are having on soil biology and structure.

Cutting out soil disturbance is probably the first principle many people consider when they think of regenerative arable farming.

And thanks to the longevity of Rothamsted Researchs Broadbalk and other experiments, researchers have been able to explore the true extent of the impact that management practices, including cultivations, are having on soil biological communities.

Prof Andrew Neal, microbial genetics expert at Rothamsted, says: Tillage and inorganic fertiliser usage is affecting microbial communities at a variety of different levels.

Damage Its well known theres physical damage that happens to soil when its ploughed because tillage breaks up mycorrhizal fungal networks. The communities of different species also change and so do the functions of the whole community.

However, Prof Neals studies have determined the ‘diet that is being fed to the soil in arable monocultures is promoting a different kind of bacterial and fungal communities.

A pasture or unmanaged system will have a very rich, diverse diet of inputs lots of different plant species and organic chemical inputs into those soils.

In a pasture with that diverse diet, most fungi make a living off the dead plant material organisms we call saprotrophs or ectomycorrhizal associations with living plants.

When you start to reduce that diversity to a continuous diet of winter wheat, we see that because of the nutritional monotony, pathogenic fungi that survive by attacking insects, plants and lichen become more abundant.

Actively building a system that encourages pathogens because of a poor diet is not a particularly sensible way to go when youre trying to grow crops. The more diverse the diet, or organic matter, the more balanced and benign the fungal communities are in the soils.

This is why broad and diverse rotations, cover crops and organic manures play such a key role in building soils with beneficial biological activity.

It is also well known that tillage results in heightened carbon losses from soil.

The extent to which organic carbon helps create structure and control soil function has recently been explored by a collaboration of researchers from Rothamsted and the University of Nottinghams Sutton Bonington campus who used non-destructive 3D imaging of soil by X-ray computed tomography to help deter mine the influence of organic carbon on soil structure and microbial communities.

It revealed that low carbon soils with a resultant poor structure, including conventionally managed arable soils, are associated with a greater number of genes associated with emission of the greenhouse gas nitrous oxide (N2O) and other biological processes which represent inefficient nutrient use.

Separately, the Sutton Bonington team has also studied the potential benefits to soil structure and the reduction in greenhouse gas emissions from no-till.

Their studies took place on 22 farms in the East Midlands with a range of soil textures where the soils managed under no-till practices had a higher bulk density, soil shear strength and moisture content.

Moisture The X-ray CT-measured porosity was higher in the tilled soils, as was pore size and the surface area of the soil pore system.

The no-till-managed soils contained higher levels of soil organic matter and more microbial biomass.

These factors influenced the potential CO2 and methane flux, which were both higher in tilled soils.

In contrast, N2O levels were higher from the no-till-managed soils.

As a result, the net global warming potential was significantly higher, by 31% on an area basis and 26% on a weight basis, from tilled soils than the no-till-managed soils.

The researchers say this work indicates that no-till practices could play a significant part in reducing greenhouse gas emissions and contribute to efforts to mitigate climate change.

The results also showed the length of the period under no-till management (between five and 10 years) did not significantly affect the net emissions of greenhouse gases from the no-tilled soils.

However, the reasons behind these greater carbon losses in cultivated soils are only just being understood at a fundamental level.

Prof Neal says: We think carbon is effectively sequestered in the soil once that rich diet is broken down into very small molecules.

Those become associated with clay particles, acting as glue to stick soil together.

This is the creation of that lovely soil tilth and structure were all conscious of as ‘good soil.

Once that structure is formed, microbes are unable to get access to the sequestered carbon and rather than being mineralised and lost to the atmosphere, it becomes locked away so microbes cant get at it anymore.

When you till, you break down that structure and a lot of that hidden carbon becomes available again to microbial activity. Gradually, over long periods of time, organic carbon becomes less abundant in the soil, unless you act to replenish it.

The challenge here is that organic carbon in soil is quick to lose, but slow to get back.

More than 170 years ago, as part of the Broadbalk experiment, soil previously managed as permanent wheat was effectively abandoned and allowed to revert to grazed pasture.

In the first 20 years, soil carbon accumulated relatively rapidly after ploughing was stopped.

However, as time went on, the rate of increase has gradually slowed.

To some extent the time frame that is required to go from a highly managed, low carbon soil to one that is rich in carbon can take well over 100 years, Prof Neal says.

Structure Its also likely that the microbial community and larger organisms in soil respond on similar time frames and you only get the final community structure once organic carbon has reached its maximum levels in those soils.

Yes, in the first few years there will be a rapid change, but when we think now about regenerative agriculture, that total regeneration takes much longer than those first 20 years.

