Soil - a dynamic process of change

Understanding the evolution of soils and landscapes is very important for an understanding of the behaviour of soils under various forms of land use. This article details how soil and its’ components arrived at your place, and how some of it moved on. Soil formation is closely connected to other landscape processes such as geology, climate, landform development, water movement, vegetation and fauna.

The processes of soil formation are dynamic and occur over an enormous range of scales in time and space (as shown in this diagram from Australian Soils and Landscapes – McKenzie, Jacquier, Isbell and Brown).

Soil Formation Processes

Soils evolve from additions, removals, transformations and translocations of material. A simple model for soil formation involving these processes is shown in this diagram, (adapted from Simonson 1959).

There are six major processes involved in the Additions process in Australian and New Zealand soils;

1. Hillslope Deposition, which is sedimentation caused by  the overland flow of material, followed by the deposition of coarser bed load and finer colloids. This is the primary cause of sequences of soils from hill crests to valley bottoms.
2. Alluvial Deposition which is sedimentation caused by flooding where the water sorts by particle size (eg sand vs. clay) and by overall soil properties.
3. Aeolian Deposition by wind, where particle size determines the travel distance and controls soil properties in wind-created dune fields. Wind is a major source of sediment and nutrients across large parts of the world.
4. Plant Litter. The organic matter deposited by plants either at the soil surface or within the soil profile by roots is the primary energy source for life in the soil. It also drives soil formation through weathering.
5. Substrate Weathering where the breakdown of primary minerals releases nutrients and other compounds and is a source of mineral nutrients for plant growth and life.
6. Soluble Salts which are added either through rainfall, dry deposition or through groundwater. They can be a significant source of nutrients and sometimes troublesome salts.

The Soil Losses process involves three types of process:

1. Erosion where water or wind remove soil from the land surface and sometimes deeper layers such as in gully erosion, is a primary control on sequences of soil types from hill crests to valley bottoms.
2. Solution Loss where various ions are leached in solution from the soil profile where the intensity of leaching influences degree of weathering and eventually fertility.
3. Volatilization where compounds such as carbon dioxide and nitrous oxide are lost to the atmosphere as gases is a cause of long-term nutrient depletion.

Three major processes involving Soil Transformations are:

1. New Mineral Formation where various minerals, especially clay minerals are created from either soluble ions or changes to existing minerals. The creation of charged surfaces on soil particles allows nutrients to be retained and exchanged with the soil solution in a process essential for the plant nutrient supply system.
2. Organic Matter Decomposition where leaf litter, plant roots, manure and dead organisms are broken down by micro-organisms to eventually produce CO2 and humus. This soil carbon cycle is essential for life on earth.
3. Iron oxidation and reduction, dissolution and precipitation of iron results in soil colour and is a useful measure of the soil water regime.

Soil Translocations involve the following five processes:

1. Clay particles move in the soil associated with percolating water and resulting from earthworm activity.
2. Iron can be translocated as well as transformed creating zones that are depleted, or zones of concentration.
3. Soluble salts, involving movement of ions in the soil solution which is largely controlled by the leaching intensity of the environment.
4. Carbonates which provide a useful monitoring tool for measuring the degree of leaching.
5. Bioturbation which is defined as the movement of materials within the soil profile caused by plants and animals which can lead to the development of contrasting A & B Horizons.

The rates of soil erosion and soil formation vary enormously under different environments, events or land uses.

For instance, the rates for soil erosion range between:

• Extreme individual storms with loss rates of 300 – 700 tonnes of soil per hectare per storm.
• Undisturbed forest catchments where soil erosion rates are as low as 0 – 1 tonnes of soil per hectare per year.
• Forests immediately after fire, by contrast, lose 10 -50 tonnes of soil per hectare per year.
• Permanent pastures on non-eroded lands can still lose up to 1 tonne of soil per hectare per year.

Other soil erosion rates include:

• Cereal cropping areas where soil erosion can range from 1 – 50 tonnes of soil per hectare per year.
• Bare fallow paddocks in temperate Australia have been recorded as losing between 50 and 100 tonnes of soil per hectare per year.
• In tropical areas on sloping lands such as in cane sugar areas, losses of between 100 and 500 tonnes of soil per hectare per year have been recorded.

By comparison, rates of soil formation are considerably slower. Examples of soil formation rates are:

• Soil forms on bedrock under favourable climatic conditions at about 0.1 to 1 tonne of soil per hectare per year.
• On alluvial soils, deposited by rivers, the rate of soil formation is only 0.4 tonnes per hectare per year even under favourable conditions.
• A common and distinctive Australian soil feature is the occurrence of soils with pronounced differences in texture between the A and B horizons.
• Nearly 20% of the soils in Australia are mantled by these soils including most regions.
• In these soils the B horizon can have from 2-10 times the clay content of the overlying A horizon. A strong textural contrast between the A and B horizons is often associated with a marked decline in permeability, which can cause waterlogging in the soil.