Soil Organic Matter (SOM): How to measure ?

I often encounter a common question on measuring soil organic matter (SOM) while discussing regenerative agriculture. It’s followed by another query on the relationship between soil organic carbon (SOC) and SOM. let me try to clarify such confusion in this article.

Soil organic matter contains plant and animal tissues at different stages of decomposition. It comprises carbon, hydrogen, oxygen and small amounts of nitrogen, phosphorus, potassium and other elements. Depending on how old the organic matter is it is categorized into passive and active types. Passive SOM consists of materials that are several decades old while residues in active SOM are few years old.

Broadly SOM fractions can be divided into 3 types. The interaction of plants and microorganisms produce byproducts during decomposition plus root exudates and sugars referred to as dissolved organic matter. It’s a continuous process that keeps happening every moment. Dissolved organic matter contributes to less than 5% of the total soil organic matter composition. The second type is fresh, decomposing plant and animal tissues called particulate organic matter. Depending on the soils it contributes up to 25% of SOM. Humus is the next category that is dark black or brown organic matter that is formed in the soil due to the decaying plant and animal residues. In fertile soils humus constitutes up to 50% of SOM and is the stable organic matter.

Soils with high organic matter offer several benefits for farmers and ranchers like,

-  Improves aeration, water holding capacity, infiltration and prevents runoff.

- Nutrient storehouse for crops and provides essential nutrients over time (cation exchange capacity CEC)

- Enhances soil microbial activity and diversity and provides nutrition for billions of diverse microorganisms.

 Soil carbon is often mentioned during discussions on carbon sequestration, carbon offsets, regenerative agriculture and climate smart agriculture. Interestingly carbon is called the ‘king of elements’. The term carbon is derived from Latin ‘carbo’ meaning coal. In the human body carbon is the second most abundant element after oxygen. Soil carbon is that carbon component of organic compounds that is measurable. It is difficult to measure soil organic matter directly in the lab, hence laboratories measure and report soil organic carbon. In other words, soil organic carbon is the measurable part of SOM.

Soil Organic Carbon (SOC) plays a vital role in sequestering carbon dioxide from the atmosphere and addressing the climate crisis.  Farm management that facilitates building soil organic carbon in agricultural and pastoral lands over a period helps to significantly reduce atmospheric carbon dioxide. It's important to know how to measure SOC and estimate SOM, without measurement it would be difficult to identify the gaps for scientifically improving farm management. 

Let me discuss how to measure SOC and estimate SOM in a hectare of farmland. 

The first step is to collect the soil samples following the standard procedures using a soil probe or auger and dispatch it to a nearby lab for analysis. From the lab report check on the total organic carbon percentage that will be used for calculating soil organic matter. On an average it has been found that about 58% of the mass of organic matter in soil is carbon. While estimating the percentage of Soil organic matter (SOM) from Soil organic carbon (SOC) a conversion factor 1.72 is used (100/58 =1.72).

 Let’s presume SOC is 2.5% of a farm. SOM is calculated as follows;

SOM (%) = SOC (%) x1.72

               = 2.5x1.72

               = 4.3%

In this example the soil organic matter is 4.3%. 

The SOM (%) can be converted into weight for a given depth and area. It helps to estimate organic matter in tons per hectare of soil. From the soil analysis data, bulk density and soil organic carbon values are required for estimation. The calculation is simple.

SOC in tons of carbon per hectare = SOC x bulk density (tons per cubic meter) x depth (meters) 

Let’s consider a hypothetical soil analysis data comprising SOC (2.5%), bulk density 1.1 grams per cubic centimeter and soil depth 10 centimeters. It’s important to convert SOC% to decimals(0.025), bulk density into tons per cubic meter (1.1 tons/cubic meter) and depth in meters (0.1 meters) per hectare (10,000 square meters) 

SOC tons/hectare = (0.025) x (1.1 x 0.1 x 10,000)

                           = 27.5 tons of carbon/hectare (t C/ha)

Further SOM per hectare can be deduced using the above conversion factor of 1.72. The amount of Soil organic matter would be 27.5 x 1.72 = 47.3 tons of organic matter per hectare.

Estimating SOM every year or at least once every 2 years to assess the management practices adopted in the farm is vital in regenerative agriculture. It helps to course correct some of the practices that are contributing to the loss of soil organic matter from farms and ranches. Soil organic matter is the lifeline of the soil, farmers need to take utmost care in preventing its loss. Growing leguminous cover crops, crop rotation, cultivating perennial forage crops, application of compost, agroforestry and silvi-pastoral systems, reduced tillage, contour planting and several regenerative agriculture practices that are appropriate to the region help in building soil organic matter and sequester carbon.