Measure active carbon to detect early changes in soil health

Measuring active carbon is a way to spot changes in soil health earlier than other indicators, including soil organic matter. Find out more below.

Soil organic matter is made up of different components or pools depending on how easily they decay. These components range from readily decomposable materials (e.g. crop residues and organic manures) through to more stable pools (e.g. humus).

Carbon makes up over half of soil organic matter. Active carbon refers to the proportion of soil organic matter (and soil organic carbon) that is most readily decomposable by soil microbes.

This can include particulate organic matter (such as crop residues), microbial biomass carbon (carbon contained within small living soil organisms) and carbohydrates (plant root exudates).

Changes in active carbon may be detectable before you can detect changes in soil organic matter (or organic carbon) following a change in soil management.

Active carbon in the soil helps build soil aggregate stability, improves water infiltration and soil porosity, and reduces erosion risk and compaction.

An increase in active carbon could indicate that pools of available carbon are building up in the soil that will promote healthy soil biology, improve nutrient cycling and build good soil structure.

Active carbon can be measured using a rapid laboratory test known as permanganate oxidizable carbon (POXC).

A dried sample of topsoil is added to a solution of potassium permanganate (a purple liquid). As the active organic matter is oxidised it reacts with the permanganate and the solution becomes less purple. The measured colour difference in the solution indicates the amount of active C (POXC) in the soil (mg C kg⁻¹ soil).

This test only measures the readily reactive carbon compounds (simple carbohydrates and microbial metabolites). The more stable soil carbon components (humus or charcoal-like materials) remain largely unaffected.

The test is commercially available in the UK as part of a soil carbon testing package (£50 to £70 per sample), but may be available individually on request.

POXC testing on the whole farm will be very expensive due to the high analysis cost. Instead use targeted POXC testing to help build an understanding of how soil management is affecting pools of soil organic matter that positively affect soil health.

Take a representative soil sample from the field area being tested using the standard soil sampling method described for measuring soil nutrients, pH and organic matter.

Sampling should be done at the same time each year (autumn or winter is preferable).

The POXC test reports results as milligrams of carbon per kilogram (mg C kg^-1) soil. The larger the value, the more active carbon content and the healthier the soil.

An increase in active carbon over time could indicate that you are building up the pools of available carbon.

There are currently no recognised thresholds for interpreting the results of POXC analyses.

However, research from the USA (conducted by the University of Missouri, 2025 and Lucas and Weil, 2012*) suggests that a value of over 800 mg C kg^-1 in the soil is considered a healthy soil for active C and microbial activity, whereas less than 200 mg C kg^-1 is seen as poor.

POXC results can be impacted by soil texture, and so a single interpretation framework that does not take this into account has limited use.

Soil texture can impact POXC results as it can influence the speed at which active carbon breaks down.

Greater infiltration in sandier soils means more rapid processing of active carbon than in less porous wetter clay soils (as evidenced by Bot and Benites, 2005*).

Before taking measures to increase active carbon first ensure that the soil is in good health.

A healthy soil has optimal interactions between:

• chemistry (pH, nutrients and contaminants)
• physics (soil structure and water balance)
• biology (earthworms, microbes and plant roots)

After establishing good soil health, anything that increases and preserves organic matter will increase active carbon.

Organic matter can be lost when exposed to air through tillage operations. Therefore, a reduction in tillage can preserve organic matter stocks.

Regular inputs of organic matter will provide stocks of active carbon.

Maintain good soil structure through remedying and preventing compaction.

A good soil structure will provide the right environment for soil biology to process the active carbon.

* Note: Full references can be found in the appendix of the original research report which can be downloaded from the project webpage: Early indicators to monitor changes in soil health.