Monday, 15 August 2016

Cover crops as drying agents - a happy accident!

We like stories of discoveries made by accident, with penicillin a prime example. 

The use of cover crops for making soil easier to cultivate isn’t a completely new idea, and it hardly comes into the penicillin category, but it was a striking finding at SPot Farm West this year which could benefit many growers.

The aim of the cover crop trial, as planned by Marc Allison of NIAB-CUF and host-farmer James Daw, was to see what reduction in nitrogen fertiliser should be made when cover crops or manure applications are made. There is a risk that manure or an incorporated cover crop could lead to excess nitrogen (N) take-up by the subsequent potato crop. 

The finding that a live cover crop at ploughing made the ground easier for all cultivations was incidental.

The cover crop recipe this year at SPot Farm West

  • 50kg/ha oats plus 7.5kg/ha vetch was broadcast onto wheat stubble in Field 35 at Thorpe Constantine in early September 2015, followed by use of a V√Ąderstad Carrier
  • This did not give enough tilth for the vetch to establish well. The oats grew to a height of 8-10 inches by early March
  • Cover crops are commonly killed before cultivations, to ensure they don’t interfere with machinery, and most of this crop was sprayed with glyphosate in mid March
  • However, a strip of 16 m (eight beds) of the oats was left alive

Oat plant before dessication

The findings
When ploughing this field the machine operator noticed that, with the same engine speed and revs, the ploughing was faster on the live crop. There was such a difference that he immediately called James Daw to come and look. 

They saw that the soil under the live crop was friable and had fallen apart nicely when ploughed, while under the dessicated crop it was more cloddy.
Friable soil on left after live cover crop, cloddy on right after dessicated cover crop
Agronomist, Mark Taplin, took samples of soil at 10cm, 20cm and 30cm depths 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 (also of NIAB-CUF) considers that it could be crucial. 

Martin Wood, a soil scientist with Earthcare International agrees that soil strength varies greatly, with soil moisture content at both the ‘too wet’ and ‘too dry’ fringes of the cultivation window.

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

Gravimetric SWC (%)
Plough speed (km/h)
Growing cover crop
Desiccated cover crop

Choice of cover crop depends on purpose 

Which cover crops are best?

In this case, with the aim being to take moisture from the soil at depth to improve cultivability, Ian Wilkinson of Cotswolds Seeds and Mark Stalham of NIAB-CUF are agreed on the answer. 

Their preferences in order of merit are rye, then oats, then wheat, then barley. They don’t recommend rye grass, which can grow too vigorously and become a nightmare when de-stoning. 

Ian found that in the Cotswolds Seeds trials the best crop for this purpose is rye, because of its good establishment and growth in low temperatures, and he considers that 90% of the rye effect can be achieved from oats.

It all depends on the year?

I can hear the well-reasoned objections: 

  • In a good early spring there may be no need to dry the soil, and then the cost of putting in a cover crop would have been wasted.”  
  • “In some years the cover crops could grow too strongly and impede cultivations”.  

There are ways to avoid these problems. A cover crop is unlikely to be wasted. 

  • Mark Stalham says that for soils with more than 10%-14% clay, i.e. sandy loam and heavier, there will only be one year in five when the soil is dry enough at depth to cultivate for potatoes without damaging soil structure. In places with higher clay content, or higher rainfall, benefits are likely to be greater. 
  • The risk of an over-dense cover crop can be reduced by a low seed rate and not sowing too early - Ian Wilkinson (Cotswolds Seeds) suggests that the first half of September is the best. 
  • If the crop is too energetic in spring it can be flailed - This is normally a better solution than dessication, which quickly stops moisture uptake. Sometimes the flailed crop material becomes slimy after wet weather and grazing can be an excellent alternative. Dessication remains an option, so the cover crop should never become too much for cultivations to tackle.

Reports from salads grower Jepco on improving cultivability with cover crops

The operations director of Jepco, Phillip Hubbert, reports increasing use by his company of overwinter cover crops prior to salad production. 

They grow in South Lincolnshire near to The Wash, on silt soils with organic matter levels of 2.5%. Land in this area is used intensively for salads and salad onions and structure is being lost due to over-cultivation.  

In the past Jepco practised ‘winter ploughing’ in October/November. 

The new pattern is that after wheat, sub-soiling is followed by a light cultivation and then sowing of either winter oats or a rye + vetch mix with a 4m corn drill. 

Oats (seed cost £25/ha), are used before cultivation in March, and a rye+vetch mix (seed cost £75/ha), before later cultivation. 

A strong crop of oats can have hard stem nodes by late April which interfere with land preparation, while rye and vetch breaks down more easily.  

Immediately before ploughing, the cover crop is flailed and the plough is set carefully to place the green material completely at the base of the inverted soil.  The next day beds are created with a triple bed-former and the salad crop is sown.

Jepco introduced cover crops for all-round soil improvement rather than for drying the soil in spring, which is not commonly a problem in their area. 

One benefit has been a reduction of 15l/ha in the diesel used by the bed-former, comparing land under cover crop with that ploughed in autumn. 

They also save some nitrogen, worth £30/ha. 

Having been convinced of the value of cover-cropping they are now refining the system. 

Initially it was not treated as a crop, but now 20kg/ha of nitrogen is applied in early spring and they would even irrigate at establishment if necessary.

Slugs are a problem in salads and Jepco used to dessicate cover crops a few weeks before ploughing, in hopes of better slug control. 

This was felt to reduce the cover crop benefit and instead bait traps are used through winter, with slug pellets applied to the cover crop and in front of the planter if necessary.

Potato growers will use cover crops for soil drying

James Daw, the SPot Farm West host, has been convinced by his own experience and will grow cover crops pre-potatoes on all of their own land this coming year. 

The Daws will also try to  put in cover crops on the land they rent for potatoes. Sometimes this is welcomed by the owner since it contributes to EFA requirements, but they don’t expect to have access to all the rented land to be able to sow cover crops everywhere they’d like.

Peter Grewar, major potato grower in Perthshire, heard about the benefits for cultivability from the unsprayed cover crops demonstrated at SPot Farm West. 

He now plans to grow oats over winter before potatoes, saying that while he has used cover crops for EFA (ecological focus area) requirements, he hasn’t yet used them before potatoes. 

Since wet soils in spring often delay planting, the Grewars are hoping to prepare the ground earlier than would otherwise be possible.

Considering cover crops on your own farm?


Your cover crop experiences, please!

It may be that the correct use of the right cover crop can build resilience to difficult spring weather into the system. 

Do you agree? 

Your experiences and views would be very welcome. 

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