BIOLOGY Volume 1 - A Guide to General Biology - 2004

10. ORGANISMS AND THE ENVIRONMENT

10.5. Factors Influencing the Environment and Habitats

10.5.2. Soil

Soil is a complex system composed of mineral and organic components. It serves as a substrate for plant growth. Understanding the characteristics and formation processes of soil is essential for successful agriculture, as it helps improve soil fertility and thus holds significant economic importance.

Soil consists of four main components:

1) mineral matter;

2) organic matter;

3) air;

4) Water, more accurately referred to as the soil solution, since it always contains various dissolved substances.

Soil Mineral Matter

Soil is made up of mineral particles of various sizes: stones, gravel, and "fine earth." The latter is typically classified, in order of increasing particle size, into clay, silt, and sand. The textural composition of soil is determined by the relative proportions of sand, silt, and clay it contains.

Soil texture strongly influences drainage, nutrient content, and thermal regime—in other words, the structural Properties of the soil from an agronomic perspective. Medium- and fine-textured soils, such as clays, loams, and silts, are generally better suited for plant growth because they contain adequate nutrients and have a higher water-holding capacity for dissolved salts. Sandy soils drain faster and lose nutrients through leaching, but they are advantageous for obtaining early yields; in spring, they dry out and warm up faster than clay soils. The presence of stones—particles with a diameter greater than 2 mm—is important with regard to the wear and tear on agricultural implements and their effect on drainage. Generally, as the stone content in the soil increases, its water-holding capacity decreases.

Soil Organic Matter

Organic matter typically constitutes only a small volume fraction of the soil, yet it is critically important as it determines many of the soil's properties. It is the primary source of plant nutrients such as phosphorus, nitrogen, and sulfur; it promotes The formation of soil aggregates, creating a crumb Structure that is especially vital for heavy soils as it enhances water permeability and aeration; and it serves as food for microorganisms. Soil organic matter is subdivided into detritus, or dead organic matter (DOM), and biota.

Humus is the organic material formed from the incomplete decomposition of DOM. A significant portion of it exists not in a free state, but bound to inorganic molecules, primarily soil clay particles. Together with these particles, humus forms the so-called soil absorption complex, which is crucial for almost all physical, chemical, and biological processes occurring in the soil, particularly for the retention of water and nutrients.

Among soil organisms, earthworms occupy a special place. These detritivores ingest large amounts of mineral particles along with DOM. By moving between different soil layers, earthworms constantly mix the soil. In addition, they create burrows that facilitate soil aeration and drainage, thereby improving soil structure and its associated properties. Earthworms thrive best in neutral and mildly acidic environments, and are rarely found at pH levels below 4.5.

Soil Air

Air fills the "pores" in the soil—the spaces between soil particles—and is easily displaced by water. In other words, waterlogged soil is poorly aerated. Soil air differs from atmospheric air in several important ways, notably containing higher concentrations of carbon dioxide and water vapor, but less oxygen.

Soil Solution

The water present in the soil contains substances that dissolve as rocks weather and DOM mineralizes. There is a continuous flow of these substances from the mineral and organic PARTS OF THE soil into the soil solution, and from there into plants.