Interplay of Life in Soil: The Vital Role of Organic Matter and Aggregates

 Organic Matter and Soil Aggregates

Organic matter exists outside of aggregates as living roots or larger organisms or pieces of residue from a previous harvest. Some organic matter is even more closely associated with soil. Humic materials may be adsorbed onto clay and small silt particles, and small to medium-sized 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.

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 mentioned 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 practices such as reduced tillage, rotations, cover crops, and applying manures and composts are used to increase soil 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