most widely accepted definition of a “soil” is;
“the soil is a natural body of animal, mineral and organic constituents differentiated in to horizons of variable depth which differ from the material below in morphology, physical make up, chemical properties and composition”
The study of soils, origins and characteristics is known as pedology.
The formation of a soil begins with the exposed parent rock.
This rock weathers to give a layer of rocky, broken material known as regolith. This may also be derived from deposition of alluvium, drift, loess, and volcanic sources.
The formation of the true soil, or topsoil, results from the addition of water, air, living organisms and decayed organic matter.
Different soils are found all over the world. Therefore, the variation in soil type must be due to locally varying factors. The variables in soil formation are as follows;
(1) Parent Materials;
· Supply of minerals to soil depends upon parent rock.
· Minerals are susceptible to different rates and processes of weathering.
· Parent rock directly contributes to control of depth, texture, drainage, and nutrient content of the soil. It may also influence colour.
· Affects the rate of weathering of the parent rock, the most rapid weathering occurring in hot, humid climates.
· Precipitation affects vegetation, which affects humus thickness and type.
· Heavy rainfall increases leaching - the movement of water transporting mineral salts through the soil.
· If evapotranspiration exceeds precipitation (i.e. if output exceeds input), then capillary action draws minerals and salts upwards.
· Leaching tends to produce acidic soils, and capillary action tends to produce alkaline soils.
· Temperature affects the length of the growing season, and the humus.
· Humidity affects evapotranspiration rates.
(3) Topography (Relief)
· Height above mean sea level (amsl) affects precipitation, cloud cover and wind positively (i.e. these factors increase with and increase in height).
· Temperature and length of growing season are negatively affected.
· Aspect (direction the slope faces) is a local variable. Facing towards the Equator generates warmer and drier slopes.
· The slope angle affects soil slope and drainage.
· Steeper slopes have faster runoff and throughflow, and are more likely to be subject to mass movement.
(4) Organisms (Biota)
· Interaction with the nutrient cycle affects the level of nutrients within the soil.
· Macro-organisms (e.g. worms) mix and aerate the soil, while micro-organisms (e.g. saprophytes, decomposition bacteria) assist in decay and decomposition.
· Humans influence development through use of fertilisers, ploughing, draining, or irrigating the land.
· Soils can take many hundreds of years to form.
· It can take between 3 and 12 thousand years to produce a soil of sufficient depth for farming.
· Young soils retain the characteristics of their parent rock, whilst older soils acquire new characteristics from the addition of organic matter and organism activity.
· Horizons develop when the soil reaches equilibrium with their environment.
· A soil profile is a vertical section through a soil, showing its different horizons.
· It is a product of input and output equilibrium, and redistribution and chemical changes of soil constituents.
· Three major horizons are recognised. They can be further subdivided in more detailed soil analysis.
· They are given letters which indicate their generic origin;
Ž A Horizon; is the upper layer. This is where biological content and humus content are at their maximum. It is also the zone most affected by the leaching of soluble materials and eluviation (downward movement) of clay particles.
Ž B Horizon; is the zone of accumulation or illuviation. This is where the material removed from the A horizon is deposited. Horizons A and B make up the “true soil”.
Ž C Horizon; consists mainly of recently weather material (regolith) which sits on the bedrock. It has been weathered in place.
PROPERTIES OF SOILS
The four major components of soil - water, air, minerals and organic matter, are all closely inter linked. The resulting inter-relationships produce a series of properties, which are listed here;
(1) Mineral (Inorganic) Matter
· Obtained by the weathering of the parent rock.
· Primary materials, e.g. sands, quartz, retain chemical characteristics of parent.
· Secondary materials are those which have been broken down by chemical weathering (oxidation, carbonation, hydrolysis and hydration)
· Forms clays and sesquioxides (oxides of iron and aluminium)
(2) Soil Texture
· Reference to the degree of coarseness of the mineral matter in the soil determined by the proportion of sand, silt, and clay particles.
· Texture controls the size and the spacing of the soil pores.
· This helps to control the water content, flow and extent of aeration.
· It also controls the availability and retention of nutrients within the soil.
(3) Soil Structure
· Aggregation of particles gives the soil its structure.
