2. Soil Forming Factors and Processes

Soil Parent materials

In a wistful allusion to the genetics of living organisms the raw materials from which soils form are called parent materials.

Vegetation established on rocky islets, Costa Rica

The formation of a new soil depends on the presence of new soil material – obtained either by denudation - the scraping clean of a surface by action of wind, water or ice - or deposition of new materials from erosion elsewhere - for example river gravels, rock-falls or blowing sand - or the formation of new rocks by uplift of the ocean bed or volcanic action. Thus there is a huge variety of possible parent materials, all with uniquely different characteristics which will be incorporated into the soils which form on them.

Here are some examples :

  • Rocks - Sandstone, siltstone, mudstone, limestone, volcanic lavas or dusts
  • Gravels - river and sea deposits
  • Scree - fallen rocks
  • Alluvium – sands, silts, clays from marine, estuarine or freshwater rivers and lakes
  • Boulder clay - can be very variable deposits laid down by scraping ice
  • Loess - blown silts and clays are common inland on the continents
  • Blown sand - forming dunes or plains
  • Man made materials – e.g. quarry or mining waste

estuarine sediments will eventually form alluvial soils (Wales, U.K.)

Soils form from these parent materials by processes of weathering - i.e. the breakdown of the material under the influence of gravity, wind and water, accelerated by the chemical and physical effects of plant roots and animal action.  Soils are also formed by the formation and decay of OM and by pedogenic processes - the leaching and evaporating effects of water and the biochemical actions of bacteria and other living things.  In the illustration above, soils form as the silty estuarine sediments are colonised by hardy plants which trap more sediment, and eventually raise the surface level beyond the reach of normal tides.

Read  pp. 22 - 26 of “The Nature & Properties of Soils”  by  NC Brady (10th edition 1990) – or similar material on the web.

 

  A Lithosere is a sequence of vegetational development - succession - on terrestrial soils which have newly formed on a particular parent material (PM).  As plant species colonise the PM, so the soil itself begins to change - a process called soil formation.

We only see the stages of the sere where new, unvegetated materials appear - e.g. naturally as when a river changes course leaving fresh alluvium or gravel, or artificially where quarrying exposes new rock, sand, gravel or clay faces.  New material is sometimes exposed within existing ecosystems - e.g.  The falling trees in forests expose new soil material for colonisation - so sometimes soil formation re-starts on material altered during a previous phase of vegetation.  Land-slips, earthquakes, floods, powerful winds or volcanic activities can also provide fresh parent materials.

2.  Formation conditions  determine the type of vegetation and the type of soil which will develop from the fresh PM.  The nature of the eventual soil is determined by:

a. PM

b. Topography - i.e. slope and land form

c. Climate - the chemical breakdown of the PM is influenced by heat and cold, ice formation, wind etc.

d. Vegetation and the activity of animals - roots bind soil particles, and cleave rocks, whilst animals burrow into the soil and mix the various components including organic matter from decaying organisms, mineral particles and chemicals produced by physical - chemical and biological activity in the soil.

e. Management (or mis-management) by people - ploughing, fertilising etc.

3.  Soil formation

can be divided into Inputs, movement / change / organisation, and Outputs.  These all change with Time.

a. Inputs include the original PM + organic matter from living and dead organisms.  As the vegetation develops it adds material to the soil - by fixing carbon from the air, organic matter or humus is added.  The fungi, algae and bacteria in the developing soil are able to free plant nutrients bound in the OM and the rocks into forms more easily taken up by plants, so the fertility gradually increases.  Certain plants, with the help of bacteria, are able to take nitrogen from the air and form it into nitrates and ammonia compounds in the soil - again essential nutrients for flowering plants a process known as nitrogen fixation.

b. Movement occurs in a number of ways - on slopes there is movement under gravity.  As vegetation covers the land surface the tendency for material to move down slopes is reduced.  Mature vegetation may stabilise steep slopes.  Change and "organisation" is related to movement and occurs in a number of ways:

Roots mould the soil into aggregates or peds, and may release sticky substances as they operate or decay.  The living materials and humus are rich in energy, and attract:

soil animals which make tunnels through the soil and ingest and egest soil materials, mixing them with their own secretions.  The soil begins to take on its own organisation or architecture, which may improve natural drainage, and make nutrients available to a competing flora and fauna. Animals also bring in materials on their feet and surfaces, especially seeds and spores, whilst the larger and more mobile animals often deliver materials from elsewhere in urine and faeces. The developing soil is affected by:

Climate.  The speed of OM decay depends on temperature and humidity, whilst rainfall washes (leaches) substances - especially soluble nutrients and iron-rich compounds - from the upper layers (horizons) into the lower ones.  In some tropical climates evaoration of soil water causes compounds like salts to accumulate in the surface horizons. On slopes, material can be washed into the soil from above.  Wind can also bring in mineral grains and OM from elsewhere.

c. Outputs comprise the materials lost from the soil.  Plant materials can blow away, or be eaten by animals and removed.  Crop plants are harvested and removed.  Soil material itself can be removed by erosion by wind water and ice.  Some of the substances moved by leaching may enter water courses above or below the ground, and be moved out of the system.