A terrestrial environment encompasses the earth's surface, both above and below ground. The soil weathering process in this environment includes complex chemical and physical processes, these processes help decompose inorganic and organic matter which eventually become soil.
A soil profile includes five different factors that influence decomposition:
- Climate: past and present (pressure, heat, water)
- Parent rock: physical and chemical characteristics of the original rock
- Hydrology: ground relief, high or low for water run-off
- Time: geological time for soil formation (over millions of years)
- Ecosystem: vegetation, animal or human activities
Decomposing Matter, Inorganic & Organic
The decomposition of inorganic mineral matter, or weathering, allows retention of moisture and exchange of gases, which are essential processes in the formation of soil.
Irons, molecules and particles, from chemical and physical processes form soil profiles from migration of the elements and organic compounds. Water, carbon dioxide, oxygen, nitrogen dioxide (NO2), salts, pollutants and particulate matter aid in soil formation. These elements and compounds are affected by changes in pH, adsorption of other nutrients and leaching of materials and are chemical processes.
Water, plus the elements and gases mentioned above are important because they transfer nutrients to plants, elements to groundwater, CO2 and NO2 into the atmosphere.
Oxygen, silica, aluminium and iron, are important inorganic elements and make up some of the, approximately 80 elements found in soil, of which, only 17 are used by plants.
Chemical Weathering and the Constituents of Soil
There are three major forms of chemical constituents in soil and soil solution components:
- Pore water pressure (porosity and pressure): potassium (K+), sodium (Na+), magnesium (Mg2+), calcium (Ca2+),chlorine (Cl-) and sulphate (SO42-) are all dissolvable irons
- Surface adsorbed constituents are minerals, nutrients, nitrates (NO3-), phosphates (PO43-) and heavy metals, for example: lead (pb)
- Potentially decomposable constituents: soluble solids, carbonates, pyrite (iron, sulphur) (FeS2), primary minerals, for example: feldspar and metal oxides
Solute concentration in soil is variable because of the condition of soil formation at the time. Parent material is different because of the differences in: deposition of gases and salts from the atmosphere, fertilizers and lime (carbonates) content in the soil (especially in farming areas) and water content.
Inorganic Minerals and Their Classification
Inorganic minerals of soil are classified into:
- Primary minerals, which still have their original chemical composition and structures created during the crystallisation process of molten lava
- Secondary minerals, which have been altered from their original structure and chemical composition by weathering
Secondary minerals usually have a larger surface area and are important in adsorption, fixation and weathering and are usually clay minerals (silicates).
The breakdown of soil is an ongoing, complex process involving various phases and chemical or physical processes. The speed of breakdown is altered by the type of rock and weathering conditions at the time, taking many millions of years to form soil.
References:
Smithson, P., Addison, K and Atkinson, K., "Fundamentals of the physical environment" Routledge, London 2002.
Environmental Chemistry 270, lecture notes, University of Auckland, Chemistry Department 2004.
Rhonda Chalmers - Rhonda has a BSc in Geology, graduating from the University of Auckland in 2007. Rhonda lives in Brisbane and works full-time as an ...
