This test, known as hot water extractable carbon (HWEC) or sometimes hot water carbon (HWC), is fairly new and needs to be done by a laboratory.
It provides a robust measure of the more labile (slippery/unstable) soil carbon fraction, and can show the subtle changes in soil quality that occur due to farm management practices and climate effects.
It is sensitive to changes in microbial biomass due to fertiliser application amounts (particularly urea) and grazing levels, as well as physical modification due to tillage.
Experiments conducted in the Waikato over several years have shown that HWEC levels are highest in drystock farming areas, lower in dairy, lower still in cropping, and least in market gardening soils (for the same soil type). Here is the link to the relevant paper.
The test has been shown to be highly correlated with microbial biomass carbon, anaerobic mineralisable nitrogen and total soil carbohydrate levels. A decline in HWEC levels also indicates a decline in the other biologically active nutrient pools of nitrogen, sulphur and phosphorus.
Until now testing for Microbial Biomass Carbon alone has been expensive, but as a result of this research Hills can now offer an Estimated Microbial Biomass Carbon (MBCest) measure whenever a HWEC test is requested. This can only be reported as a derived result, but is thought to be of value in monitoring the status of soil over time.
HWEC tests should be done at the same time as more traditional tests on samples collected at the same time of year. Records of HWEC kept over time will be helpful in understanding trends and the impact of changing farming practices (and perhaps also longer term from climate change) so that adjustments can be made to ensure sustainable farm management.
Typical measurements on New Zealand soils can range from 350mg/kg (very low) to more than 7500mg/kg (very high).
Early experiments were somewhat complicated, but laboratory experience by such as Hill Laboratories has shown that an accurate measure of non-purgable organic carbon can be obtained through a test using a sample of dry, sieved (<2mm) soil that is extracted in 80degC hot water for 16 hours, then filtered and the filtrate measured by a Total Organic Carbon analyser. A quoted cost for this is about $20 + GST. Depths for soil sampling vary according to soil usage, with 75mm for pasture and 150mm for horticultural soils.
Labile carbon in your soils is the basis of the reactions of most of soil life, so checking that it is actually there, and in sufficient quantities to achieve planned aboveground growth, will help farmers to actually use their soils to best advantage.
Who am I? Sue Edmonds is a farming and science writer from Waikato.