3.9: 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. In addition to using cropping systems that promote organic matter accumulation, 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 of low potential for aggregation (which shelters organic matter from microbial attack) and limited protective bonds with fine minerals. 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 given their texture and drainage condition.

Figure 3.6. Soil organic matter (carbon) levels vary in different soils and cropping systems, analogous to glasses filled with variable amounts of water. Illustration by Vic Kulihin.

Starting Point

It is good to consider the soil’s current status when you build up organic matter in a soil. A useful analogy is the three glasses of water in Figure 3.6 that represent organic matter levels in different cropping systems. We are generalizing here, but some soils that are severely degraded (case 1, say from severe erosion or intensive tillage, etc.) have low organic matter levels (empty glass) and have the potential to increase and store much more. Another soil (case 3) may be in a cropping system that has for a long time been cycling much of the organic matter or has received a lot of external organic inputs as we discussed previously. Here the glass is nearly full and not much additional organic matter can be stored. In such cases we should focus on protecting the existing organic matter levels by minimizing losses. The in-between scenario (case 2) may be a conventional grain or vegetable farm where organic matter levels are suboptimal and can still be increased. In the context of carbon farming and raising overall soil organic matter levels, benefits will accrue more in cases 1 and 2 than in case 3, where the soil is already close to being saturated with organic matter. Moreover, if farms that fit case 3 are located near those that fit cases 1 or 2, there are potential gains from transferring the excess organic residues, like manure from a livestock farm to a farm growing only grain crops. Note: The amount of stored organic matter also depends on the soil type, especially clay content, and you may imagine a larger glass for a fine soil than a coarse soil, and the fullness of the glass is similarly proportional.

How Much Organic Material is Needed to Increase Soil Organic Matter by 1%?

To increase organic matter in your soil by 1%, let’s say from 2% to 3%, requires a lot of organic material to be added. This usually takes the form of plant roots, aboveground plant residues, manures and composts. But to give an idea of how much needs to be added for such a seemingly small increase (and is actually a LARGE increase), let’s do some calculations. A surface soil to 6 inches weighs about 2 million pounds. One percent organic matter in this soil would then weigh 20,000 pounds. But when organic material is added to soil, a large percentage is used as food by soil organisms, so a lot is lost during decomposition. If we assume that 80% is lost as soil organisms go about their lives and 20% eventually ends up as relatively stable soil organic matter, some 100,000 pounds (50 tons!) of organic materials (dry weight) would be needed. Because smaller amounts of residue are usually added to soils, large soil organic matter increases usually take time. In addition, soils with different amounts of clay and with different degrees of drainage have different abilities to protect organic materials from decomposition (see Table 3.4).

Adding Organic Matter

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 3.7. 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, such as the outer layer of dead soil microorganisms or fragments of relatively fresh residue. 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 can accumulate. But because there is little protection for the free particles of organic matter, they tend to decompose relatively rapidly under normal (oxidized) conditions.

Figure 3.7. Organic matter changes in soil as practices favoring buildup are implemented. Redrawn and modified from Angers (1992).

The reverse of what is depicted in Figure 3.7 occurs when management practices that deplete organic matter are used. First, free particles of organic matter are depleted, and then physically protected organic matter becomes available to decomposers as aggregates are broken down. What usually remains after many years of soil-depleting practices is organic matter that is tightly held by clay mineral particles and trapped inside very small (micro) aggregates.

Equilibrium Levels of Organic Matter

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 when it reaches an equilibrium of gains and losses. This model allows us to see interesting trends that reflect the real world. To use the model you need to assume reasonable values for rates of addition of organic material and for soil organic matter decomposition rates in the soil. Without going through the details (see the appendix to this chapter for sample calculations), the estimated percent of organic matter in soils for various combinations of addition and decomposition rates indicates some dramatic differences (Table 3.4). 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 3.4 Estimated Levels of Soil Organic Matter after Many Years with Various Rates of Decomposition (Mineralization) and Residue Additions*
		Fine texture, poorly aerated <--> Coarse texture, well aerated
Annual organic material additions**	Added to soil if 20% remains after one year	Annual rate of organic matter decomposition
		1%	2%	3%	4%	5%
----pounds per acre per year----		---equilibrium % organic matter in soil ---
2,500 5,000 7,500 10,000	500 1,000 1,500 2,000	2.5 5 7.5 10	1.3 2.5 3.8 5	0.8 1.7 2.5 3.3	0.6 1.3 1.9 2.5	0.5 1 1.5 2
*Assumes the upper 6 inches (15 centimeters) of soil weighs 2 million pounds. **10,000 pounds per acre addition is equivalent to 11,200 kilograms per hectare.

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.

In addition to the final amount of organic matter in a soil, the same simple equation used to calculate the information in Table 3.4 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. We 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 previously, with these assumptions, after many years a soil will end up having 1.7% organic matter at equilibrium. 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 3.8a). 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 the soil was very depleted and the additions started when it was only at 0.5% organic matter content, a lot of organic material can accumulate in the early stages as it is bound to clay mineral surfaces and inside very small- to medium-size aggregates that form—preserving organic matter in forms that are not accessible to organisms to use. In this case, it is estimated that the net annual gain in the first decade might be over 600 pounds per acre (Figure 3.8a).

The soil organic matter content rises more quickly for the very depleted soil (starting at 0.5% organic matter) than for the soil with 1% organic matter content (Figure 3.8b), because so much more organic matter can be stored in organo-mineral complexes and inside very small and medium-size aggregates. This might be a scenario where a very degraded soil on a grain crop farm for the first time receives manure or compost, or starts to incorporate a cover or perennial crop. 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.

Figure 3.8. Net organic matter additions and changes in % organic matter content for soils. Estimated for soil starting at 0.5% or 1% organic matter, receiving a total of 5,000 pounds of residue per acre per year; 20% remains after one year, and soil organic matter decomposes at the rate of 3% a year.

Increasing Organic Matter Versus Managing Organic Matter Turnover

Increasing soil organic matter on depleted soils is important, but so is continually supplying new organic matter even on soils with good levels. It’s important to feed a diversity of soil organisms and provide replacement for older organic matter that is lost during the year. Organic matter decomposes in all soils, and we want it to do so. But that means we must continually manage the turnover. Practices to increase and maintain soil organic matter can be summarized as follows:

• Minimize soil disturbance to maintain soil structure with plentiful aggregation (reducing erosion, maintaining organic matter within aggregates);
• Keep the soil surface covered 1) with living plants if possible, planting cover crops when commercial crops are not growing, or 2) with a mulch consisting of crop residue (reducing erosion, adding organic matter);
• Use rotations with perennials and cover crops that increase biodiversity and add organic matter, including some crops with extensive root systems and plentiful aboveground residue after harvest;
• Add other organic materials from off the field when possible, such as composts, manures or other types of organic materials (uncontaminated with industrial or household chemicals).