Are Cover Crops a Valid Addition to Arable Rotations?

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After the cold and wet spring of 2016 many are struggling to see a benefit in the use of cover crops. Are they a valid addition to arable rotations, or just an enigma?

“Results in the field are a far cry from the promise of transformed soil structures, biological enhancements and significant improvements in weed control that were promoted so strongly,” says Dick Neale, Hutchinsons technical manager.

“For many, the result has in fact been wet, cold, slug infested seedbeds that refused to dry. “The high cost with little apparent return will cause many growers to question the value of using cover crops this coming season.”

“However, it’s so important to understand why you are growing a cover crop be that as a green manure, for weed suppression, as forage or for generally improving soil structure and organic matter.”

None of these are mutually exclusive - a cover crop grown to improve soil structure will also store nutrition, stabilise the soil surface, most likely suppress weed growth and increase soil organic matter,” he says.

He points out that it’s important to remember that a cover crop is the same as any other crop in the soil - it is just being grown in a different context. The multiple benefits of cover cropping are cumulative over years and one year’s beneficial or negative impacts should not be overstated on the one hand, or condemned on the other.
“Most cover crops will be grown on soils destined for spring cropping. For lighter soils without black grass, this means they can be sown as soon as the ground becomes available in the autumn.“

“The primary objective on light soils will be increasing organic matter, soil stabilisation and fertility building. Control of difficult weeds like annual nettle will also be a consideration.

“Seed beds are easy to achieve in light soils, so growing plenty of biomass is not a problem and generally should be a priority on these soils.”

However when it comes to heavier soils the issues are very different; clay soils are wetter soils, and moisture management is critical says Mr Neale.

“On heavy soil growers should take the view that they are growing a ‘conditioning’ crop not a cover; we do not want to actually fully cover the ground. Instead sun and wind should reach the soil surface through early March, so the soft friable surface will break down during the drilling process and allow good seed coverage and good seed-to-soil contact.”

“The cover crop will extract moisture over winter and the lower soil horizons will always be drier than uncovered clay. The issue is that, if the soil surface is in shade from a dense cover canopy, the 50mm zone you want to drill into will remain wet. It is soft, so a drill disc or tine will easily penetrate, but the drill will fail to close the seeding slot and it dries open and the crop fails.

Cover crop worm structure

“Heavy soils almost inevitably have black-grass these days, and spring cropping is beginning to feature primarily as a control tool for this weed. Cover crops in this situation have to combine with the overall black grass control strategy, so there is no point in trying to establish a cover until the late September/early October flush has been sprayed out.”

Sowing in October means the cover must be of robust, late establishing crops like peas, beans and oats. Although brown mustard, phacelia and spring linseed are surprisingly robust as part of a mix at this time, the use of the exotic species should be questioned.

Late sowing is only important where you have grass weeds to manage out, but in heavier soils the more robust plants do better than small seeded types. Linseed, phacelia and white mustard seem to be the small seed exceptions to this rule.

Nuffield scholar, Russell McKenzie, who has a keen interest in cover cropping, has been working closely with Hutchinsons at the Brampton site looking specifically at the effects of the cover crop on soil structure and conditioning, as well as nutrient capture.

“When deciding on what to grow, it’s really important to look at the mix of crops and choose those that are not going to impede nitrogen availability in order to avoid any pitfalls in terms of speed of crop establishment and crop vigour,” he says.

“Look to use a variety with a penetrating tap root which will have a good effect through the soil structure, also think to include a fibrous rooting species to open up the surface structure and friability in the surface layers.  For example, the mix that I have used at my home farm contained phacelia, linseed, buckwheat, sunflowers, maple peas and fodder radish,whilst that used at Brampton consisted of oats, peas and beans,” he says.

Cover Crop -fodder radish

Mr McKenzie explains that to allow the natural recycling of nutrients, the cover crops were grazed off with sheep in November reducing potential nitrogen lock-up, whilst allowing sun and wind to dry the surface which is critical on clay soils in the spring.”

 “We have learnt from our work in tillage systems for black grass and general weed control, that roots draw moisture toward them.  Water is drawn toward the root by capillary action via micro pores in the soil resulting in the wettest zone being immediately around the root ball, so in clay soils the cover crop species and seed rates chosen should allow for good soil surface drying in the early spring.”

“Covers which are too thick should be killed off pre-Christmas on clay based soils. Once sprayed off, the root ball moisture dissipates back through the soil, while sun and wind dries the surface.”   

Neil Watson, southern regional technical manager for Hutchinsons adds that the high number of slugs seen in 2015/2016 should not be attributed to cover crops. “The mild winter has seen little mortality in the slug population and fresh food and retained moisture in the base of covers has supported slug survival. However, it is the lack of overwinter kill and the wet spring that are the primary drivers of slug survival - the cover is not the deciding factor here.”

Oats, cereal rye and black grass produce allelopathy that negatively impacts on following cereal crops is known, but has been largely ignored. “Apart from slower growth, the symptoms of negative allelopathy in spring barley and wheat are very similar to residual herbicide damage - this issue is easily avoided by planning species choice pre cereals, the level of seed inclusion, date of sowing and date of destruction prior to sowing of the spring cereal. The cold soils in spring 2016 will also have slowed the degradation of the allelopathic exudates,” he says.

Neil Watson
Monitoring soil condition, Neil Watson, Hutchinsons

The use of a soil augur allows you firstly to look at soil texture, by examining soil cores from down through the soils profile.

This may help to identify any issues such as a slow draining permeable subsoil overlying a more permeable top soil which is causing the ponding for which there is little you can do. However, if it is a structural issue caused by compactions it will also be evident.

With the augur it’s possible to see how deeply  the roots are penetrating the soils profile, and look to see if there are any obvious structural impediments and at what depth.
A second tool used to look at soil structure is a soil penetrometer, says Mr Watson.

A soil penetrometer aims to mimic the root development as it moves through the soil profile, by measuring the resistance encountered as it is pushed to depth. The actual amount of resistance encountered and at what depth it occurs is a useful indicator of potential compaction issues.

An infiltrometer measures how easily water is able to move through the soil profile; this effect can  be mimicked  by using small sections of  drain pipe filled with water pushed into the soil, then measuring how long it takes for water to move through the soil.

The important point about all three pieces of kit is that they allow you to examine your soils critically and quantifiably in a manner that would not otherwise be outwardly visible.  Doing this before putting a crop in the ground is a less expensive option than waiting for crop yield to reflect any costly structural issues.