Soil Organic Matter (SOM)

Soil organic matter makes up a small part of all soils. In this area of Nebraska, cropland fields will average around 3% organic matter. SOM is a small but powerful part of any soil. SOM can be thought of as the Soil Bank where carbon and other organic compounds are processed and stored below our feet. Remember that carbon is always cycling, moving from air to vegetation, to soil, to rivers, back and forth, always in flux. In the soil bank, we find fast moving carbon along with carbon that will be locked away for a very long time.

The pie chart above represents the organic matter portion (3%) of a typical soil. This chart indicates that about 15% of the carbon and other organic compounds in a soil will be quickly moving through living organisms and fresh residue (just as money flows quickly between bank tellers and individual checking accounts). Around 40% of SOM at any one time will be decomposing. During decomposition, organic materials will be part of the soil for a little longer period of time, maybe 1 to 10 years depending on climate.

100+ Years of Cropping

Actually, I think it is fairly amazing that our cropland fields in this area still contain 3% organic matter after 100+ years of cropping. In fact it is rare to find a field below 2% OM in this area. The reason for this is that the humus fraction (40% of SOM) is so stable that it is very hard to remove this portion of OM even under repeated tillage. It is locked tightly with soil minerals.

The humus/stable portion of SOM represent the ultimate goal for all soil health enthusiasts. Folks wanting to build healthy soils are implementing principles to move and lock as much organic material and carbon as possible into the stable humus fraction of SOM. Luckily, using proper regenerative farming techniques we can quickly build the fresh and active portions of OM. These quick gains help farmers see benefits in nutrient cycling and water holding capacity in a few short growing seasons.

So how do we build SOM?

Traditionally we thought keeping soils covered with residue and mulches was the main option to increase the organic matter level of soils. Referring back to our pie chart, crop residues would move in the following order through the Soil Bank:

1. Bank Tellers (soil organisms)
2. Checking Account (fresh OM)
3. Savings Account (decomposing OM)
4. Retirement Account (stabilized OM)

Actually this process is very inefficient or leaky along each step. The following graphic adapted from “The Nature and Properties of Soils” by Brady & Weil, gives an example of growing a corn crop. The corn crop produces a total of 7,500 lbs of organic matter per acre. Obviously we take off the grain immediately losing a third of the produced OM. We are left with 2,500 lbs. of OM in the above ground corn stalks and 2,500 lbs. of OM in the roots.

As the graphic indicates, almost 90 percent of the above ground residue never makes it into soil organic matter. It is digested, respired and oxidized before making it into SOM. Since the roots are buried in the soils, the conversion rate is a little better with only 70% of the produced biomass lost before reaching the soil organic matter pool.

When you run the numbers, it get’s a little depressing. To build soil organic matter from 3% to 4%, (a 1% increase) you would need to add 20,000 pounds of residue! Under optimum conditions this would take years.

Reports of significant SOM gains in short order

So how do regenerative farmers such as Gabe Brown report multiple percent increases in SOM over just a few years? Dr. Christine Jones, founder of Amazing Carbon, says this is possible through what she calls the liquid carbon pathway. Carbon is rapidly moved in liquid form via plant roots in association with fungi and locked away into the humus/stable portion of SOM.

Once again referring back to our pie chart of SOM, the liquid carbon pathway skips the leaky Checking Account and Savings Account steps. Soil organisms (mycorrhizal fungi in association with roots and mineral soil) move organic material directly into stable SOM compounds! In this process new soil healthy soil aggregates are formed.

Trees turn CO2 into wood. Soils turn CO2 into humus.

Christine Jones

Why does this just work in regenerative Ag?

The missing link with most of our agriculture lands is the required soil biology. As previously reported, most of the U.S. and World soils are currently degraded. The liquid carbon pathway first requires regenerating our soils with fungi. Most cropland is currently bacteria dominant, refer to the ecological succession image below.

The goal of regenerative agriculture is to get degraded (bacteria dominant) soils to the most ecologically productive part of the ecological succession chart. In our part of the world, that is at the edge of perennial grasses and forest. This is where we see wildlife flourish, at the edge. These soils have a near balance of fungi to bacteria ratio.

This requires building a home for soil fungi with the proper plants to advance succession from bacteria dominance. In addition fungi find it difficult to survive under annual tillage and inorganic fertilizer applications. Reducing soil disturbance and detrimental chemical applications is a key step in the process. Note that for areas where trees have taken over, management needs to be applied to set succession back!

At DS Family Farm we utilize the cattle herd to manage ecological succession. Currently we are building plant root and soil fungi associations with a long term goal of getting and keeping our pastures near the “Sweet Spot” of succession. We do have the challenge in some areas of keeping woody species from advancing into our perennial grass pastures.

We thank God that the overall system He put in place is very forgiving and even provides for an abundance while we begin the regeneration process. An abundance that we can share with others in the form of pasture grazed meat. Call or stop by to see the herd in motion and the steps we are taking to closer reach our goals in ecological succession.