Counting the cost of soil compaction

Independent soils advisor and cultivation specialist, Philip Wright, based in Lincolnshire in the UK, has been studying the effects of compaction for years. Recently he has looked closely at how wheelings in the field impact yield.

Philip delivers independent advice to farmers and companies across the UK. This enables him to conduct his own scientific trials and studies in the field, gathering evidence from commercial farms in real life working situations.

Farming practices have changed

“Interestingly when you look at scientific papers about curing compaction they are now quite out of date – many conducted up to 30 years ago. We have learned a lot in the intervening period and farming practices have changed along with the size and type of machinery, but the science hasn’t always kept up,” he says.

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Modern tyre technology combined with variable pressure systems alleviate compaction with reducing pressure increasing yield in wheelings by 10% to 15%.

Yield loss caused by wheelings in the field

During the past few years Philip, together with a farmer colleague, has been carrying out his own replicated trials into yield loss caused by wheelings in the field, often called ‘trafficking’. “At certain times of the year you can see this in the crop – lines clearly show up where the drill tractor wheelings are suppressing growth.

“This won’t be seen on yield maps, because the combine usually goes along the row and the header width averages them out. If you combined across the tramlines they would appear as waves – if the monitoring system was sensitive enough. “But look at the headland – an area of heavy trafficking. The yield is always lower there, even if you cultivate and sow it last,” he says.

The contrast is clear – commonly the yield is 15% to 25% less in the wheelings

Philip’s in-field studies are assessing the difference in yield in identified wheelings, compared with un-trafficked land on either side. “The contrast is clear – commonly the yield is 15% to 25% less in the wheelings – where drilling tractor ground pressures are within the ‘normal’ range seen on farms generally,” he says.

“If you add that up over the whole farm it’s a significant loss. But this can be reduced by alleviating the localised compaction by simply using good quality tyres and always maintaining the lowest pressure for the load,” he adds.

Types of cultivations

These figures are quite variable – influences include the types of cultivations involved, how loose and damp the soil is when drilling, and the soil type, of course. Philip has compared yield effects in conventionally farmed, reduced till, and direct drilling situations, and these days focusses primarily on minimum till and direct drilling scenarios. The latter has arguably the most supportive structures, but much to gain from a ‘prevention rather than cure’ approach.

A farm using a 4m wide drill – which is quite common in the UK – could be suffering an overall yield loss across up to 40% of its cropped area, says Philip.

A drilling tractor on 710mm wide rear tyres with a ground contact patch of about 0.75m/tyre, will be running over 1.5m of ground in every pass. Including the headlands this usually equates to over 40% of the land being trafficked.

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This field has been ploughed, pressed and drilled with a conventional disc cultivator drill, eradicators set deep, tractor tyre pressures at 1.6 bar. Taking the yield in the un-trafficked area as 100%, measured yields in the wheelings are down to 85%.

Overall yield loss of 5%

“My tests have found the compaction commonly reduces the yield by up to 15% in these wheelings. That’s a very conservative figure – losses are often much higher. If this is applied across the whole cropped area it’s amounts to an overall loss of 5%, or more, straight off the bottom line,” he explains.

Eradicators are also not the solution – once soil has been compressed, and its pore structure compromised, loosening does not restore this immediately – natural actions of growing roots and other biological factors are needed over time, which is not available in the case of the crop just established.

Correct tyre pressure

Setting the correct tyre pressure can have a big impact on cutting compaction and reducing the subsequent yield loss it causes, says Philip. “High pressures not only cause compaction, but will also reduce traction and increase wheelslip that leads to smearing. The denser, wetter soil in these areas can also stimulate blackgrass growth,” he says.

VF (Very high Flexion) tyres

Modern technology, such as VF (Very high Flexion) tyres provide a large contact area and are able to operate at very low pressures. “But many operators will compromise and set a ‘reasonable’ pressure of 1.2 to 1.5 bar, which allows them to travel on the road up to 50km/hr at pressures, as well as work in the field. This really defeats the object and doesn’t allow them to make the most from the tyre technology,” he explains.