· Aggregates form different shapes known as peds.
· Shape and alignment of peds, combined with the particle size and texture, determines the number of pore spaces.
· There are six different types of ped.
(4) Organic Matter
· Derived from the decomposition of organic material.
· Splits in to a three layer profile.
· Humus gives the soil a black or dark brown colour. High amounts of humus are found in the chernozems, of the US Prairies, and the Russian Steppes.
· The organic matter is a major source of nutrients; combines with clays to form a clay-humus complex which is essential for a fertile soil because it provides a high water and nutrient retention capacity.
(5) Soil Moisture
· This affects the movement of water and nutrients.
· It helps in the development of horizons, supplies water for living organisms, provides a solvent for nutrients, controls soil temperature and determines the incidence of erosion.
· Drainage depends on the balance between water retention capacity and infiltration rate.
· Micro pores - retain water, restrict air. Found in clays.
· Macro pores - low water retention, high infiltration rate. Found in sands.
· Soil water is classified by the tension at which it is held. When infiltration of a saturated soil ceases, water with a low cohesive strength drains rapidly under gravity. It is known as gravitational water.
· Once it has drained, the remaining water which the soil can hold is called the field capacity. Moisture at field capacity is held either as hygroscopic water, or as capillary water. Hygroscopic water is always present as a thin film around the soil particles to which it sticks because of a high cohesive tension.
· Capillary water is attached to, and films around, the hygroscopic water, but has a lower cohesive strength. It is freely available to plants, but can be lost by evapotranspiration.
· Fills the pore spaces.
· Essential for the growth of living organisms.
· Soil air contains more carbon dioxide and less oxygen than normal air.
· Gases are exchanged by diffusion with biota.
(7) Soil Organisms (Biota)
· Include bacteria, fungi and earthworms.
· More active and plentiful in well drained, warmer and aerated soils (mull) than in colder, more acidic and less well drained and aerated soils (mor).
· They are responsible for decomposition (by detritivores), fixation (bacteria convert nitrogen in the air to nitrates in the soil) and the development of soil structure.
(8) Soil Nutrients;
· A term given to chemical compounds in the soil which are essential for plant growth and the maintenance of the fertility of a soil.
· There are four Primary elements, which are required in large quantities, and are essential to plant growth. These are Carbon, Hydrogen, Nitrogen and Oxygen.
· There are five Secondary elements, which are required in lesser quantities, and aid plant growth. These are;
· Calcium - helps the growth of roots and shoots,
· Magnesium - component of chlorophyll,
· Potassium - help in the formation of starches and oils.
· Sulphur - required for leaf growth
· Phosphorous - required for leaf growth.
· There are also several trace elements which are required in minute amounts, e.g. Molybdenum, to activate enzymes.
· Nutrients may be obtained from;
(i) Rainwater (in solution). This is readily available to plants, but may be leached from the soil in groundwater, especially the secondary elements.
(ii) Artificial fertiliser.
(iii) Minerals released from the parent rock. These are soluble and dissolve in to the soil solution, forming positively charged Cations.
(iv) Nutrients attached to the clay-humus complex provide the major reserves. Particles develop negatively charges anions on their surface which attract and absorb the mineral cations from the soil. The resultant double layer is known as the Gouy layer.
· Cation exchange takes place between soil and soil solution, but can occur between soil particles and plant roots. For example, cations of Magnesium, Calcium or Sodium are released from the clay-humus particles and replaced with an equal number of hydrogen cations found in the soil or attached to plant roots.
· This forms a dual function. It provides nutrients and increases the acidity of the soil. This accelerates the rate of weathering of the parent rock and releases more minerals to replace those used by plants or lost by leaching in gravitational water.
· Different soils have different cation exchange capabilities (CEC), i.e. different abilities to retain and recycle cations for plant use.
(9) Acidity (pH)
· Is the degree of concentration of positively charged hydrogen cations.
· Measured on the pH scale, which is logarithmic.
· Acid soils need liming for successful farming. In the UK, slightly acidic soils are the best, as they help to release secondary elements.
· If soils are too acidic, then they release iron and aluminium, which become toxic. High acid content also increases leaching, because it makes organic matter more soluble.
