Soil-Forming Processes

Minerals vs. nutrients

  • A mineral is a naturally occurring solid substance formed through geochemical processes with a characteristic chemical composition. Rocks are composed of several minerals.
  • Nutrients are chemical elements required by organisms to live and grow. Nutrients can be produced by the organism or taken up from its environment.
  • Plants absorb nutrients from dissolved minerals in the soil which, in turn, are consumed by herbivores and then by the people who eat the herbivores. People can also obtain nutrients directly from fruits and vegetables. In this way, minerals move up the food chain.

Organic vs. mineral soils

Soil material is referred to as organic if it contains more than 20 % organic matter.

Mineral soils, by contrast, contain less than 20 % organic matter but can possess organic surface horizons. Soil organic matter is derived from the remains and exudates of living organisms (predominantly plants). Organic matter is utilised by a variety of soil organisms as both a source of energy (to function) and materials for building their bodies. During this process, water, carbon dioxide (CO2) and various organic compounds such as sugars, starches, proteins, carbohydrates, lignins, waxes, resins and organic acids, are converted through a process known as mineralisation, into inorganic compounds, such as ammonium (NH4+), phosphate (PO4 3-) and sulphate (SO4 2-). This process, together with the release of CO2 from the soil, is vital for plant growth. Some of these compounds are immobilised by being incorporated into the bodies of soil organisms, and are only available after the death of the organism.

The annual return of plant and animal residues to the soil varies with climate, vegetation type and land use. The effect can be easily seen when comparing soils of grasslands and forests. The organic matter content, moisture-retention and nutrient-holding capacity of grassland soils are generally much higher than those of forests. In addition, the type of vegetation can also affect soil characteristics. The litter of coniferous trees tends to be low in calcium, magnesium and potassium, which tends to lead to acidic conditions in the soil. Conversely, soils under natural grasslands favour nitrogen fixers, such as Azotobacter.

Tropical rainforests generally return about 15 tonnes of litter per hectare each year, compared to around eight tonnes for temperate grasslands, two tonnes for agricultural soils and 0.1 tonnes for alpine forests. Root decay contributes a further 30 – 50 % of the amount produced from leaf fall.

Soil organic matter and carbon

  • Carbon is an important constituent of all living matter.
  • All soils contain varying amounts of the element carbon (C) in both organic and inorganic forms.
  • The term soil organic matter (SOM) is used to describe the organic constituents in the soil (e.g. cells and tissues of soil organisms and plant and animal residues at various stages of decomposition).
  • With the exception of calcareous soils, the majority of C in soils is held as organic carbon (OC).
  • The term ‘soil organic carbon’ refers to the C occurring in SOM.
  • On average, 58 % of SOM is carbon.
  • Living organisms play a key role in the C cycle.

Ref: Global Soil Biodiversity Atlas p22; 27