Revision Notes

Soil Formation – Introduction

The formation of soil happens over a very long period of time, often taking thousands of years. Soil is formed from the weathering of rocks and minerals. The surface rocks break down into smaller pieces through a process of weathering and these are then mixed with moss and organic matter. Over time this creates a thin layer of soil.

Mechanical Weathering

Mechanical weathering occurs when rocks break down into smaller and smaller pieces. The main cause of mechanical weathering is water, which causes rocks to split apart as it freezes and thaws. This occurs over a long period of time. The activities of plant roots and animals can also contribute to mechanical weathering.

When water freezes, it has an unusual property. Most materials expand when heated and contract when cooled. This is true of water as well, except when it is cooled from 4oC to 0oC. At these temperatures, water expands and solidifies into ice, taking up 9%-10% more space than it did as water. This expansion exerts incredible pressure on rocks and eventually splits them apart. When water gets into tiny pores in rocks it is known as ice wedging or frost action, and over time can cause pot holes. Plants such as lichens, mosses, and tiny roots also find their way into tiny pores and crevices in the rock, expanding as the plant grows and eventually causing cracking. Animals, such as earthworms and small burrowing mammals, act by transporting particles that have been broken down.

Abrasion, where rocks rub against each other, also contributes to mechanical weathering. This occurs mostly in streams where fragments bounce off each other and wash against each other.

Chemical Weathering

Rainwater, oxygen, carbon dioxide, and plant decay acids can all contribute to mechanical erosion. Hydration and Hydrolysis are chemical reactions of water together with other substances. Hydration is when water combines with another substance, for example, the hydration of anhydrite creates gypsum. Water can also form hydrogen ions (H+) and (OH-). When these ions replace the minerals, the reaction is called Hydrolysis. Minerals such as feldspar, hornblende, and augite combine with water to form clay.

Oxidation occurs when oxygen combines with another substance, and most often occurs when the oxygen reacts with iron-bearing minerals such as such as magnetite, pyrite, and mafic silicates such as hornblende, and augite. When oxygen combines with the iron, chemical bonds are broken down, weakening the structure. Other elements such as Aluminium and Silicon can act in the same way.

Carbonic acid is created when CO2 dissolves in water. Carbonic acid attacks many types of rock and over years can cause them to dissolve.

Climate has an important influence on the process of weathering. Warm climates favour chemical weathering while cold climates favour physical weathering. The more moisture there is, the more weathering occurs. Chemical reactions occur at a faster rate when the temperatures increase.

Soil and top soil are produced naturally at a rate of 1mm in 200-400 years, averaging at about 1 ton/ha/y (tons per hectare per year). A full soil profile (which we shall be looking at below) takes 2,000 – 10,000 years to develop. Soil is being lost at a rapid rate all over the world – in some places almost forty times faster than it can be replaced naturally. The USA has lost 80mm since farming began 200 years ago, which amounts to 18 t/ha/y, while China appears to be losing 40 t/ha/y. Statistics of this sort remind us that soil is valuable resource that deserves to be cared for and managed effectively.

When we think about soil formation, there are five key aspects to consider:

1. parent material,

2. climate,

3. topography,

4. biota,

5. time.

Understanding these five factors can give us an important insight into the nature of a particular soil, as well as giving us some useful clues as to how it might be managed.

Parent material – The nature of the soil parent material has an important influence on soil characteristics. Soil texture is often a direct result of the type of parent material and soil texture profoundly influences many other soil characteristics, such as permeability and the availability of plant nutrients. Parent material also determines the types of clay minerals present in the soil. The degree of influence of the parent material on soil characteristics depends on the type of parent material and the developmental age of the soil. There are five basic types of soil parent materials.

1) Alluvium: Alluvial parent material is transported and deposited by water, and may be found particularly in valley basins. Flood plain alluvium is laid down parallel to river courses when the rivers overflow their banks.

2) Colluvium: Colluvial material is transported primarily by gravity. These are the deposits of debris found on hillslopes and at the base of cliffs.

3) Eolian (loess): Eolian material is transported by the wind. Eolian material refers to material of all sizes that is capable of being moved by the wind. Loess refers strictly to silt-sized or smaller material that is wind transported.

4) Lacustrine and marine sediments: These sediments are materials carried by streams and deposited in fresh or salt-water basins. The former are referred to as lacustrine deposits and the latter as marine deposits.

5) Residual materials: Residual parent materials are those that develop or weather in place from the underlying bedrock. These materials are not transported.

Climate – The temperature and precipitation environment of a soil determine to a large degree the rate of weathering of that soil, by controlling the rates of chemical and physical processes. Climate also determines the amount and type of organisms (mainly plants) which also influences soil formation. Water is essential in the soil, being responsible for moving minerals and organic matter through the soil, and also leaching them away. Water is also the medium for all the chemical reactions that take place in the soil. Lack of moisture often results in an accumulation of soluble components in the soil, for instance, calcium carbonate. Temperature is also important to the processes of soil formation. Soil weathering, leaching, and erosion all occur at a significantly slower rate when the soil is frozen. Conversely, soil weathering and leaching take place at an accelerated rate in tropical environments. This is due to heavy rainfall and high rates of activity by soil-dwelling organisms, including bacteria.

Topography – Topography has a huge influence on the soil environment. For example, steeply sloping areas in desert regions will erode very quickly unless protected by plant cover. The direction in which a slope faces, known as its ‘aspect’, influences the amount of sunlight it receives, which in turn, can determine the type of plant community that grows on the soil. For example, a north-facing slope receives much less sunlight and is much colder than a south-facing slope and this will influence the types of plant that grow and also the amount of activity in the soil.

Biota – The contribution of living organisms to soil formation can be considerable and is something we shall be considering in more depth later in this module. Both animals and plants are responsible for mixing the different layers in soil, moving nutrients, and producing organic material. Plant cover is especially important in controlling soil erosion.

Time – Young and old soils are very different from each other. New geological surfaces are constantly being created through sedimentary deposition of materials or deposition of entirely new material. For instance, through volcanic activity. As these new deposits are laid down, soil begins to form and development takes place at a pace that combines the effects of time and the other soil-forming factors.

Further Information

The Five Factors of Soil Formation Video – Soil Web UBC

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