Measuring soil organic carbon

Methods measuring soil organic carbon

While other methods exist, a common analyses for organic carbon in soil in Australia is the Walkley–Black wet oxidation method. This method measures only the organic component of carbon in soil.

Where carbon is derived by dry combustion using LECO or Elementar analyses then the value represents the total carbon pool including any carbonates. Carbonates can be tested for by observing the reaction (fizz) from the application of a weak hydrochloric acid solution, such as vinegar, to the soil. Highly alkaline soil (pH > 8.5) is usually indicative of the presence of carbonates. Where carbonates are identified using a fizz test, soil can be acid pre-treated to remove any carbonates, with the resultant analytical value measuring remaining organic carbon. In soil with no carbonate, no acid pre-treatment is required.

Total soil organic carbon provides a measure of the amount of organic carbon present in a soil, but provides no information on its characteristics, function or stability. It is composed of four different fractions (dissolved, particulate, humus and resistant organic carbon), which vary in their properties and decomposition rate. Understanding how each of these fractions change within the soil provides information on sequestration potential, nutrient turnover, biological function and soil properties such as water holding capacity. The following methods differ in their development, relative difficulty and expense, with the most promising commercial application associated with successful calibration of the mid-infrared technology for soil organic carbon.

Size fractionation

The size of soil mineral particles is often used to attribute the organic carbon contained within them to particulate, humus or resistant organic carbon fractions that differ in their properties and decomposition rates. However, this is a time and labour-intensive method for quantifying organic carbon fractions. Size fractionation and 13C solid-state nuclear magnetic resonance spectroscopy has been used to determine the particulate, humus and resistant components of organic carbon in a range of soil to calibrate against other methods such as mid-infrared spectroscopy.

Density fractionation

Density separation of soil carbon fractions uses water to isolate less decomposed organic materials (the ‘light fraction’), with a density less than 1 gram per cubic centimetre, and increasingly dense liquids such as sodium iodide to separate aggregates or particles (the ‘heavy fraction’) linked to biological processes and soil functions such as nutrient cycling and cation exchange capacity. This method is time consuming and is used primarily by researchers to link organic carbon fractions to soil function.

Permanganate labile carbon

The carbon fraction measured using the permanganate oxidation method has been linked to biological function, but can arguably be less sensitive to differences in soil organic carbon based on changes in management or land use.

Working with soil test results

Stepping through the calculations

1. Soil test results are usually given as percent organic carbon, but can also be expressed as the weight of organic carbon per weight of soil.

1.5% soil organic carbon = 1.5 carbon per 100 g soil = 15 g carbon per 1 kg soil

2. Since soil organic matter is about 58% organic carbon, if you know the percent organic carbon in a soil sample, you can calculate the percent soil organic matter by multiplying by 1.72 (100/58 = 1.72). For example, if a soil sample has 1.5% organic carbon: soil organic matter % = 1.5 x 1.72 = 2.58%

soil organic matter % = soil organic carbon % x 1.72

3. To calculate tonnes per hectare of soil organic carbon you will need to know your mass of soil, which is calculated using the bulk density of your soil and sampling depth in metres.

soil organic carbon (t/ha) = soil organic carbon (%) x soil mass (t/ha)

soil mass (t/ha) = bulk density (g/cm3) x sampling depth (m) x 10 000

• When using this equation, you will need to use soil organic carbon % as a fraction or decimal. If you have 1.5% organic carbon, use 1.5/100 or 0.015 in the equation.
• Many topsoil samples are taken to 10 cm depth. This is equivalent to 0.1 m depth and you would use 0.1 as the sampling depth in the equation to calculate tonnes per hectare of soil organic carbon in the top 10 cm of soil. If you sample to 30 cm, use 0.3 m to calculate tonnes per hectare of soil organic carbon in the top 30 cm of soil.
• The multiplication factor of 10 000 is used because there are 10 000 square metres (m2) in one hectare.

For example, if soil sampled to 10 cm depth (0.1 m) has 1.5% organic carbon and bulk density of 1.4 grams per cubic centimetre: soil organic carbon (t/ha) = 0.015 x 1.4 x 0.1 x 10 000 = 21 t/ha

4. The above principles are used to determine tonnes per hectare of soil organic matter.

soil organic matter (t/ha) = soil organic matter (%) x soil mass (t/ha)

• In the example for this equation, percent soil organic carbon (used as a decimal) is converted to percent soil organic matter by multiplying by 1.72.

For example, if soil sampled to 10 cm depth (0.1 m) has 1.5% organic carbon and bulk density of 1.4 grams per cubic centimetre: soil organic matter (t/ha) = 0.0015 x 1.72 x 1.4 x 0.1 x 10 000 = 36.12 t/ha

Importance of bulk density

Bulk density is the weight of soil in a known volume. Different soil and soil depths have different bulk densities, but a high bulk density can indicate compacted areas, which slow the growth of roots. In the context of organic carbon, a measure of bulk density is needed to calculate ‘stock’ values per unit area (t/ha) and to calculate an equivalent mass of soil.

Estimates of bulk density allows us to calculate real changes in soil organic carbon through time. This is because over a number of years, changes in bulk density can occur. For example, compaction or management practices such as tillage, can change bulk density.

The percentage of organic carbon at any depth increment can be adjusted for bulk density and reported as a mass of carbon per unit area (t C/ha). This means that when you go back to a particular area a number of years later, even if significant compaction or soil mixing has occurred, you are still able to calculate the amount of carbon present for an equal soil mass, taking into account any changes in bulk density through time or space.

For example, a soil with an initial bulk density of 1.2 grams per cubic centimetre and an organic carbon concentration of 1% (12 t C/ha to 10 cm) was exposed to farming practices, which used over a period of ten years, resulted in topsoil compaction. This increased the bulk density of the topsoil to 1.4 grams per cubic centimetre but did not change the percentage of organic carbon in the soil. Sampling to 10 centimetres would now sample soil not previously analysed (dark brown area on diagram) for this depth layer. Without adjusting for the change in bulk density to an equivalent soil mass, it would seem as if a change in organic carbon stock of 2 tonnes per hectare had occurred. If the stocks are adjusted to an equivalent soil weight, then results show no change in organic carbon stocks.

Impact of gravel

Laboratory measures of soil organic carbon are generally done on sieved soil samples, which in many cases exclude any materials larger than 2 millimetres in size. Consequently, if a soil has a significant amount of gravel or stone material, this fraction is removed before analysis with the final soil carbon or nutrient assessment being only representative of the mineral component of the remaining soil. To correct this, laboratory results need to be adjusted to reflect the original composition of the soil sample.

For example, if the laboratory result is 1.4% organic carbon, but 10% of the original sample volume was gravel or stone, then the actual soil organic carbon content of that soil is equivalent to 1.26% organic carbon (90% of 1.4%). Such adjustments are sometimes overlooked and can lead to reports of rapid or unusually large changes in total soil organic carbon.