The guide lists and answers key questions for robust on-farm monitoring of soil carbon and associated indicators of soil health.
This is particularly significant given the recent announcement of the Sustainable Farming Incentive scheme, which is reliant on a consistent approach to soil sampling.
The guide was produced by the Duchy College, Plymouth University, Rothamsted Research and the Farm Carbon Toolkit.
Stephen Roderick, project management at Duchy College, said: “In the Soil Carbon Project we were trying to understand the relationship between farm management practices and levels of soil organic matter, as well as issues around how we monitor soil carbon."
Carbon sequestration plays a key part in climate change mitigation, but soil carbon’s importance goes beyond sequestering as much as possible, added Andy Neal, research scientist at Rothamsted.
“What’s much more important are the co-benefits of getting organic matter into soil – organic matter affects how much water and nutrients the soil can store, and can limit the carbon footprint,” he said.
Becky Willson, business development and technical director at Farm Carbon Toolkit, said that, traditionally, the sampling periods are in the spring and autumn.
“We have been looking at what’s happening at different depths, zero to 10cm, 10 to 30cm and 30 to 50cm; the important thing is to avoid sampling after cultivation, particularly if it has been ploughed.
"Leave the fields to settle after cultivation for at least three months,” she added. Farmers should sample at the same time each year because seasonality can affect results.
“If sampling just for organic matter, in theory the soil can be sampled at any time of year but be consistent with that approach.
Measuring soil health through carbon dioxide flush
"And if you’re sampling for nutrients at the same time, think about when that fits in best in rotation management.”
When it comes to sample location, it’s best to select fields which represent the variation across the farm, including differences in soil texture, cropping and management, she said.
There are three main options for sampling patterns within a field: Sampling in a ‘W’ configuration, in a linear transect or a grid formulation.
The guide recommends at least five sample points although 15 is preferable. Samples can be aggregated but only on a field or zone basis and they must be well mixed before bagging.
In terms of equipment, a soil auger is the best implement for the job – but digging a hole and removing soil by hand is fine. Clean buckets and sandwich bags are also necessary for collecting the samples.
Farmers should send the samples off as soon as possible, although up to two weeks’ delay is acceptable if they are refrigerated – this slows the organic matter breakdown.
In the lab, there are a couple of options for testing organic matter and soil carbon – Loss on Ignition (LOI) and DUMAS.
Jasper Newman, researcher at the University of Plymouth, said: “LOI provides a rough idea of the soil organic carbon content and is generally slightly cheaper, however it is not standardised between labs making it crucial to stick with the same lab."
The researchers behind the guide say it will help farmers and advisers to adopt the most consistent techniques for monitoring soil carbon – which will become more important as focus on carbon intensifies.
Mr Roderick added: “Robust estimates of soil carbon stocks can be a complicated subject; this guide is designed in collaboration with our research partners to answer those key questions."
