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Timely and well-chosen post-harvest cultivations will help make soils more resilient to wet and dry conditions, says agronomy firm Hutchinsons. Be clever about post-harvest cultivations

Timely and well-chosen post-harvest cultivations will help make soils more resilient to wet and dry conditions, says agronomy firm Hutchinsons.

In the rush to prepare for drilling, it can be tempting to go straight in with the cultivator or subsoiler as soon as the combine leaves the field. But doing so may not be best for soil health or crop establishment.

“Before doing anything, it is important to stop, take a step back and consider what the soil needs,” says Hutchinsons head of soil health Ian Robertson. Growers should adopt a flexible cultivation strategy, he says.

“Clever cultivation can mean anything from not cultivating at all to subsoiling or ploughing where necessary. As a general rule, never cultivate at the same depth every year and make sure whatever you do delivers what the soil actually needs.”

Low disturbance subsoilers are increasingly popular for rectifying structural issues in shallow tillage systems. Such implements are often needed to break up distinct layers that can form where ground has been repeatedly cultivated at a shallow depth..

Soil requirements

Cultivating repeatedly at about 50mm can restrict water infiltration and root growth. But  the need for remedial action can often be avoided by adopting a more varied approach to cultivations in the first place.

The first step in deciding whether any cultivation is required is to dig a few holes. Then identify whether any structural issues need addressing. These can include problems such as compaction or poor drainage.

Soil assessments are usually best done in spring or autumn when ground is moist and warm, with active root growth and biological activity. When assessing soils in summer, care is needed not to mistake dry, hard soil for being compacted.

The bubble test (see panel) is a simple way of identifying whether dry soils are compacted. Infiltration tests are also useful, but when conducted in summer, make sure water does not flow straight down cracks, says Mr Robertson.

“Typically, 50% of soil is made up of air and water, so it may be that rock hard ground just needs wetting-up again to return to a friable surface that can be drilled straight into.

“In the past two years, we’ve seen examples where growers have rushed to create a seedbed after harvest, only for heavy rain to make it unworkable and un-drillable later in autumn. In some cases it may have been better not to touch it.”

Root networks left by crops do a fantastic job of stabilising soil aggregates. They also improve the porosity and structure of the top layer that crops are drilled into, so leaving this undisturbed can often be a better choice.

Nine times out of 10, the top 50mm is actually in good condition, agrees Hutchinsons technical manager Dick Neale agrees. “Stubbles generally handle moisture much better than a cultivated surface,” he says. 

“If you’ve got a nice friable surface that has managed moisture well, most modern drills are capable of drilling directly into stubble, so there’s no need to cultivate. Cultivations destroy aggregate structure, which takes time to rebuild.”

Not disturbing the surface offers significant benefits for moisture conservation too. This can make all the difference when establishing crops such as oilseed rape or early-sown wheats in dry autumns, adds Mr Neale.

“Moisture conservation and managing moisture within the seedbed have got to be an absolute focus. Oilseed rape in particular is better direct-drilled with a disc or tine-based implement to minimise soil movement and conserve moisture.”

Given the importance of even sowing depth for such a small seed, Mr Neale advises against seeder units on subsoilers. Growers should plan rotations and cultivations carefully to ensure any structural issues are rectified in preceding seasons, he says.

Target cultivations

If soil assessments reveal some form of cultivation is required, both experts urge growers to select operations and implements suited to the specific soil requirements.

“If compaction is identified, consider where it is, how extensive it is and what depth it is at, so machines can be setup correctly to address this,” says Mr Neale.

“Don’t assume poor water movement from the surface is due to deep compaction and poor drainage; it may be a surface issue that’s easier and cheaper to rectify.”

Many soil water management problems in recent seasons have been caused by issues of consolidation, slumping or capping in the top 100-125mm of soil, not by deeper compaction.

In such situations, running a subsoiler through at 250mm deep could make the situation worse. Soil moisture content is critical to operations such as subsoiling and mole ploughing, adds Mr Robertson.

“Subsoiling needs soil to be dry enough for natural fissures and cracking, but if it’s too dry, there’s a risk of bringing up large slabs and creating an uneven surface. If conditions aren’t right, don’t rush into doing it.”

It is also important to remember that cultivation stimulates germination by bringing fresh weed seed to the surface. This can be used to manage blackgrass, where shallow  cultivation encourages a chit that can be sprayed off before drilling.

“Remember though, blackgrass won’t want to grow until September or October, so timing is key,” says Mr Robertson. And ploughing can be useful in some situations, such as where there has been a high black-grass seed return that year.

That said, any soil movement increases moisture losses in a dry autumn. Where seedbeds are prepared in advance of drilling, they shouldn’t be cultivated too fine to make them more resilient to heavy rainfall.

Cover crops

Whenever soil is moved it is vital to encourage new root growth as quickly as possible – whether from a commercial crop, catch crop, or cover crop. This stabilises soil and helps rebuild soil structure, porosity and organic matter.

Six weeks or more is generally required for a catch crop to be worthwhile. So the main opportunity is between early-harvested crops like oilseed rape or winter barley and a following winter wheat.

Oilseed rape volunteers offer a simple, cheap and effective catch crop. But Mr Neale says dense regrowth can dry soil significantly. This has the potential to cause issues for establishing following crops.

He suggests raking out some rape volunteers and sowing other catch crop species into the stand, such as buckwheat, berseem clover and vetch, to bring more diverse root structures and improve soil moisture.

Where land is going into spring cropping, over-winter stubbles will offer some soil protection. But Mr Robertson’s his preferred approach is to have something growing, such as a multi-species cover crop, to stabilise soil, build structure, retain nutrients and add organic matter.

If land requires cultivation, this should be done when soil conditions are suitable after harvest before sowing the cover crop. “Treat cover crops like any other crop. Establish them properly and understand how to manage them according.”