Soil

I

Introduction

Soil, thin surface covering that overlies the bedrock of most of the land area of the Earth. It is a resource that, along with water and air, provides the basis of human existence. Soil develops when rock is broken down by weathering and material is exchanged through interaction with the environment. Organic matter becomes incorporated into the soil as the result of the activity of living organisms. Soil also contains water, minerals, and gases. The soil system is dynamic and it develops a distinct structure, often with recognizable layers or soil horizons arranged vertically through the soil profile.

Soil is essential for the development of most plants, providing physical support and nutrients. Plants are anchored in the soil by their roots. Nutrients, dissolved in soil water, are necessary for the plants’ growth. Soil contains various organic matter, including dead material from plants and animals as well as animals that choose to live in the soil. The soil is therefore a store of major nutrients such as carbon and nitrogen and plays an important role in global nutrient cycles and in regulating hydrological cycles and atmospheric systems.

Soils vary from place to place due to various conditions such as climate, rock type, topography, and the local soil-forming processes. Over time soils develop characteristics specific to their location, which relate closely to the climate and vegetation of the area. The major world biomes reflect a clear association between vegetation and soil that has developed in response to the prevailing climate. Each soil type has a distinct combination of soil horizons and associated soil properties.

People depend on the soil for agriculture, and as such it is a valuable natural resource. Soils form continuously as the result of natural processes, and can therefore be regarded as a renewable resource. However, the soil-forming processes operate very slowly and the misuse or mismanagement of the soil may lead to damage or erosion, or can disrupt the processes by which the soil forms. If this happens the resource can be degraded or even lost. Many human activities cause damage to soils. These include bad farming techniques, overgrazing, deforestation, urbanization, construction, mining, wars, contamination, pollution, and fires. The most critical result of these is soil erosion. With growing populations, the need for productive soils is increasing. Soil loss in many developing countries is a major cause for concern and will become a major issue in the future. The process of soil loss can have a detrimental effect on other systems as it produces sediment that can cause siltation of river systems and reservoirs, set off flooding downstream, and contribute to pollution and damage to estuaries, wetlands, and coral reefs. Soils need to be managed carefully in order to remain in good condition.

In order to maximize the potential of soils it is important to understand soil systems and the processes that operate within them. This creates a better appreciation of the type of land use that would be sustainable and continue to be productive. The knowledge of how different soil types have developed results in the recognition of the dynamic equilibrium between soils and their environment. This makes it possible to make informed decisions about the best ways in which soils can be utilized, and how they may respond to changes in land use.

II

Soil Formation

Soils are complex and varied, and do not merely consist of random assemblages of particles. A vertical section through the soil reveals layers known as horizons. Usually three main horizons, overlain by organic matter, can be identified. The extent to which each horizon has developed depends on many local factors and the time over which the soil has been forming. Not every horizon will appear in every soil; some soils may have few or very indistinct horizons while others have clear, well-defined horizons.

A

Soil Profiles

The process of soil formation begins with the breaking down of bedrock, which produces a layer of loose material called a regolith. Water, gases, living organisms, and decayed organic matter (humus) are added over time. This leads to the development of a recognizable vertical structure. A section through that structure is known as a profile. This reveals the different soil horizons, at different depths, which will differ in their physical, chemical, and biological attributes. The horizons are designated by capital letters. The organic material overlying the soil is termed the “O” layer; the top layer of the soil, “A”; the middle layer, “B”; and the lowest layer, “C”. Below the soil lies the parent material, or bedrock, which is termed either the “D” or “R” layer. Further subdivisions of the soil can be made according to the detectable variations and processes.

The top layer of soil (the O-horizon) consists entirely of accumulated organic matter and contains three distinct layers of humus. The top layer (L) is composed of newly deposited organic material or litter, such as leaves and animal remains, which are easily recognizable on the surface. The layer below is known as the fermentation (F) layer, where the breakdown of the dead organic matter occurs and partly decomposing litter is found. Once totally decomposed, so that the original plant structures are not visible, the material forms the deepest layer of humus, the H horizon, which is usually very dark, and no plant and animal remains are identifiable.

In the A-horizon, the humus from above is mixed with mineral particles, so that the top of this horizon is also dark. As water passes down through the soil, it can remove, or translocate, humus, clay particles, and nutrients (bases) by a process known as eluviation (E). This often results in the A-horizon becoming paler towards the bottom.

The B-horizon is essentially a mineral horizon characterized by in situ weathering of the original parent material. Particles may be carried down into this horizon from above and accumulate. This is known as illuviation, and clay, iron, aluminium, or humus may be found. B-horizons are the most variable of soil horizons.

The C-horizon represents the weathering zone where the regolith is being produced from parent material. The D- or R-horizon is the unweathered bedrock or parent material from which the mineral portion of the soil is derived.

B

The Soil System

Scientists widely use the system-modelling approach to look at the processes that operate in a soil. Materials and energy are gained and lost, and so the system can be seen as a series of inputs, outputs, stores, processes, and recycling. Any change in the inputs or outputs to the system has a profound effect on the way in which the processes within the soil can operate and, consequently, on the character of the resulting soil.

Inputs to the system include: water from precipitation or from further up the slope; gases from the atmosphere and from respiration of soil organisms; nutrients released from decaying and weathered rock; organic matter from decaying plants and animals; and solar energy and radiation. Outputs include: nutrients taken up by plants; nutrients taken away by water as it passes downwards through the soil in leaching; water lost at the surface of the soil by evaporation; and soil particles lost by soil movement down-slope and erosion. Materials to be stored and recycled comprise: dead organic matter deposited in the soil by plants and animals; dead organic matter decomposed by soil organisms; and nutrients taken up and stored by plants.

The relationship between the vegetation of an area and its soil is a very important one as the stores and recycling of materials in soils are closely linked to the vegetation. The vegetation and animals that feed on that vegetation provide the input of dead organic matter to the soil. This in turn produces the humus and also the nutrients upon which the plants depend. Therefore the development of soils is closely linked to the process of primary succession. As plant succession occurs and the ecosystem becomes more complex, the vegetation exerts an increasing influence on the soils until they reach a stage of maximum development in equilibrium with the climax vegetation. The organic content of the soil builds up over time and this stabilizes the soil and ensures sufficient nutrients to support the increasing vegetation, which in turn also becomes more stable.