Organic Matter and Aggregates Organic Matter and Aggregates Organic matter occurs outside of aggregates as living roots or larger organisms or pieces of residue from a past harvest. Some organic matter is even more inti-mately associated with soil. Humic materials may be adsorbed onto clay and small silt particles, and small to medium-size aggregates usually contain particles of organic matter. The organic matter inside very small aggregates is physically protected from decomposition because microorganisms and their enzymes can’t reach inside. This organic matter also attaches to mineral particles and thereby makes the small particles stick together better. The larger soil aggregates, composed of many smaller ones, are held together primarily by the hyphae of fungi with their sticky secretions, by sticky substances produced by other microorganisms, and by roots and their secretions. Microorganisms are also found in very small pores within larger aggregates. This can sometimes protect them from their larger preda-tors—paramecium, amoeba, and nematodes. There is an interrelationship between the amount of fines (silt and clay) in a soil and the amount of organic matter needed to produce stable aggregates. The higher the clay and silt content, the more organic matter is needed to produce stable aggregates, because more is needed to occupy the surface sites on the minerals dur-ing the process of organic matter accumulation. In order to have more than half of the soil composed of water-stable aggregates, a soil with 50% clay may need twice as much organic matter as a soil with 10% clay. ACTIVE ORGANIC MATTER Most of the discussion in this chapter so far has been about the factors that control the quantity and location of total organic matter in soils. However, we should keep in mind that we are also interested in balancing the different types of organic matter in soils—the living, the dead (active), and the very dead (humus). We don’t want just a lot of humus in soil, we also want a lot of active organic matter to provide nutrients and aggregating glues when it decomposes. It also supplies food to keep a diverse population of organisms present. As men-tioned earlier, when forest or grassland soils were first cultivated, organic matter decreased rapidly. Almost the entire decline in organic matter was due to a loss of the active (“dead”) part of the organic matter. Although it decreases fastest when intensive tillage is used, the active portion increases relatively quickly when prac-tices such as reduced tillage, rotations, cover crops, and applying manures and composts are used to increase soil organic matter. AMOUNTS OF LIVING ORGANIC MATTER In chapter 4, we discuss the various types of organisms that live in soils. The weight of fungi present in forest soils is much greater than the weight of bacteria. In grasslands, however, there are about equal weights of the two. In agricultural soils that are routinely tilled, the weight of fungi is less than the weight of bacteria. The loss of surface residues with tillage lowers the number of surface-feeding organisms. And as soils become more compact, larger pores are eliminated first. To give some perspective, a soil pore that is 1/20 of an inch is considered large. These are the pores in which soil animals, such as earthworms and beetles, live and function, so the number of such organisms in compacted soils decreases. Plant root tips are generally about 0.1 mm (1/250 of an inch) in diameter, and very compacted soils that lost pores greater than that size have serious rooting problems. The elimination of smaller pores and the loss of some of the network of small pores with even more compaction is a problem for even small soil organisms. The total amounts (weights) of living organisms vary in different cropping systems. In general, soil organisms are more abundant and diverse in systems with complex rotations that return more diverse crop residues and that use other organic materials such as cover crops, animal manures, and composts. Leaves and grass clip-pings may be an important source of organic residues for gardeners. When crops are rotated regularly, fewer parasite, disease, weed, and insect problems occur than when the same crop is grown year after year. On the other hand, frequent cultivation reduces the populations of many soil organisms as their food sup-plies are depleted by decomposition of organic matter. Compaction from heavy equipment also causes harmful biological effects in soils. It decreases the number of medium to large pores, which reduces the volume of soil available for air, water, and populations of organisms— such as mites and springtails—that need the large spaces in which to live. HOW MUCH ORGANIC MATTER IS ENOUGH? As mentioned earlier, soils with higher levels of fine silt and clay usually have higher levels of organic matter than those with a sandier texture. However, unlike plant nutrients or pH levels, there are few accepted guidelines for adequate organic matter content in particular agri-cultural soils. We do know some general guidelines. For example, 2% organic matter in a sandy soil is very good and difficult to reach, but in a clay soil 2% indicates a greatly depleted situation. The complexity of soil organic matter composition, including biological diversity of organisms, as well as the actual organic chemicals present, means that there is no simple interpretation for total soil organic matter tests. We also know that soils higher in silt and clay need more organic matter to produce sufficient water-stable aggregates to protect soil from erosion and compaction. For example, to have an aggregation similar to that of a soil with 16% clay and 2% organic matter, a soil with close to 50% clay may need around 6% organic matter. Organic matter accumulation takes place slowly and is difficult to detect in the short term by measurements of total soil organic matter. However, even if you do not greatly increase soil organic matter (and it might The question will be raised, How much organic matter should be assigned to the soil? No general formula can be given. Soils vary widely in character and quality. Some can endure a measure of organic deprivation . . . others cannot. On slopes, strongly erodible soils, or soils that have been eroded al-ready, require more input than soils on level lands. —Hans Jenny, 1980 take years to know how much of an effect is occurring), improved management practices such as adding organic materials, creating better rotations, and reducing tillage will help maintain the levels currently in the soil. And, perhaps more important, continuously adding a variety of residues results in plentiful supplies of “dead” organic matter—the relatively fresh particulate organic matter— that helps maintain soil health by providing food for soil organisms and promoting the formation of soil aggre-gates. A recently developed soil test that oxidizes part of the organic matter is thought to provide a measure of active carbon. It is more sensitive to soil management than total organic matter and is thereby an earlier indi-cator for soil health improvement. Interpretation of the test is currently an active research area. (See chapter 22.) 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 poten-tial for aggregation (which protects particles of organic matter) is limited, as are the fine minerals that form pro-tective 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. 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 sur-faces that are available for forming bonds with organic matter will form organic-mineral bonds. Small aggre-gates 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 fol-lowed 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 bond-ing 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 (oxydized) 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 reason-able values for rates of addition of organic materials and SOM decomposition rates in the soil. Without going through the details (see the appendix, p. 34, for sample calculations), the estimated percent of organic matter in soils for various combinations of addition and decompo-sition 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. 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 3.2 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 mat-ter content reaches a steady state. If it was a very 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 aggre-gates—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 mat-ter) 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.
