Plans to financially reward farmers for increasing soil carbon could run aground on an inconvenient fact – not all soils have the same ability to sequester carbon.

In the wake of Defra's announcement that farm support in England and Wales is to be linked to soil carbon levels, two leading soil scientists have warned a major farming conference that this may lead to ‘postcode lottery’ where some farmers will be doomed to fail.

Presenting their data at the University of Cambridge, Rothamsted’s Professor Andrew Neal and Professor David Powlson said that 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 farmers.

Professor Neal 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 up when they come to test your soil.”

Common ways to improve soil carbon include the addition of manures or crop residues, growing cover crops in the winter and the rearing of grazing livestock and crops together.

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The problem, said Professor Neal, was that 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.

“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.”

Speaking at the annual IFS Agronomic Conference, Professor Neal and Professor Powlson proposed an alternative system that measures the flow of carbon through soil, rather than the actual amount in soil at a given time – what Professor Neal called a 'dynamic rather than static' view of soil.

“If a system of policy requirements or financial incentives for increasing soil carbon is to be instituted, we propose an alternative approach using carbon models to predict probable changes in soil carbon taking account of the farmer’s soil type, local climate, cropping practices and starting soil carbon content. This could be combined with closer monitoring at a network of benchmark sites.”

Careful thought needs to go into designing replacements for current EU farm subsidies, added Professor Powlson, as there were 'formidable challenges' to doing this in ways that are both fair and practical.

“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," he said. "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.”