Biological Properties of Soil
Soil Fauna
Soil fauna play an important role in the decomposition and cycling of organic matter in soil systems. Insects, earthworms, and other arthropods churn the soil and promote formation of soil structure, aeration, reduction in bulk density, and nutrient cycling. They also enhance water movement. Some soil fauna ingests organic matter by taking in soil, applying digestive enzymes, and excreting the soil/ organic matter complex, thus providing a more readily available food source for other soil fauna and soil microorganisms.
Earthworms
Earthworms are integral to a healthy soil ecosystem, improving soil structure, increasing the rate of nutrient cycling, enhancing water infiltration, and soil aeration (Figure 6.6). Their food consists of decaying plants and other organisms; as they eat, however, earthworms also ingest large amounts of soil, sand, and tiny pebbles. It has been estimated that an earthworm ingests and discards its own weight in food and soil every day.
Benefits of Earthworms
By their activity in the soil, earthworms offer many benefits. First, they feed on organic matter and are critical to incorporating organic matter from the soil surface into the soil profile. Second, earthworms speed nutrient cycling in the soil. The shredding action of earthworm digestion increases the surface area of the organic matter, making the organic matter more readily available to soil microorganisms that feast on the earthworm feces, or “castings.” The third significant way earthworms improve the soil is by developing soil structure. Their passages are like superhighways for air and water, as well as for root growth.
Conditions Affecting Earthworm Activity
Earthworms prefer cool, moist, but well-aerated soils well supplied with decomposable organic materials, preferably supplied as surface mulch. In temperate regions they are most active in the spring and fall, often curling into a tight ball to ride out hot, dry periods in summer. They normally avoid water-saturated soil (i.e., anaerobic conditions). Most earthworms thrive best where the soil is not too acid (pH 5.5–8.5) and has an abundant supply of calcium (which is an important component of their mucilage excretions).
Nematodes
Nematodes are a large and diverse group of microscopic non-segmented worms that occur in many habitats, and are abundant in most soils (Figure 6.8 ). Nematodes feed on decaying plant litter, bacteria, fungi, algae, protozoa, insect larvae, living roots, and other nematodes as predators. Like protozoa, nematodes feeding on bacteria and fungi release plant-usable ammonium nitrogen into the soil because their bodies do not need all the nitrogen the microbes provide.
Arthropods
Soil arthropods represent one of the most important components of soil-living communities and play an important role in maintaining soil quality and health and providing ecosystem services. It is a well-known fact that soil arthropods are involved in many processes such as organic matter translocation, breaking and decomposition, nutrient cycling, soil structure formation, and consequently water regulation. Arthropods range in size from microscopic to several inches in length. They include insects, such as springtails, beetles, and ants; crustaceans such as sowbugs; arachnids such as spiders and mites; myriapods, such as centipedes and millipedes; and scorpions.
Large Soil Animals
Very large soil animals, such as moles, rabbits, woodchucks, snakes, prairie dogs, and badgers, burrow in the soil and spend at least some of their lives below ground. Their population is highest in undisturbed soils such as pastures and forests. Moles are secondary consumers; their diet consists mainly of earthworms and other small invertebrates found in the soil.
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