In general, more organic matter is present near the surface than deeper in the soil (see figure 3.5). This is one of the main reasons that topsoils are more productive than subsoils exposed by erosion or mechanical removal of surface soil layers. Some of the plant residues that eventually become part of the soil organic matter are from the aboveground portion of plants. In most cases, plant roots are believed to contribute more to a soil’s organic matter than the crop’s shoots and leaves. But when the plant dies or sheds leaves or branches, depositing residues on the surface, earthworms and insects help incorporate the residues on the surface deeper into the soil. The highest concentrations of organic matter, however, remain within 1 foot of the surface.
Litter layers that commonly develop on the surface of forest soils may have very high organic matter contents (figure 3.5a). Plowing forest soils after removal of the trees incorporates the litter layers into the mineral soil. The incorporated litter decomposes rapidly, and an agricultural soil derived from a sandy forest soil in the North or a silt loam in the South would likely have a distribution of organic matter similar to that indicated in figure 3.5b. Soils of the tall-grass prairies have a deeper distribution of organic matter (see figure 3.5c). After cultivation of these soils for 50 years, far less organic matter remains (figure 3.5d).
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 intimately 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 predators—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 during 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.