Origin and Formation of Soil

Scientifically, soil is derived from rocks through a variety of physical, chemical and biological processes. These processes are known as weathering. The soils so formed bear resemblance, especially in their chemical make-up, to the rocks from which they originate.

Soil formation generally involves two stages or phases.

a) The first stage involves the accumulation of parent material (PM) for the soil. This process is generally described as rock weathering.

b) The second stage involves the formation of the soil from the parent material.

The parent material may result from in-situ degeneration and decomposition of rocks and minerals. It may also result from the accumulation of transported and deposited rock debris by the various agents of denudation – water, ice, wind.

Soil formation begins when variations in climatic conditions lead to variations in temperature and pressure. These variations will then cause rocks to crack and chip, breaking it into smaller pieces. Other climatic factors like wind and rain will result in the translocation of the weathered rock materials if relief (topography) favours it; otherwise, it will remain in-situ. At the place of deposition, the combined actions of living organisms (plants and animals) and other factors like heat and mineral acids, will result in the physical and chemical disintegration of the deposited rock materials, reducing them into very small mineral particles. Further exposure of these small mineral particles to the physical and chemical factors in the atmosphere over long periods of time, will lead to their extensive disintegration into the various soil separates, ultimately resulting in agricultural soils.

Generally, the following influences the predominant characteristics of soils:

a)   Type of rock (parent material) from which the soil is derived: Regardless of their classification; rocks that are rich in basic elements (Ca, Mg, K and Na) give rise to soils that are natively (naturally) productive. On the other hand, rocks that contain silica (Si), salts and non-nutrient elements such as heavy metals, weather into soils of low native fertility.

b)   Extent of the activity of living organisms: Through their activities, living organisms such as earthworms, soil insects and arthropods greatly modify soil composition and properties by tunnelling and mixing the topsoil and subsoil materials. On the other animals often leave their waste on or in the soil, thereby, contributing to soil organic matter. Dead plants, dropped leaves and animals left to decompose on or in the soil also contribute to soil organic matter. More so, the growth and penetration of plant roots also has a large impact on the modification of soil structure. While grasses promote granulation, trees foster the recycling of subsoil materials to the topsoil. This is why the topsoil is usually thicker in areas with lots of living organisms.

c)   The intensity of other soil-forming factors: Besides the influences of parent material and time, the more intense of the other soil forming factors shapes the predominant characteristic of a soil. The predominant type of weathering (physical or chemical), the rate of weathering, effective depth of soil, rate of soil development, among others are affected by the more intense of the other soil-forming factors.

d)   Duration of the process: This influences the extent of soil horizonation. Older soils have more horizons with distinct differentiation, while younger soils have fewer, less distinct horizons.