Soil and Soil Forming Processes
A.Soil as a Dynamic Body:
Soils constitute a major element in the natural environment, linking climate and vegetation, and they have a profound effect on man's activities through their relative fertility.
Soil is a dynamic layer in which many complex chemical, physical and biological activities are going on constantly. Soils become adjusted to conditions of climate, landform and vegetation, and will change internally when those controlling conditions change.
Soil contains matter in all three states: solid, liquid and gaseous.
1.Solid:
The solid portion is partly organic and partly inorganic. The inorganic, or mineral, part of the soil is made up of particles derived from the parent material, the rocks which weather to form the soil. The organic portion consists of living and decayed plant and animal materials such as roots and worms. The end-product of decay is humus, black amorphous organic matter.
2.Soil Water
Soil water is a dilute but complex chemical solution derived from direct precipitation and from runoff, seepage, and groundwater.
3.Soil Air
The soil air fills the pore spaces of the soil when these are not occupied by water.
After heavy rain, the ground will initially be waterlogged, with the pore spaces entirely filled with water. But the water moves out rapidly by gravitational movement until the coarser pores are empty and water is no longer supplied to gullies and field drains. The soil is then said to be at field capacity. Further removal of water from the soil may occur by evapotranspiration, until the pore spaces are largely air-filled and the soil becomes parched.
B.Physical and Chemical Properties of Soils
1.Soil Colour:
Colour varies considerably in soils and can tell us much about how a soil is formed and what it is made of.
Soil can range from white to black, usually depending on the amount of humus. In cool humid areas, most soils contain a relatively high humus content and are generally black or dark brown, whereas in deserts or semi-deserts little humus is present and soils are light brown or grey.
Reddish colours in soils are associated with the presence of ferric compounds, particularly the oxides and hydroxides, and usually indicate that the soil is well drained, although locally the colour may be derived from a red-coloured parent material.
In humid climates, grevish or bluish colours reflect the presence of reduced iron compounds, eg. Fe(OH)2 indicating poor drainage conditions.
2.Soil Texture:
The texture of a soils refers to the sizes of the solid particles composing the soil. The sizes range from gravel to clay (The following table). The proportions of the different sizes present vary from soil to soil and from layer to layer.
Name of Grade / Diameter (mm)Coarse gravel
Fine gravel / Above 2
1.0 - 2
Coarse sand
Medium sand
Fine sand
Very fine sand / 0.5 - 1
0.25 - 0.5
0.1 - 0.25 0.25
0.05 - 0.1
Silt / 0.002 - 0.05
Clay / Below 0.002
Soil Texture Grades
Standard soil textural classes can be defined according to the ratio of sand, silt and clay, and can be represented on a triangular diagram. Any point within the diagram defines the percentage proportions of the three grades.
Texture largely determines the water-retention properties of the soil. In a sandy soil, pore spaces are large and water drains rapidly: in a clay soil, the individual pore spaces are too small for adequate drainage. Generally speaking, loam textures are best for plant growth.
3.Soil Structure
Because of the cementing action of ions in the soil, individual particles in a soil tend to aggregate together in lumps or peds. According to the shape of the peds, soil can be described as having a blocky, platy, crumb or granular, or prismatic or columnar structure.
The soil structure has an important bearing on its ease of cultivation. Soil with a crumb structure are best for seed germination and are said to have a good tilth.
4.Soil Colloids, Bases and Acidity:
Included in the clay fraction of the soil are soil colloids - tiny particles with unusual chemical properties. The colloids may be organic, made up of a very finely divided humus, or mineral, in which case they are referred to as clay minerals. Together, the two types make up a clay-humus complex.
Most soils have more clay minerals than organic colloids. The clay minerals are minute thin flakes but they are of great importance because they are in a state of continuous chemical change, which is fundamental to soil formation.
Clay minerals have a complicated atomic structure and a vast surface area in relation to their weight. Overall, they are negatively charged. This is invariably neutralized by the attraction to their surfaces of positively-charged ions (cations) of calcium, magnesium, potassium and sodium. These are known in soil science as bases.
They are only held loosely in an exchangeable position by the clay minerals, and may be given up in the process of base exchange to plants which require them for growth. Some bases are more readily given up than others. In particular, the metallic cations, such as potassium and sodium tend to be replaced by hydrogen ions.
Under natural conditions, the bases are recycled to the soil by the decomposition of plants and animals. Where the vegetation is removed by man by cutting or cropping, the bases can only be fully replenished if supplied artificially in the form of fertilizer.
Soft calcareous rocks are often naturally fertile because the rate weathering of the calcium in the parent material is sufficient to replace the loss of leaching of exchangeable calcium.
