Source: ANP Inc.

By Mike Petersen, ANP Lead Agronomist

Even as a soil scientist, I can be confused by the numbers/values related to Soil Organic Carbon (SOC) compared with the term Soil Organic Matter (SOM). So a bit of refresher on what is what is not a bad thing. Each time your results come back from a soil laboratory what is it that is being expressed to you? Yes it is a percent, but of what?

As a good soils man, I dug deeper to find out what are we meaning by SOC vs. SOM? As it turns out the friends from ‘Down Under’ (Australia) give a good explanation that may help you all not have to scratch your heads in confusion.

Soil Organic Carbon is a component of SOM. SOC is made up of four biological types or fractions in the soil:

  1. Crop residues in the soil that are less than 2 mm in size, such as fine roots, bits and pieces of leaf, cob, shucks and stems. 
  2. Particulate sized plant debris between 0.053 mm and 2 mm that is partially decomposed, cellulosic and lignin fibers are less perceivable.
  3. Humus fraction dominated by decomposed molecules glued to the soil minerals, generally smaller.
  4. Recalcitrant organic carbon that is stable—and silicified lignin or charcoal. There is also carbon in carbonates/silicates in soils of the western states.

SOM includes all carbonaceous and silicified materials, earthworms, insects, fecal materials, plant debris larger than 2 mm, root bits and pieces, leaf matter that has been partially chewed by small insects or ants, fungal hyphae, glomalin (a complex polysaccharide given off fungal hyphae) and sticky secretions from roots, bacteria (ie: actinomycetes) or earthworms.

Scientists believe SOC makes up about 58-60% of what is SOM and reported as a percentage of a known quantity of soil, ie: 100 grams. SOC has the components of humic and fulvic acids in particulate forms that adhere to clay particles, see Figure 2.

Throughout the world, soils are tested with either of two dominant methods to determine SOC. The first has been around since the 1930's, Walkley-Black Method of wet oxidation. This wet method detects about 80% of what is truly in the soil.

The other is dry combustion — oxidizing soil at high temperature and measuring the off-gassing of carbon dioxide. This over estimates the amount due to it includes inorganic forms such as carbonates or bicarbonates. More often to obtain an accurate number or value in western United States, soils would be to have the lab say if they wash the soil sample with acid to oxidize the carbonates then run either the combustion or potassium permanganate wet method.

It can be a good consideration (not an absolute) to get a bulk density of the same sampling site and then correct for texture and pore space.

It has been said and written by numerous scientists, that soils with less than 2% SOC should be considered unstable for soil aggregates and not sustainable into the future. This is particularly true with full width tillage methods. More tillage puts a great deal of the surface soils in the western states falling backwards.

The fraction of SOC that is labeled as easily decomposed and susceptible to carbon dioxide release, it is some of the most desirable fraction to add to the SOC level. As mentioned above it is the material larger than 0.053mm, especially when tilled and exposed to the sunlight — lost. This is the material that helps soils have a great affinity for water and can also hold onto more nutrients. In sandy soils (less than 10% clay) that is a real good thing.

In order to make the number from the lab of SOC relate to SOM, take the lab result times 1.72 which estimates SOC 58% of the SOM total. Results from below 30 inches in a soil profile use the number 1.334 to get a SOM related number. My suggestion as you sample in the western 10 states and your pH levels are higher than 8.0, ask if they ran an acid wash first to remove the carbonates.

Quick in summary, SOC is part of the SOM fraction in the soil, about 58% of it. Microorganisms, earthworms, mucus, plant exudates, plant roots and <2mm fecal matter all make up SOM. Knowing this can help you have a better idea of the SOM amount in your soils.