Soil Organic Matter #3: analysis options

Author: Alli Grundy

14th June 2022

NRM

Agriculture

Simple measures of organic matter analysis are needed for soil evaluation, management, and monitoring. There are a few analytical options that all arrive at a soil organic matter (SOM) result. It is important to recognise that no single laboratory technique exists that provides an absolute measure of organic matter. Each laboratory method estimates the SOM content in a slightly different way based on various assumptions, and the method of choice depends on purpose of the analysis.

NRM offers three analytical options for assessing SOM: loss on ignition (LOI), organic carbon by DUMAS combustion, or CarbonCheck, our comprehensive soil package that also includes a full soil carbon audit.

Loss on Ignition (LOI)

In layman’s terms, LOI is the difference in the weight of soil before and after it has been ignited. The difference between the weights is the assumed organic matter content. The ignition temperature used must be sufficiently high enough to burn off the organic matter content but not inorganic carbonates and the structural water molecules that are bound within clay particles. The soil also needs to be properly and consistently pre-dried. This is to drive off any residual moisture that the soil contains when it arrives at the laboratory.

Typically, LOI temperature methodologies range between 350 to 650ᵒC. Over the last 50 years, numerous studies have attempted to pinpoint the critical temperature at which structural water/carbonates disappear, and compensatory factors have been derived to avoid over estimating SOM. There is no “one factor fits all” that works for all soil types. However, based on internal research, NRM uses an ignition temperature of 430ᵒC. This is within the optimal range where the organic matter components are the only ones removed.

The LOI methodology is adequate for the purposes of monitoring changes in organic matter over time. It is the most cost-effective choice and, for the purposes of the SFI scheme, is entirely acceptable. It is ideal for assessing a large volume of samples and gives a good indication of the health and fertility of a soil overall.

DUMAS

The alternative test commonly used for organic matter is the DUMAS lab method, which measures organic carbon and uses a factor to convert to organic matter. It is acceptable to assume a relationship between SOM content and soil organic carbon, and a universal conversion factor has been derived between organic matter and soil carbon. The conversion factor of 1.724 assumes that 58% of soil organic matter is carbon. Whilst this is acknowledged as being acceptable, variation within and between soil types does exist. The accuracy of the factor also relies on the accurate analytical determination of organic matter and organic carbon.

Generally, calculating organic matter content from soil carbon data results in its underestimation when those calculated values are compared to the same samples analysed via LOI analysis. This may be due to the sensitivity of the conversion factor.

Analysing soil carbon by the DUMAS method involves heating the soil in oxygen rich air until it starts to burn and the amount of carbon dioxide (CO₂) produced is measured. Over the decades this has been proven as a reliable method across a wide range of soil types.

The DUMAS method is more expensive to conduct due to sample preparation, analysis time, and the consumables used in the process.

CarbonCheck

CarbonCheck is the method of choice when it is important to understand the carbon stocks of any given soil for future carbon market opportunities.

Developed by NRM in 2021 following consultations with industry experts, customers, and government advisors, CarbonCheck is rapidly becoming the industry standard for soil carbon analysis. This service uses the DUMAS combustion technique to determine the soil organic carbon content. By measuring the bulk density (kg/l) of the soil sample, tonnes per hectare of carbon can be calculated. This service also measures active carbon, which is a measure of readily available carbon for soil microbes, total carbon, soil inorganic carbon, total nitrogen, C:N ratio, and a calculated organic matter content.

Analysis choice

Before sending samples for organic matter content, it is important to be clear of the purpose of the analysis. Understanding this will determine which analytical method is appropriate. Being aware of which test measures organic matter and which measures organic carbon is important, particularly if a conversion factor is applied to derive organic matter content.

The rule of thumb is to stick to the same method and laboratory. We suggest using LOI for a quick assessment of soil organic content from a larger number of fields where you plan to monitor levels over time. The DUMAS method may be preferred where an indicative organic matter content can be calculated from total organic carbon, but be aware of the variation around the conversion factor.

If you need an assessment of soil carbon content for sequestration purposes and benchmarking for future carbon markets, then you should choose the CarbonCheck method, which also calculates carbon stocks.

Seasonal variation and sampling strategies are important considerations when measuring organic matter to ensure that the variation in the result is kept to a minimum. Good quality data is required to monitor changes in organic matter content. It is therefore best practice to be clear what you want measuring, sample at the same time of year, remove any obvious organic residues from the sample (roots/vegetation), and, if possible, take replicated samples to build a more accurate picture of the content and be consistent.

For more information on choosing the appropriate organic matter test or soil carbon content in time to meet the obligations of the published SFI standards before its launch at the end of June, please contact NRM or discuss with your agronomist.