The bases in the soil are essential as nutrients for plant growth. Some nutrients - such as carbon, hydrogen, oxygen and calcium are required in relatively large quantities: others such as - iron, copper, sodium and magnesium - are only needed in traces but are nonetheless equally important. Plants also obtain some of their essential elements from the atmosphere. In turn, animals derive their elements through the plants.
Soil acidity is a property related to the proportion of exchangeable hydrogen in the soil in relation to other elements. The degree of acidity is measured on the logarithmic pH scale which ranges from 0 (extreme acidity) to 14 (extreme alkalinity).
5.Soil Water:
Soil water, the water temporarily held in the soil, is in reality a complex chemical solution. It is a dilute solution of such substances as bicarbonates, sulfates, chlorides, nitrates, phosphates, and silicates of calcium, magnesium, potassium, sodium and iron.
- Soil Profile:
The term soil profile denotes the arrangement of the soil into layer-like horizons of differing texture, colour and consistency.
Soils are recognized and classified into broad groups on the basis of the parts of the profile that are present.
Basically, there are three parts to the soil profile. Horizons A and B represent the true soil, or solum; horizon C is the subsoil, or weathered parent body. Below this is the parent bedrock or other underlying rock, designated as horizon D.
The A horizon in humid climates is composed of two very different parts. The upper, or A1, horizon is rich in organic matter and is dark coloured. The lower, or A2, horizon is a zone of leaching. The B horizon is usually a zone of accumulation of soil colloids and is dark in contrast to the A2 horizon above it.
D.Factors Affecting Soil Formation
Many types of processes and influences, known altogether as soil formers, act together to develop a soil. Some of these are passive conditions: others are active agents.
1.Passive Soil Formers:
a.Parent Material:
It is the residual or transported overburden of disintegrated rock making up the bulk of the soil. Certain of the original rock forming minerals have been thoroughly changed chemically into new compounds and reduced to colloidal size. Yet, the type of parent material does not alone determine the kind of soil that is present.
b.Topography/ Landform
Where slope is steep, surface erosion by runoff is more rapid and water penetration is less than on gentle slopes. As a result, the soil will be thinner on steeper slopes. Flat bottom lands likewise have thick soils, but they are poorly drained and dark coloured. Gentle slopes where drainage is good but erosion is slow are considered the norm for soil formation.
Another influence of landform is the slope aspect or direction of exposure of the surface to the slanting rays of the sun. In middle latitudes it is common to find that south-facing slopes (in the northern hemisphere) exposed to the warming and drying effects of sunlight, have different conditions of vegetation and soils from north-facing slopes, which retain cold and moisture longer.
c.Time:
A soil is said to become mature when it has been acted upon by all soil forming processes for a sufficient long time to have developed a profile that changes only imperceptibly with further passage of time. Soil that are evolving from recently deposited river alluvium or glacial till, for example, are considered young.
In young soils the characteristic horizons are absent or poorly developed. A mature soil might be to say that it is in equilibrium with the many processes and forces acting upon it.
2.Active Soil Formers
a.Climate:
Of the active soil formers, climate is perhaps the most important. Climatic elements involved in soil development are: moisture, temperature and wind.
i.Moisture:
Precipitation provides the soil water, without which chemical ad biological activities are not possible. When soluble chemicals are dissolved in water they ionize, or dissociate into positively and negatively charged particles. Without ionization the many complex chemical interchanges of elements necessary to soil development and plant growth cannot take place.
An excess of precipitation, however, tends to leach away the colloids and ions. This process of downward migration of soil components by water percolating through the soil is known as eluviation.
A distinctive leached horizon of the soil, the A2 horizon, results from this process. The deposition of colloids and bases in the underlying B horizon is a process known as illuviation.
Rainfall and evaporation controls result in the formation of two major groups of soils: pedalfer soils show pronounced leaching and occur in areas where the rainfall is more than 600 mm annually; pedocal soils have an excess of calcium carbonate and occur in areas where annual rainfall is less than 600 mm.
ii.Temperature:
It acts in two ways:
-Chemical activity is generally increased by higher temperatures but reduced by cold, and it ceases when soil water is frozen. Thus, tropical soils have a parent material which is thoroughly altered chemically.
-Bacterial activity is increased by warmer soil temperatures. Where bacteria survive, as in the humid tropics, they consume all dead plants that lie upon the ground.
