Understanding more about organic matter and soil carbon can help growers improve soil health, get more from crop nutrition inputs and build greater overall resilience into their crop production, believes Agrii agronomist Will Francis.
“So many aspects of profitable crop production are affected by the condition of your soil that it’s worth spending a bit of time getting to know the type of soil you have, what its organic content is and what management you can focus on to get it working to its full potential,” he says.
“Organic matter is basically plant and animal residues at various stages of decomposition with three known pools in the soil depending how far this process has proceeded.
“The first is ‘active’ organic matter which is made up of recent organic matter inputs and soil organisms such as those from previous crop residues and is not very decomposed such that you can still tell what it is made of.
“Next is ‘slow’ organic matter which refers to organic compounds derived from the active pool which are slightly more decomposed and identifying the origin of these inputs/organisms is more difficult.
“Finally, ‘stable’ means fully decomposed organic matter where you can no longer identify the origin inputs and organisms as is the case with humus.”
Soils with higher levels of organic matter are generally seen as healthier and more resilient with several benefits resulting from this, he says.
“High organic matter soils tend to store nutrients better and act like a sponge when it comes to water retention as well providing the essential energy for soil biology which is responsible for the decomposition process converting carbon from ‘active’ to ‘stable’ organic matter.
“During this process, the soil biology secretes the sticky substance Glomalin which binds soil particles together, aggregation, and this also improves overall soil structure which, in turn, facilitates greater aeration and water infiltration through the soil profile.”
Improving carbon use efficiency and C:N ratio
The soil microflora also feeds on the carbon from the residues/organisms as the organic matter decomposes and the efficiency of this dictates what the overall carbon use efficiency (CUE) of the soil is, Will Francis explains.
“CUE is the percentage of carbon consumed that is used to make biomass. The higher the number, the more carbon has gone into the soil and the less into the air as CO2.
“It is an important factor when building soil organic matter and soil health and like nitrogen use efficiency (NUE), high CUE can lead to lower emissions and a faster increase in soil organic matter.
“Soils with a higher proportion of larger, more recalcitrant molecules, such as those found in straw for example, have a lower CUE with fungi tending to favour these types of molecules in the decomposition process.
“Simpler molecules with a lower carbon to nitrogen ratio have a correspondingly higher CUE with bacteria more active on these easier to digest molecules. In addition, some microbes are more efficient at decomposing than others and therefore contribute to a higher CUE.
“By considering carbon use efficiency and the carbon to nitrogen ratio (C:N) of crop inputs and their effect on organic matter, growers can also influence the microbial makeup of the soil, particularly the fungi:bacteria ratio and this can have a significant impact on the profitability of crops.
“The C:N ratio is also important for managing NUE, which is a key factor in economic productivity and crop performance and a key consideration when checking soil sample results and deciding on the most appropriate inputs for nutrient management, such as cover crop species, organic amendments and fertilisers.
“The ideal C:N ratio from a soil test is around 12:1, but this differs from the ideal C:N ratio for inputs, as not all the carbon will remain in the soil.”
Measuring soil carbon for revenue generation
In the current production climate, there are other benefits to understanding more about soil organic matter and carbon content, he says.
“Soil organic carbon is different from organic matter and it is this which carbon markets tend to look when evaluating payments.
“Organic carbon is a measurable component of organic matter and it usually assumed to be 50-58% of total organic matter with the other 42-50% deemed to consist of other organic compounds such as hydrogen, oxygen and nitrogen.
“Carbon markets tend to look at the amount of organic carbon in soils in tonnes per hectare (t/ha) to a defined soil depth, usually 0-30cm, 30-60cm and 60cm depths, so it is important to check guidelines with a recognised market before undertaking samples.
Soil carbon tends to be analysed most commonly using the Dumas method which is favoured by laboratories due to its improved accuracy, he adds.
“The sampling strategy and number of samples taken needs to align with the carbon scheme being joined and this will depend on factors such as soil type, management practices and the potential for change. Samples will usually need to be geo-referenced.
“Fine dry bulk density measurements also need to be taken using a corer. This cannot be taken using a standard soil auger or trowel in the field and remember, only certain labs offer bulk density tests.”