· The relationship between evapotranspiration and precipitation affects the acidity of the soil. If there is a balance, then the soil is neutral, e.g. the American Prairies.
· Incoming solar radiation can be absorbed.
· Topsoil absorbs heat more rapidly, but then loses heat more rapidly than the subsoil.
· Warm soil will have greater biota activity, therefore a more rapid breakdown of organic matter. It is therefore more likely to contain nutrients, because weathering will be faster.
PROCESSES OF SOIL FORMATION;
· “the process by which soluble constituents in the solution are removed from the soil”
· Where precipitation exceeds evapotranspiration, readily soluble salts are dissolved by downward percolating rainwater, which contains oxygen, carbonic and organic acids.
· Over a long period of time, soluble materials, e.g. calcium carbonates, are removed from the soils in humid regions.
· Leaching also attacks bases, e.g. Calcium, potassium, magnesium, held as exchangeable ions in the clay-humus complex, and replaces them with hydrogen ions.
· Thus, a major effect of leaching is to leave the soil more acidic, leading to the development of the cambic or ‘weathered’ B horizon.
· The leaching process is checked when soils are limed for farming, or slowed by the introduction of plants, as they bring the minerals of the subsoils to the surface.
· “specific loss of material in suspension from a soil horizon”
· Finely dispersed humus and clay can move colloidal suspensions from eluvial to illuvial horizons.
· Process is encouraged by a climate in which vertical cracks develop in the soil profile by drying. Eluviation then occurs on the rewetting of the soils through these cracks.
· The result of eluviation is the development of a B horizon enriched in clay, which is referred to as an argillic horizon.
· Clay content of the illuvial horizon can be increased considerably with the amount remaining in the E horizon of the same soil, recognisable by the paler colour.
· Eluviation is frequently referred to under its French name of lessivage and the soils are sols lessives.
· This is a process characteristic of low rainfall areas in continental interiors.
· Leaching is slight, and despite downward movement, soluble materials are not removed.
· Soils are only wetted to a depth of between 1 - 1.5m when the moisture begins to re-evaporate.
· A calcic horizon of calcium carbonate accumulates in the B or upper C horizon.
· Soils are relatively unleached, and the exchange capacity is dominated with calcium ions (and, to a lesser extent, magnesium ions).
· The presence of the ions stabilises colloids, and limits movement in the soil.
· Humus is mull, produced by the natural grass vegetation.
· Intensive root systems provide large amounts of organic matter to the soil when dead.
· Characteristic of soils in the tropical humid regions of the world.
· Process involves the relative accumulation of sesquioxides of iron and aluminium with the loss of silicon.
· Prolonged exposure to the humid environment produces a highly weathered, low base status oxic horizon.
· Ferralitization is accompanied by strong leaching of the soil, so pH values are low.
· Rapid decay and recycling of the elements in leaf fall keep bases and nutrients in circulation.
· Clay formation is restricted to kaolinite group, which is frequently associated with iron as cement.
· The resulting soil is usually freely drained, red in colour, and does not disperse easily in colour.
· Salinisation is the enrichment of the soil with salt.
· It is usually acheived by the evaporation of surface moisture.
· Salts in solution are drawn upwards by capillary action, then deposited as the water is evaporated.
· Soils develop a surface encrustation of salt and are known as white alkali or solonchak soils.
· Salt can be derived from a salt rich geological substratum, or from salt spray blown inland which gradually accumulates in the unleached soils of the arid/semi arid areas.
· Explained as a more intense form of leaching, podsolisation occurs in cool climates, where precipitation is greatly in excess of evapotranspiration, and where soils are well drained or sandy.
· It is defined as the removal of sesquioxides under extreme leaching.
· As the surface is often coniferous forest, heathland or moors, rain percolating through it becomes progressively more acidic, and may reach a pH of below 4.5.
· This dissolves a number of bases (Ca, Mg, Na, and K), and ultimately the sesquioxides of iron and aluminium.
· The resultant podsol soil has two distinct horizons; a bleached A horizon, drained of coloured materials by leaching, and the reddish-brown B horizon where sesquioxides have been illuviated.
() Often the iron deposits form a hard pan which is characteristic of a podsol.