The specific properties of an individual soil type are determined by pedological processes that operate during its lifetime. These physical, chemical and biological actions add, transform, move (translocate) and destroy or remove material within the soil. It is important to recognize that soil-forming processes can evolve and change over time in response to factors such as climatic variability and land use.
Below the soil, solid rock or unconsolidated sediments can be found in (or on) which the soil has developed. In reality, all sediments are derived from solid rock by a process known as weathering. In principle, there are two main types of weathering: physical and chemical.
Physical weathering
In physical weathering, rocks disintegrate without changing their chemical composition. Typical examples of these processes are the splitting of rocks through daily warming of the sun and cooling during the night or by the repeated freezing and thawing of water causing tremendous pressures if it occurs in confined spaces, such as crevices in rocks. Physical weathering produces a layer of loose material, which covers the underlying solid rock. This material is known as regolith and can vary from a few millimetres to tens of metres thick.
Chemical weathering
Chemical weathering is a gradual and continuous process. It is driven primarily by the reaction between water or an acid and elements within the parent material, which lead to the creation of secondary minerals from the original compounds present in the rock. Chemical weathering is much stronger if temperature and humidity are high. Water is the key factor in chemical weathering as rainfall is slightly acidic with a pH of around 5.6 in unpolluted environments. Atmospheric carbon dioxide dissolves in rainwater to produce a weak carbonic acid. Some minerals, due to their natural solubility e.g. highly soluble salts and gypsum or inherent instability relative to surface conditions e.g. silicate minerals, slowly dissolve to form secondary products, such as clay minerals, iron and aluminium (hydr)oxides, carbonates and essential plant nutrients, such as calcium and potassium.
One of the most well-known solution-based weathering processes is de-calcification, which occurs on parent materials that are rich in calcium carbonate, such as limestone and chalk. The weak carbonic acid in rainfall reacts with the calcium carbonate in the limestone to form calcium bicarbonate, which is then removed. This process can be even stronger if gases, such as sulphur dioxide and nitrogen oxides, are present in the atmosphere. These oxides react in the rainwater to produce stronger acids with a pH as low as 3. Water molecules can break up into positively charged hydronium (H3O+) and negatively charged hydroxyl (OH-) particles which can actually penetrate the crystal lattice of silicate and carbonate minerals. Those with a positive charge disrupt the balanced state of the mineral in question causing various cations to be released into the soil. This process is known as hydrolysis and is one of the underlying factors of soil fertility. Another chemical process involves the simultaneous loss, referred to as oxidation, and gain, referred to as reduction of electrons in substances. These exchanges are referred to as redox reactions. As materials become oxidised, the unbalanced charge degrades a material’s structural composition.
Biological weathering
Biological weathering is caused by the activities of living organisms and has both physical and chemical aspects. Examples of physical biological weathering include the loosening of rock by roots growing into cracks and burrowing creatures, such as termites that mix, or churn, the soil. Chemical biological weathering can be caused by bacterial activity or by strong organic acids from plant roots or litter. A recent study demonstrated a three-four-fold increase in weathering rate under lichen-covered surfaces compared to recently exposed bare rock surfaces.