In a farmers lifetime there will be appreciable changes and improvements, but really what theyre doing now will benefit whoever inherits the land after them and future society as much as it benefits them today.

Weighing up grant options

With financial incentives for reduced tillage emerging in the Governments Sustainable Farming Incentive plans and with grant schemes available for new drills, farmers who have so far resisted the no-till movement are likely to be weighing up their options.

However, AHDBs Harry Henderson warns that although cutting out cultivations may be the first idea farmers have in becoming ‘regenerative, they should be wary of jumping into making the leap from a plough or power harrow simply to benefit from the productivity scheme.

He says: The drill is just one step in about 10 to reduce your soil movement and its often said to be about step seven to get to a minimal disturbance or no disturbance situation.

However, theres no doubt that savings can be made from moving to this type of management system, he adds.

Machinery isnt going to get cheaper, even with new policy and the Environmental Land Management scheme coming in.

Cost For those wanting to start this journey, ease of transition will be dictated by a few underlying factors, with soil type, region and rainfall having the biggest influence on suitability.

On top of this, a pragmatic approach will also be key, says soil health expert Philip Wright.

Some soils are more challenging than others and may take longer to get to where you want to be to do an effective job and I think being aware of that is a big help.

Clearly if you have wetter conditions, it is more challenging because we are relying on the soil and nature to fix stuff rather than putting metal in the ground.

Ground types that are less suited to no-till systems include non-calcareous soils with a high clay content and a high silt content, and sandy soils that are non-humic and low in organic matter.

These fields will take longer to convert because they do not self-structure so well, Mr Wright says.

Good field drainage, whether it is natural or man-made, is also fundamental and residue management must also be consistent across the field to achieve consistent results.

Considering the main pillars of conservation agriculture residue management is one and growing roots in the soil at all times.

You have got to be prepared to consider growing cover crops, particularly to create lots of cycles of wet and dry as the season progresses.

This will help to self-structure soils that are less self-structurable.

How you manage compaction and traffic is also hugely significant, he adds.

You have got to be prepared to control where you drive.

This naturally allows a transition to direct drilling much quicker and helps soils to recover quicker from deep cultivations.

Yield ‘lags are not uncommon following a big change in growing systems, but Mr Wright says there are a number of steps that can negate the impact.

There can be a lag due to biological processes kicking in.

Do not accept yield lags do something to minimise them.

For example, a turning headland or an area that has had compost/manure on it will take longer so it may be you decide to do something to remediate them.

Finally, mindset is also going to have a strong influence on success, he says.

You need to be willing and eager to make it work.

Use other peoples experience in the area or on similar soil types and speak to contractors who are experienced in direct drilling.

Start with a field in good structural condition.

The most important thing is to be pragmatic. I do not believe we can be prescriptive in farming. If something needs doing, we ought to do it.

In the field Paul Davey, Lincolnshire

Over the past six years, north Lincolnshire mixed farmer Paul Davey has farmed all 440 hectares of his land under no-till, learning a number of lessons along the way.

The land, which stretches from Market Rasen to the River Humber, features a wide variety of soil types, from permeable subsoils to deep clays, and produces a range of crops including cereals, legumes, linseed and lucerne.

Mr Davey says: These all respond differently in different seasons.

The single biggest change I have noticed is in soil structure. We talk a lot about the biological benefits of no-till, but we do not make enough noise about understanding how stable soil aggregates are created and retained.

Infiltration rates across the farms have increased ‘enormously, leaving the land relatively dry, even in autumn 2019, he says.

On the whole we could travel through late winter and early spring with our equipment.

However, the journey has not been without its challenges.

Mr Davey says: We would be lying if we did not say that water and slugs did not provide headaches.

Too much moisture does not suit the system because you get anaerobic conditions and you are managing pests associated with previous farming systems.

Disturbance Some of the transition has involved a low disturbance subsoiler in some places, which complements the system quite well to give the soil time to build stable aggregates through plant rooting.

A yield lag was seen on one farm which Mr Davey says may have been converted too quickly.

It was a farm which seemed to respond well to its previous system. Moving it into a different rotation, it was maybe too fast. It was three years of shallow cultivations and then zero-till.

However, now in its fifth season, the farms soils are becoming more friable.

There are difficulties, but you do not learn if you do not try anything.

Economically, there were lessons in machinery, which is not a small investment, but the resource management in using less fuel is very welcomed.

It does not tend to offset the yield dip in the first couple of years but as time went on it was clear there were other input savings to be made, particularly from crop inputs, which slowly but surely help to increase the margin on that style of production.

Clawing some of that margin back for yourself does start to make a real difference.

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