Potato Review

30 POTATO REVIEW JANUARY 2022 SOIL HEALTH G ROWERS face a ‘postcode lottery’ if plans to reward them financially for increasing soil carbon stocks are to be introduced, two leading soil scientists have warned. Rothamsted’s Professor Andrew Neal and Professor David Powlson say geological history alone, which determines the type of soil found on a farm, will make capturing more carbon in the soil “nigh on impossible” for some growers. Andy said: “If you farm on sandy soils, as is the case for farmers in places such as Bedfordshire or Nottinghamshire, then you will struggle to increase the carbon content of your soil. You might be doing all the right things to increase the carbon flowing through your soil, but it won’t show ‘Not all soils are equal’ Sustainable Farming Incentive scheme may lead to ‘postcode lottery’ over soil carbon say experts. up when they come to test your soil.” Under proposals put forward by the Government as part of its post-Brexit shake up of UK agriculture, farmers would be paid to manage their farms using environmentally- friendly practices, often termed ‘regenerative agriculture’ but Andy says the potential for carbon sequestration is strongly influenced by soil type — particularly texture — and the starting carbon content, which will be a legacy of past farming practices. Flow should be measured “Changes in soil carbon in response to alterations in management practice occur slowly. It’s also difficult to measure the likely small changes in soil carbon within a short time period. Indicators such as soil microbial biomass are useful in showing whether organic carbon is increasing or decreasing in response to a change of management - but these measurements provide neither an estimate of soil carbon stocks, nor a prediction of absolute changes in carbon,” said Andy. Andy and David believe it would be better to introduce an alternative system that measures the flow of carbon through soil, rather than the actual amount in soil at a given time – what Andy calls a “dynamic rather than static” view of soil. David said: “There is much interest in sequestering carbon in soil as a means of mitigating climate change by making payments to farmers in return for the amount of carbon sequestered. There are several ‘early warning’ methods that may be successful in detecting whether or not a soil is increasing its carbon content, and this is helpful. But they will probably not provide direct evidence of the absolute change in total carbon that has occurred.” Soil organic matter contains about 50% carbon and influences virtually all soil properties. Recent research using Rothamsted soils by Andy and colleagues has shown how the structure of the soil is affected by the processing of organic matter inputs by soil microbes as metabolites form associations with mineral particles. The resulting structure in turn impacts oxygen diffusion through the soil and the microbial processing of carbon and other nutrients important for crops. “Imaging of pore networks clearly shows how a clay-rich soil adapts to differences in organic inputs, but a sandy soil shows little adaptation. Thus, the physical structure and resulting changes in biological activity, are less influenced by organic inputs in sandy soils. This is consistent with the observation that sandy soils have much less capacity to sequester carbon than soils of finer texture,” he said. David Powlson (left) and Andy Neal (right)