34.17 : The Soil Ecosystem

Plants obtain inorganic minerals and water from the soil, which acts as a natural medium for land plants. The composition and quality of soil depend not only on the chemical constituents but also on the presence of living organisms. In general, soils contain three major components:

1. Inorganic mineral matter, which constitutes about 40 to 45 percent of the soil volume.
2. Organic matter, also known as humus, which makes up about 5 percent of the soil volume.
3. Water and air, covering about 50 percent of the soil volume.

Healthy soil has an adequate quantity of air, water, minerals, and organic matter to promote plant growth.

Based on its physical structure, the soil is composed of four distinct layers:

1. O-horizon or topsoil
2. A-background
3. B-horizon or subsoil
4. C-horizon or base soil

O-horizon comprises freshly decomposing organic matter - a result of the decomposition of plants, animals, or microorganisms. It is also known as topsoil. This humus layer is significant in improving soil fertility, moisture, and air retention. Though the humus is a smaller percentage of the overall soil volume, it is nevertheless essential.

A-background is a mixture of organic and inorganic components and is the beginning of true mineral soil formed by the weathering of rock.

The B horizon, or subsoil, is a layer of fine clay that is less fertile than the topsoil. It is rich in moisture and displays less biological activity than the topsoil.

The C horizon comprises the underlying weathered rock. Beneath the C horizon generally lies the bedrock that acts as a parent material in soil formation.

Soil Formation

Soil forms as a consequence of physical, chemical, and biological weathering processes acting on the parent bedrock material. In the case of physical weathering, the earth forms from mechanical actions such as temperature change, wind, frost, abrasion, or earthquakes, any or all of which can cause the breakdown of the bedrock. In chemical weathering, the bedrock reacts with water, acid, or other chemical components. Biological weathering, in contrast, is influenced by burrowing animals and plant roots that grow into the cracks of the rock, making it split.

Overall, the type of soil that will be produced is governed by five major interacting factors - composition of the parent material, the type of living organisms present, the climatic conditions, topography, and time. Interaction among these factors produce an infinite variety of soils across the earth.