“If in doubt, involve the tyre manufacturer, working to their lowest safe advised pressures ensures that expensive damage is avoided,” he advises.

Reducing pressure to 0.5-0.6 bar produced an increase in yield of between 10% to 15%, compared with the standard 1.0 bar pressure

Philip’s more recent trials show how reducing pressure has a direct effect on increasing yield in the wheelings. “In the tests the farm ran the same tractor and (direct) drill in the same field at various pressures from the ‘normal’ right down to 0.5 bar. The results were clear – reducing pressure to 0.5-0.6 bar produced an increase in yield of between 10% to 15%, compared with the standard 1.0 bar pressure,” he explains.

These results were very significant – on the back of a difficult season with high rainfall following drilling, which exacerbated the effects of such pressure differences. “Clearly, such benefits confirm the advantages of controlling traffic generally. They also highlight the importance of attention to detail for key operations such as drilling.”

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Field trials revealed reducing pressure to 0.5-0.6 bar produced an increase in yield of between 10% to 15%, compared with the standard 1.0 bar pressure.

Variable tyre pressure

In field practicalities mean operators rarely adjust the tyre pressures, even though they know it’s beneficial. “But variable pressure systems, such as the Fendt VarioGrip and others, do make this possible to do relatively easily. While they cost about £12,500, there is an argument this will be recouped in higher yields, before considering benefits to fuel consumption and field efficiency through reduced wheel-slip” comments Philip.

“The soil must be dry enough to crack and make fissures below the surface. If it is too moist it will simply smear and make matters worse,” says Philip. “You need to know the actual depth of the hard layer to ensure the machine is set correctly – just below the critical depth. Text continues underneath image “Working too deeply will smear the soil upwards, as opposed to lifting and shattering. In turn this will also require high power because, effectively, the machine is now mole draining. But in most cases this extra effort will all be wasted because the smeared land at depth simply presents a greater barrier to root growth and the passage of water,” he explains.

Curing compaction

While prevention is always better than a cure, there are times when it can’t be avoided. Wet autumn root crop and maize harvests result in high soil structure damage. But in better conditions, Philip says the emphasis should always be on prevention.

“Compaction is directly related to the weight of machines and axle loads on the soil. While equipment is getting bigger and loads are higher, with modern tracks and tyres at sensible pressures the problem is actually getting better. If compaction exists it is not always deep and is now much easier to sort out,” he explains.

“Despite this some farms will continue to sub-soil to depths of 350mm, simply because that’s what they have always done in that field. There is also evidence that such deep loosening doesn’t have a long term effect, with the ground being too loose, and closing up again quite quickly.

“Aim to create structured columns allowing roots and water to penetrate, rather than a loose, unsupportive mass of soil likely to slump back. In effect, the aim is to replicate a more naturally structured soil found after a significant dry period”.

Soil is quite resilient

Soil is actually quite resilient, he says. “You can find, even after the wettest weather, when the ground dries it also naturally restructures – particularly the heavier soils with higher clay contents. This may mean deeper cultivations are simply not required.”

“It’s always important to check what is actually happening below ground, by digging in a number of areas per field. This will show you the extent of the damage and its depth – digging holes is a lot cheaper than sub-soiling unnecessarily!”

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Poor drainage is a yield robber. Check drains are working when it is raining and if outfalls are not flowing, suspect problems in the system.

Compaction easy to find

Compaction may be lurking underground, but it’s not a hidden problem, because it’s actually quite easy to find. If it’s not as obvious as water lying on the surface or in short, compact roots it will show up in lower than expected yields, which should direct you to areas that need further investigation.

Soil structure problems need to be cured with a combination of ‘metal and roots’

“There’s never going to be a single answer, but I feel soil structure problems need to be cured with a combination of ‘metal and roots’. While subsoiling and cover crops both bring benefits and one may be better in some situations, it’s more likely a combination of the two will bring the best results,” he explains.

“The lightbulb moment for me, many years ago, was understanding that it is actually plant roots that structure the soil. Metal, however, carefully targeted, can get the best out of such roots where they are struggling to cope. Recently, and importantly, this I am finding applies to all roots – be they of a commercial crop, or a cover crop.”