Definition of Soil

Soil, which is the solid part of the earth's surface, is the major element of land. Through physical, chemical and biological processes, soil is derived from rocks. Although soil originates from rocks, it is essentially different from them in certain respects.

Unlike rocks, the soil is made up largely of new secondary minerals. These secondary minerals are continually being formed from the underlying rocks through the processes of weathering and pedogenesis, while at the same time, the top part of the soil is continually  being removed by denudation.

The term 'soil' has various meanings and connotations to different professional groups and practitioners. Consequently, there are many definitions of soil but each borders on the respective field of consideration. This leads to the differences in the treatment and classification of soil. For example, a mining engineer sees soil as debris covering the rocks or minerals which must be quarried and as such, soil is a nuisance. To the roadway engineer, soil is the material on which a road bed is to be placed. If its properties are unsuitable, then it has to be removed and replaced with rock and gravel. Other physical scientists, physicians and geologists view soil as a collection of matter - solid, liquid and gas - having recognizable mechanical properties in relation to the rock from which it was formed and the use to which it is intended.

Agriculturally, soil may be regarded as the collection of natural bodies occupying parts of the earth's surface that supports plants and have properties due to the integrated effects of climate and living matter acting upon parent materials (rocks) as conditioned by relief over periods of time.

Generally, soils are grouped into mineral or inorganic soils and organic soils.

Mineral or Inorganic Soils:

These are soils consisting predominantly of and having its properties determined predominantly by mineral matter. They usually contain less than 20% organic matter but may contain an organic surface layer up to 30cm thick.

Organic Soils:

These are soils which contain a high percentage of organic matter throughout the solum usually greater than 20%, for example, muck and peat soils commonly found in swamps, bogs and marshes.

Soil Boundaries

The upper limit of soil is air or water and its lateral boundary is water, rock or ice. The lower limit or boundary of soil is the most difficult to define as it depends on usage. Overall, soil scientists consider the lower boundary of the soil to be between 1.5 and 2.0 meters (150 to 200 centimeters) where there is no obstruction. Where an obstruction such as underlying bedrock, an impenetrable layer of soil, or a body of water, makes it impossible to reach a depth of 2.0 meters, then the depth to the obstruction will become the depth of soil for that area.