101 Questions answered
Choosing the Right Soil Organic Carbon Test: Monitoring vs Carbon Farming
This depends on what you are trying to achieve. If you already test soil carbon and are monitoring levels on your farm, use the same test method you have used in the past so you can compare results.
Walkley Black is the most common test method offered and most cropping research has used this method. Walkley Black measures about 80-90% of soil organic carbon (SOC), and doesn’t measure carbonates, which is helpful on many SA soils. Soil texture plays a role in how much SOC this method measures, with the test measuring more SOC on sandy soils and less on heavier textured soils.
The dry combustion method, sometimes called the DUMAS test, measures total organic carbon. This is the test method required for carbon accounting. The lab will pre-treat the sample to remove the inorganic carbon, then only measure the soil organic carbon.
If your plan is to measure soil carbon for carbon farming, note that this is not something you can do yourself. See Question 87. ‘How do I get an accurate carbon baseline and can I do it myself?’
Regardless of the test method, make sure the result is adjusted for gravel content. If the soil has 15% gravel, the SOC result is adjusted down by this much e.g. 1.2% – (1.2 * (1-0.15) = 1.02%.The Soil Quality website has an online calculator to adjust soil carbon results for bulk density and gravel content.
The table below summarises the main test methods and their benefits and limitations. On soils where there can be high levels of inorganic carbon, methods that measure both organic carbon and inorganic carbon need to be interpreted with care. Labs in SA that are used to carbonate soils might do a pre-test to remove the inorganic carbon, but it’s best to request the pre-test every time if you have calcareous soils.
