As organic material is decomposed, essential minerals are released into the soil, where they may

he absorbed by plant roots. Organic matter also in­creases the soil's water-holding capacity by acting like a sponge.

The pore space between soil particles occupies roughly 35 to 60 percent of a soil's volume and is filled with varying proportions of water (called soil water) and air (called soil air); both are necessary to sustain all the organisms living in the soil.

Soil water contains low concentrations of dis­solved mineral salts that enter the roots of plants as they absorb the water. Soil water not absorbed by plants moves down through the soil, carrying dis­solved minerals with it. The removal of dissolved materials from the soil by water percolating down­ward is called leaching. The deposit of leached material in the lower layers of soil is known as illuviation. Iron and aluminum compounds, humus, and clay are some illuvial materials that can gather in the subsurface portion of the soil.

Some sub­stances completely leach out of the soil because they are so soluble that they migrate all the way down to the groundwater. It is also possible for water to move upward in the soil, transporting dis­solved materials with it, as when the water table rises.

Soil air contains the same gases as atmospheric air, although they are usually present in different proportions. Generally, as a result of respiration by soil organisms, there is more carbon dioxide and less oxygen in soil air than in atmospheric air. Among the important gases in soil air are (1) oxygen,   required by soil organisms for respiration; (2) nitrogen, used by nitrogen-fixing soil organisms; and (3) carbon dioxide, involved in soil weathering.

Soil Horizons

A deep vertical slice, or section, through many soils reveals that they are organized into horizontal lay­ers called soil horizons. A soil profile is a section j from surface to parent rock, showing the horizons.

The uppermost layer of soil, the O-horizon, is rich in organic material. Plant litter, including dead leaves and stems, accumulates in the O-horizon and gradually decays. In desert soils the O-horizon is often completely absent, but in certain organic-rich soils it may be the dominant layer.

Just beneath the O-horizon is the topsoil, or A-horizon, which is dark and rich in accumulated humus. The A-horizon has a granular texture and is somewhat nutrient-poor due to the gradual loss of many nutrients to deeper layers by leaching.

The B-horizon, the light-colored layer beneath the A-horizon, is often a zone of illuviation in

which minerals that leached out of the topsoil and litter accumulate. It is typically rich in iron and aluminum compounds and clay.

Beneath the B-horizon is the C-horizon, which contains weathered pieces of rock and borders the solid partner rock. The C-horizon is below the extent of most roots and is often saturated with ground water.

Soil Organisms

Although soil organisms are usually hidden under­ground," their numbers are huge. Millions of soil organisms may inhabit just 1 teaspoon of fertile ag­ricultural soil! In the soil ecosystem, bacteria, fungi, algae, worms, protozoa, insects, plant roots, and larger animals such as moles, snakes, and ground­hogs all interact with each other and with the soil.

Earthworms, probably one of the most familiar soil inhabitants, eat soil and obtain energy and raw materials by digesting humus. Castings, bits of soil that have passed through the gut of an earthworm, are deposited on the soil surface. In this way, min­erals from deeper layers in the soil are brought to upper layers. Earthworm tunnels serve to aerate the soil, and the worms' waste products and corpses add organic material to deeper layers of the soil.

Ants live in the soil in enormous numbers, constructing tunnels and chambers that help to aerate the soil. Members of soil-dwelling ant colo­nies forage on the surface for bits of food, which they carry back to their nests. Not all of this food is eaten, however, and its eventual decomposition helps increase the organic matter in the soil. Many ants are also indispensable in plant reproduction because they bury plant seeds in the soil. Seeds bur­ied by ants typically have special structures, called oil bodies that are very nutritious. Ants bring the seeds underground, eat the oil bodies, and dispose of the rest of the seeds in underground refuse piles, along with their droppings and members of the col­ony who have died. Thus, the ants not only bury the seeds away from animals that might eat them, but also place the seeds in well-fertilized soil that is ideal for seed germination and seedling growth.

Plants are greatly affected by the properties of soil, although most plants can tolerate a wide range of soil types. Soil, in turn, is affected by the types of plants that grow on it. As a result of the complex interactions among plants, climate, and soil, it is hard to specify cause and effect in their relationships. For example, are the plants growing in a certain locality because of the soil that is found there, or is the soil's type determined by the plants?

One very important symbiotic relationship in the soil occurs between fungi and the roots of vas­cular plants. These associations, called mycorrhizae, enable plants to absorb adequate amounts of essential minerals from the soil. The threadlike body of the fungal partner, called a mycelium, extends into the soil well beyond the plant root. Min­erals absorbed from the soil by the fungus arc trans­ferred to the plant, while food produced by photosynthesis in the plant is delivered to the fun­gus. Mycorrhizae have been demonstrated to enhance the growth of plants.

Nutrient Recycling