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Arable

Careful considerations for cover crops There’s still much to be learnt about growing cover crops – the key benefits and the pitfalls – as Dominic Kilburn discovers. The decision to grow cover crops is not one to be taken lightly. They have the potential to bring undesirable effects such as an increase in pests and diseases, weed issues, depletion of some mineral nitrogen and nutrients (which can affect the following crop) and, of course, there is the cost of the seed and herbicide. In addition, their benefits are not entirely understood. However, a reduction in soil erosion and nitrate leaching, an increased supply of nutrients and organic matter, weed smothering abilities and soil structural improvements can all be listed as potential positives. Agrii’s Dr Syed Shah (right) admits to being a little sceptical surrounding some of the benefits cover crops can bring. While he agrees that they will enhance the soil in terms of organic matter levels, for example, there’s a big question mark as to how long this might take to achieve. “If your existing soil organic matter level is at two per cent and you want to raise it to four per cent, then you will need to apply 18t/ha of carbon. “That would take 30 years if the cover crop delivered 600–800kg/ha of carbon, depending on the type of crop and how it was incorporated. “And if you were to grow a cover crop on a particular field just once every three years in the rotation, it could take 90 years to get the required increase in organic matter,” he stressed. Having said that, Dr Shah acknowledged that species like

Why cover crops? • Reduce soil erosion • Trap nitrogen and reduce •

oil radish, and mixes such as oats and vetch, which have deep and lateral rooting, have the potential to improve soil crumb structure and soil biology. But recent work he has undertaken at AgriiFocus trials has highlighted that a cover crop’s ability to hold on to nutrients for the next crop, and also reduce leaching, may be their key advantages.

be done in good time to ensure the following crop gets the benefit at the right time. “Cover crops will also remove readily available potassium from the soil and this needs to be taken into account when considering the next crop. Potassium takes about a year to become available so this should to be thought about well in advance.”

Increased yield Trials in 2012/2013 on sandy loam soil set out to investigate the effect of cover crops on the yield of spring barley. The crop of NFC Tipple was drilled 21st March (2013) into a plot which had remained as stubble over winter, and into another following a fodder radish crop which had been drilled the previous August (and sprayed off at the end of January before being ploughed in). A range of nitrogen treatments was applied during the season to both plots. Taken to yield in August, the plot which had featured a cover crop, on average, out yielded the overwintered stubble plot by 0.8t/ha (7t/ ha compared with 6.2t/ha). The improvement in yield was seen across the full range of nitrogen application comparisons which ranged from 0–180kg N/ha, pointed out Dr Shah. Further trials measuring nitrogen accumulation in six different cover crops (2014/2015) demonstrated considerable reductions in nitrate leaching compared with the (stubble) control. “Cover crops can trap nitrogen and release it for the next crop, but it must be the right cover crop

leaching Improve organic matter – but this takes a long time Can control weeds through smothering

Oil radish has a good tap root and shows some attempt at breaking through compacted soil.

grown at the right time and in the right conditions,” he commented. “In our trials all cover crops accumulated a significant amount of nitrogen in the above-ground biomass where they converted anything from 30-90kg/ha N. “However, the highest biomass was found in crops drilled on the 15th August on loamy sand, and the poorest biomass was in crops drilled on the 19th September on silty clay loam. “You must treat a cover crop as a crop. It needs a good seedbed and it must be drilled in time or else its GAI will be down, and also its nutrient retention. “Sunlight makes a difference,” he added. Dr Shah also suggested that it is important that growers don’t leave it until the last minute to spray off a cover crop. “Get rid of the biomass early otherwise it can affect the establishment of the next crop in terms of nitrogen availability and slug populations. It takes about two months after the cover crop’s destruction before nitrogen starts leaching out, and so this needs to

Nutrient status His Agrii colleague Andrew Richards concluded that growers must consider the nutrient status of their soils following cover crops. “Although cover crops do trap nutrients, not all are made available for the spring crop and so it’s important to compensate where necessary. But at the same time, they must consider that they are already spending money on growing a cover crop and do they want to be spending more on nutrient compensation?” he questioned. “If you want to improve organic matter, then apply organic manure. But if you want to reduce leaching and improve soil biology and structure, then cover crops have a role to play.” ■

Oats and vetch breaks through compaction well.

Cover crop dynamics – at a glance • Oil radish – Good tap root which shows some attempt to break through • • Phacelia has a system of lateral roots creating a good crumb structure within the top 10cm of the soil.

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the compacted layer. Lateral roots which enhance crumb structure. Oats and vetch – Good crumb structure. Combination of both lateral and deep roots and breaks through compaction well. Phacelia – Good system of lateral roots creating a good crumb structure within the top 10cm. Lacks ability to break through compaction below this.

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