One of the challenges in implementing bio-agriculture as a solution to climate change has been to be able to accurately measure changes in the amount of soil carbon, at an economical cost.
Soil carbon has been difficult to measure up till now for a number of reasons:
- Soil carbon varies significantly at short distances—the carbon difference between samples collected within a meter can be larger than many-years worth of sequestration.
- The relatively small amount of carbon in relation to the total amount of soil to be measured. To get a full profile of carbon content, soil needs to be measured to a depth of 60cm .
- The difficulty of re-measuring in the exact same spot year after year makes it hard to determine any changes.
To solve these problems in a practical, cost effective way, a mobile on-farm testing and measuring laboratory has been successfully developed. This work was undertaken in cooperation with Kansas University USA.
Researchers found that visible and near-infrared reflectance (VIS-NIR) measurements of soil samples correlate to important soil properties such as carbon and nitrogen.
The measuring equipment is mounted on a truck or tractor that moves through the fields at 7-8 km/hr taking 384 soil measurements every second. The collected measurements are turned into maps of the field by the instrument's computer.
Important developments of this technology came by combining the benefits of a small number of soil samples for accurate laboratory analysis and using these for calibrating the large number of sensor-based stratified samplings. The sensors can simultaneously measure three levels of soil depth up to one meter deep.
These methods for measuring, monitoring and verification of soil carbon change achieve a 90% or better confidence interval, with less than 10% error. Confidence intervals are based on the variability within the sampled area and the number of samples. Farm test results of various soil types, different climates and varied produce show a bias free accuracy of 90% confidence interval, with a great cost efficiency compared to conventional soil sample laboratory testing. Conventional testing has the added difficulty of relocating earlier measurement spots.
Veris Technologies, Inc., USA is a supplier of the VIS-NIR – Spectrometer ready for immediate use.
The measuring equipment can take 384 soil measurements every second. The collected measurements are turned into maps of the field by the instrument's computer.
Measuring Very Small Changes In Soil Carbon
On one hectare of land the soil down to 60cm weighs approximately 2,500 tonnes – calculated by weight and density. In this soil there is about 50 tonnes of soil carbon (184 tonnes of CO2-e) - calculated by weight, density and lab analysis.
This 50 tonnes of carbon is 2% of the total 2,500 tonnes.
If the yearly carbon increase is 1 tonne of carbon/hectare/year ( the equivalent of taking 3.67 tonnes of CO2-e out of the atmosphere. - 1 tonne carbon = 3.67 tonnes of CO2-e). then this one tonne carbon increase is 0.04% of the total soil. Over ten years that is a 0.4% increase in carbon level. These figures have been verified in tests spanning 10 years carried out by the GRDC in Australia,
Measuring such small changes is one of the difficulties in making accurate measurements. If we only measure every third year, then we are measuring a 3 x 0.04% = 0.12% change and we get a higher accuracy and confidence interval.