Arable News

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Nitrogen savings uncovered

Could cover crops planted after cereals and ahead of potatoes save growers the cost of some nitrogen fertiliser use as well as deliver benefits to the environment?

Could cover crops planted after cereals and ahead of potatoes save growers the cost of some nitrogen fertiliser use as well as deliver benefits to the environment? Dominic Kilburn reports on trials work undertaken by ADAS which suggests this could be the case.

Overall, fertiliser response results demonstrated on average that the equivalent of 50kg N/ha of cover crop nitrogen was recovered by the following potato crop.

A winter wheat crop will typically only use two thirds of the nitrogen applied to it in a growing season, with the remainder, plus anything extra gained from soil mineralisation, having nowhere to go after harvest and with the potential to be lost through leaching before the next crop is planted. This is particularly so when planting potatoes following winter wheat, where up to a five month window between harvest and planting can occur.

ADAS scientist Dr Martyn Silgram (pictured left) points out that any loss of nitrogen is a potential danger to the environment and a waste of money for growers. “We grow potatoes at a stage in the rotation where soils can remain uncultivated for several months over winter during the wettest time of the year, but there is an opportunity of using a cover crop to capture some of the remaining soil-based nitrogen rather than it being lost through leaching,” he suggests.

Martyn says that the amount of nitrogen which can be scavenged from the soil depends on the type of cover crop planted as well as the weather conditions over winter. “The key aspect for an autumn-planted cover crop is to get it in the ground early – by mid-September – to allow the crop to get a decent rooting depth in order to catch nitrogen as it moves down through the soil profile.

“The earlier the better,” he suggests. The weather also plays an important role in the success, or otherwise, of a cover crop; dry and cold winters are less favourable in terms of nitrogen uptake compared with warmer and wetter ones. “Our research and historic data suggests that in a dry year, about 15kg N/ha can be taken up by a cover crop but can increase up to 50kg N/ha or more in a warm wet winter,” he adds.

A cover crop takes up nitrate from the soil and converts it into plant material, with typically up to half the scavenged nitrogen then becoming available to the next crop and the other half helping to build soil fertility and improve soil structure by increasing organic matter content.

“We’ve looked at different species of cover crops, their scavenging of autumn soil mineral nitrogen, and the subsequent recovery of that nitrogen by the following crop,” continues Martyn. “How much nitrogen is available to the following spring-sown crop depends on which cover crop species, sowing date, weather and destruction date are used. For example, if you leave the cover crop in the ground too long into the spring before it’s destroyed, then it can get woody and its carbon to nitrogen ratio widens. This has the effect of locking up nitrogen over the short term, rather than releasing it to the next spring crop,” he explains.

“Some crops are better at scavenging nitrogen than others; for example brassicas tend to take up more nitrogen than grasses. Most cover crops however have to be disced-in to be destroyed, or sprayed with glyphosate, and so the costs of the cover crop seed, sowing and destruction have to be traded off against the financial benefits that can be made from fertiliser savings and improved soil structure and fertility,” he explains.

In an on-going project funded by the Potato Council, Martyn has explored the ability of cover crops to conserve nitrogen which would otherwise be lost via leaching in water draining from soils over winter, and optimise the value of that scavenged nitrogen for the next potato crop.

ADAS trials work that took place in Telford, Shropshire and in Scotland during the 2011/12 season looked at a range of cover crops planted following cereals and ahead of potatoes. The cover crops assessed included oilseed rape, mustard, oil radish and winter rye to determine the extent of nitrogen uptake on light textured soils.

At the Telford site, crops were drilled in late September with mustard dying off in mid-January and the remaining crops sprayed off in February. Residues were disced-in as part of standard cultivations prior to bed-forming.

“We measured 50-60kg N/ha locked up in the cover crops at destruction in mid-February from rye, mustard and radish; all proving to have a consistently good nitrogen uptake level across four replicates,” he says. “It was a dry winter too and so maybe they didn’t have to scavenge too deep for the nitrogen,” he adds.

With soil samples tested on the 24th February, 19th March, 10th April and 8th May, soil mineral nitrogen (SMN) levels were greatest from the mustard crop which, because it died earlier than other crops in the trial, released a higher percentage of nitrogen to the potato crop compared with the other crops. “Once cultivated in, the cover crops released most of their nitrogen over a four month period following destruction,” points out Martyn.

Overall, fertiliser response results demonstrated on average that the equivalent of 50kg N/ha of cover crop nitrogen was recovered by the following potato crop – with the mustard and radish delivering up to 60kg N/ha each, although a very wet May meant that some of the released nitrogen could have been washed away before the potato crop had a chance to take it up later in the season.

Martyn comments that the main associated costs with cover crops include seed; mustard and rye at approximately 40/ha, although if cereal volunteers are to be used as a cover crop then a farmer can use tail corn to reduce the expense.

He estimates 20/ha to broadcast the crop while destruction costs can be absorbed completely if they coincide with stubble cultivations or if crops which are not frost-hardy, such as mustard, are grown which will tend to die off naturally well before cultivations are planned. Costs can be partially offset through the points available in option EJ13 in the Entry Level Scheme under Environmental Stewardship.

“It’s clear that the cover crops did a very good job of taking up nitrogen in the first place, and then releasing it. I was expecting 30kg N/ha to be released by the cover crops at best, but we got 50kg N/ha on average and there were no effects on the potato crop in terms of disease, pests or quality.  “As well as capturing valuable crop-available nitrogen which would otherwise be lost through leaching (contributing to water pollution), the cover crop nitrogen also helps contribute to organic matter reserves. This is particularly useful on the lighter soils potatoes are often grown in as it improves soil structure, nutrient retention and water holding characteristics”, he concludes.

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