Building Soils for Better Crops, Third Edition

The Dynamics of Raising and Maintaining Soil Organic Matter Levels


It is not easy to dramatically increase the organic matter content of soils or to maintain elevated levels once they are reached. It requires a sustained effort that includes a number of approaches that add organic materials to soils and minimize losses. It is especially difficult to raise the organic matter content of soils that are very well aerated, such as coarse sands, because the potential for aggregation (which protects particles of organic matter) is limited, as are the fine minerals that form protective bonds with organic matter. Soil organic matter levels can be maintained with lower additions of organic residues in high-clay-content soils with restricted aeration than in coarse-textured soils because of the slower decomposition. Organic matter can be increased much more readily in soils that have become depleted of organic matter than in soils that already have a good amount of organic matter with respect to their texture and drainage condition.

Graph illustrating organic matter changes in soil
Graph illustrating organic matter changes in soil

When you change practices on a soil depleted in organic matter, perhaps one that has been intensively row-cropped for years and has lost a lot of its original aggregation, organic matter will increase slowly, as diagrammed in figure 3.6. At first any free mineral surfaces that are available for forming bonds with organic matter will form organic-mineral bonds. Small aggregates will also form around particles of organic matter. Then larger aggregates will form, made up of the smaller aggregates and held by a variety of means—frequently by mycorrhizal fungi and small roots. Once all possible mineral sites have been occupied by organic molecules and all of the small aggregates have been formed around organic matter particles, organic matter accumulates mainly as free particles—within the larger aggregates or completely unaffiliated with minerals. This is referred to as free particulate organic matter. After you have followed similar soil-building practices (for example, cover cropping or applying manures) for some years, the soil will come into equilibrium with your management and the total amount of soil organic matter will not change from year to year. In a sense, the soil is “saturated” with organic matter as long as your practices don’t change. All the sites that protect organic matter (chemical bonding sites on clays and physically protected sites inside small aggregates) are occupied, and only free particles of organic matter (POM) can accumulate. But because there is little protection for the free POM, it tends to decompose relatively rapidly under normal (oxidized) conditions.

When management practices are used that deplete organic matter, the reverse of what is depicted in figure 3.6 occurs. First free POM is depleted, and then as aggregates are broken down physically protected organic matter becomes available to decomposers. What usually remains after many years of soil-depleting practices is the organic matter that is tightly held by clay mineral particles.

Assuming that the same management pattern has occurred for many years, a fairly simple model can be used to estimate the percent of organic matter in a soil. It allows us to see interesting trends that reflect the real world. To use this model you need to assume reasonable values for rates of addition of organic materials and SOM decomposition rates in the soil. Without going through the details (see the appendix for sample calculations), the estimated percent of organic matter in soils for various combinations of addition and decomposition rates indicates some dramatic differences (table 3.2). It takes about 5,000 pounds of organic residues added annually to a sandy loam soil (with an estimated decomposition rate of 3% per year) to result eventually in a soil with 1.7% organic matter. On the other hand, 7,500 pounds of residues added annually to a well-drained, coarse-textured soil (with a soil organic matter mineralization, or decomposition, rate of 5% per year) are estimated to result after many years in only 1.5% soil organic matter.

Table 3.2 Estimated Levels of Soil Organic Matter after Many Years with Various Rates of Decomposition (Mineralization) and Residue Additions*
Annual rate of SOM decomposition (%)
Fine textured, poorly drainedCoarse textured, well drained
Annual organic material additionsAdded to soil if 20% remains after one year12345
—–lbs per acre per year—–—–final % organic matter in soil —–
*Assumes upper 6 inches of soil weighs 2 million pounds.

Normally when organic matter is accumulating in soil, it will increase at the rate of tens to hundreds of pounds per acre per year—but keep in mind that the weight of organic material in 6 inches of soil that contains 1% organic matter is 20,000 pounds. Thus, the small annual changes, along with the great variation you can find in a single field, means that it usually takes years to detect changes in the total amount of organic matter in a soil.

Graphs illustrating net organic matter additions and changes in percent organic matter for soils
Graphs illustrating net organic matter additions and changes in percent organic matter for soils

In addition to the final amount of organic matter in a soil, the same simple equation used to calculate the information in table 3.2 can be used to estimate organic matter changes as they occur over a period of years or decades. Let’s take a more detailed look at the case where 5,000 pounds of residue is added per year with only 1,000 pounds remaining after one year. Let’s assume that the residue remaining from the previous year behaves the same as the rest of the soil’s organic matter—in this case, decomposing at a rate of 3% per year. As we mentioned above, with these assumptions, after many years a soil will end up having 1.7% organic matter. If a soil starts at 1% organic matter content, it will have an annual net gain of around 350 pounds of organic matter per acre in the first decade, decreasing to very small net gains after decades of following the same practices (figure 3.7a). Thus, even though 5,000 pounds per acre are added each year, the net yearly gain decreases as the soil organic matter content reaches a steady state. If it was a very much depleted soil and the additions started when it was at only at 0.5% organic matter content, a lot of it might be bound to clay mineral surfaces and so help to form very small aggregates—preserving more organic matter each year. In this case, it is estimated that the net annual gain in the first decade might be over 600 pounds per acre (figure 3.7a).

The soil organic matter content rises more quickly for the very depleted soil (starting at 0.5% organic matter) than for the 1% organic matter content soil (figure 3.7b), because so much more organic matter can be stored in organo-mineral complexes and inside very small and medium-size aggregates. Once all the possible sites that can physically or chemically protect organic matter have done so, organic matter accumulates more slowly, mainly as free particulate (active) material.

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