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The potential for cover crops and improving cultivations

Publication Date: 
4 August 2016

James Daw, host farmer of the Strategic Potato (SPot) Farm, West sited at Thorpe Constantine in Staffordshire has clear ideas about cultivations.

He has expressed these frequently at meetings and seminars but has now gone further and demonstrated the potential for improvement and the latest technical know-how to fellow growers using his own farm as a proving ground.

One of the features of the SPot Farm is that trials are not replicated over small plots but rather conducted on a field scale as part of normal farm procedures, giving confidence that changes can be realistically integrated into current practice.

In summary, Mr Daw, who also offers up his time as an AHDB board member, believes that cultivations for potatoes are often carried out at too great a depth. Apart from being over-costly on fuel and labour he feels that too fine a tilth at depth is doing nothing for yield and potentially harming following crops. “This was especially the case in 2012,” he told a group of visitors from Scotland. “In many cases growers are using the horsepower available to them now just because they have it rather than because it is needed.”

That is not to say however that he skimps on primary cultivations. All operations are carried out in the spring just before planting. In some years, such as this one when the spring was wet, he uses the plough over the 800 acres devoted to potatoes. More typically he uses a Simba SL working to a depth of 270mm with tines set at 380mm width.

For this second season at SPot Farm West, cultivation best practice continued to be tested, but with some incidental benefits arising from trials newly added this year – particularly the introduction of cover crops over winter.

The cover crop demonstration

In early September 2015 a mix of 50kg/ha of oats and 7.5kg/ha of vetch was seeded into wheat stubble. It was part of a test of whether nitrogen fertiliser should be reduced after application of extra organic matter such as manures or a green manures.  

Nitrogen applications have to be finely balanced. According to Marc Allison and the team at NIAB’s Cambridge University Farms (NIAB CUF) supervising the trial, the extra nitrogen that many growers tend to add ‘for insurance purposes’, has previously been shown to cost money and reduce yield. At the same time under-applying by 20kg/ha could take around six tonnes per hectare off yield. Therefore understanding the impact of cover crops or other added organic matter on the nutritional profile of the soil for the following crop is important.

Cover crops can improve cultivability

Most of the cover crop was subsequently sprayed off mid-March as is common practice, but a strip of 16m (eight beds) was left live right up to ploughing (see image below).

The vetch germinated poorly and the oats only grew to about 25cm, but the effects of a rather unimpressive looking crop were much greater than expected. The operator reported that during ploughing, when he reached the live strip of oats, the tractor was able to go up a couple of gears.

James Daw’s Agronomist, Mark Taplin, took samples of the soil at 10cm, 20cm and 30cm depth and it was found that under the live crop the moisture content was about 16% at all depths, compared with 17% under the dessicated crop. This is a small difference, but Mark Stalham of NIAB CUF considers that it could be crucial, particularly on soils prone to delayed planting following a wet spring.

Effects of cover crop treatments on soil moisture on April 20th and ploughing speed on April 23rd


Friable soil on left after live cover crop, cloddy on right after dessicated cover crop.


Continuing conventional cultivation explorations

Other trials in an adjacent field look at the differences between shallow ploughing, deep ploughing and the non-inversion cultivation which is preferred farm practice on the mixed soils at Thorpe Constantine.

Mr Daw believes it is vitally important not to over-cultivate or stone-separate at too great a depth. If he does have to use the bed-tiller he instructs the tractor driver to use it only in the parts of the field that really need it. The rest of the run can either be done shallower at a greater speed or the machine can simply be lifted out of the ground where it is not needed.

Considerations when using cover crops to aid cultivations

If opting to explore the impact of cover crops on cultivability, there are a few things to consider:

  • This point in time, early August, is the critical point at which to make a decision on over winter cover crops.
  • Ensure that early access is possible to fields intended for potatoes.
  • Choice of cover crop is linked to its purpose. If attempting to reduce soil moisture to improve cultivability, preferences in order of merit are rye, then oats, then wheat, then barley.
  • Allow time to source cover crop seed at a reasonable rate. For this purpose the cheapest seed will be sufficient.

James Daw continues to open up the SPot Farm to visitors interested in discussing the implications of the demonstrations for the benefit of the whole industry, with Farm Walks set for 19th August and 8th September (register at 

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