-Thus there is no layer of decomposing vegetation on the ground and little humus within the soils of the humid tropics.
iii.Wind
Wind is of minor importance as a climatic factor in soil development. Winds may increase the evaporation from soil surfaces and may remove surface soil in arid regions lacking a plant cover. Windblown dust may accumulate and thereby provide the parent material of a soil.
b.Biotic Factor
The organisms affecting soil development range from microscopic bacteria to large mammals, including man.
Besides providing much of the humus, vegetation influences the soil in several other ways. By intercepting direct rainfall and binding the soil with roots, plants check soil erosion. They counteract percolation by transpiration, reducing the effectiveness of the rainfall.
Most important of all, plants help to maintain the fertility of soil by bringing bases such as calcium, magnesium and potassium from the lower layers of the soil into stems and leaves, and then releasing them into the upper soil horizons.
Different types of vegetation require different proportions of basic nutrients; trees, especially conifers, use little calcium and magnesium, whereas grass recycle abundant quantities of these.
The influence of animals on soils is both mechanical and chemical. For example, earthworms rework the soil by burrowing, and also change its texture and chemical composition by passing it through their digestive systems.
E.Soil Forming Processes
1.Podzolization:
The regime of podzolization dominates in climates having sufficient cold to inhibit bacterial action, but sufficient moisture to permit larger green plants to survive. Such conditions exist only in middle and high latitudes, and at high altitudes.
In its extreme development podzolization is associated with coniferous trees. These plants do not require the bases and hence do not restore them to the soil surface. The result is that humic acids, produced form the abundant leaf mold and humus, leach the upper soil strongly of bases, colloids, and the oxides of iron and aluminum, leaving a characteristic ashgrey A2 horizon composed largely of silica (SiO2). Colloids, humus, and oxides of iron carried out of the A2 horizon accumulate in the B horizon, which may be dark in colour, dense in structure, and in some cases hardened to rocklike consistency.
PodzolizationIron podzol soil
2.Laterization:
The pedogenic regime of laterization is in some respects a warm climate relative to podzolization, in that both are associated with climatic regimes of ample precipitation and with forest. Laterization takes place in a warm climate having copious rainfall well distributed throughout the year.
A high mean annual temperature and a lack of severe winter season permit sustained bacterial action which destroys dead vegetation as rapidly as it is produced. Consequently little or no humus is found upon or in the soil.
In the absence of humic acids the sesquioxides of iron (Fe2O3) are insoluble and accumulate in the soil as red clays, nodules, and rocklike layers (laterite). Silica, on the other hand, is leached out of the soil and disposed of eventually by stream flow in the process of desilication.
No distinctive soil horizons are developed. In the absence of silicate colloids the soil tends to be firm and porous rather than sticky and plastic, and will transmit water readily. Laterization results in very low soil fertility because bases are not held in the soil and humus is lacking.
LaterizationLateritic soil
3.Calcification:
It is a pedogenic regime of climates in which evaporation on the average exceeds precipitation. Calcification is associated with a continental climatic regime with low total annual precipitation and with a tropical wet-dry climatic regime with a short wet season. Rainfall is not enough to leach out the bases, so that calcium and magnesium ions remain in the soil.
Grasses, which use these bases, restore them to the soil surface. Colloids remain essentially in place and are not leached out, but are in a dense (flocculated) state and hold the soil into aggregate structure.
Calcium carbonate, brought upward by capillary water films and evaporated in dry periods, is precipitated in the B horizon of the soil in the form of nodules, slabs, and even dense stony layers (caliche). Microbial activity is restricted and humus may be abundantly distributed throughout the A and B horizons.
Humus occurs in progressively smaller amounts as one traces the soil into climate zones of increasing aridity. Calcification is characteristically associated with grasslands - the steppes and semi-deserts.
CalcificationChernozem (Cool Climate)
Aridisoils, soils of the desert climate, are dry for long periods of time. Because the climate supports only a very sparse vegetation, humus is lacking and the soil colour ranges from pale gray to pale red. Soil horizons are weakly developed, but there may be important subsurface horizons of accumulation of calcium carbonate or soluble salts (salic horizon) The salts, of which sodium is a dominant constituent, give the soil a very high degree of alkalinity.
Saline soil
4.Gleization:
The pedogenic regime of gleization is characteristic of poorly drained (but not saline) environments under a moist and cool or cold climate. Gleization is thus associated with the climatic regime of polarization (tundra climate) but is also effective in bog environments of continental climates with cold winters.
Low temperatures permit heavy accumulations of organic matter to form a surface layer of peaty material. Beneath this is the glei horizon, a thick layer of compact, sticky, structureless clay of bluish gray colour. The glei horizon lies generally within the zone of ground water saturation; consequently the iron is in a partially reduced condition and imparts the bluish-gray colour.