Fundamental Soil-Forming Processes

The fundamental soil-forming processes involved in the development of soil profile are described below:

(i) Gleization:

This process takes place in wet and cold Tundra regions where saline conditions do not exist. In this process there develops a compact structure less and sticky surface layer.

This layer is blue-green in colour, poorly aerated and has reduced content of iron compounds. It favours surface accumulation of peat materials and undergoes a series of chemical, physical and biological changes which produce a characteristic soil.

(ii) Podzolisation:

In temperate zones where climate is cold and moist, the rate of decomposition is slow and in the decomposition process acids are produced which make the soil acidic. The acidic unproductive soil is called podzol and the process is called podzolisation. In the process of podzolisation humus and some minerals including dissolved Si, Fe and Al salts from A horizon move downwardly with percolating water and accumulate in the lower horizons. This process is more effective in sandy base-poor parent materials under intense leaching and thick vegetational cover.

(iii) Laterisation:

In tropical and subtropical regions, when rainfall occurs the organic matter and minerals are leached away and hydroxides of aluminium and iron are precipitated in the form of residue which is called laterite. This process is termed as laterisation. In this process, silica is completely removed. Laterites usually do not show well differentiated horizons. Podzol and laterites are collectively described as pedalfer group’ (iron accumulating group).

(iv) Calcification:

In subhumid and dry regions, due to lack of excessive moisture the soil accumulates considerable amount of soluble materials, and carbonates of calcium and magnesium are deposited in B horizon. The soil having such features is called pedocal (calcium accumulating soil). Pedocals are not acidic and calcium content in them is very high. This process is common in grasslands.

Hydromorphic profile development:

Such soil-forming processes result in swamp, bog, marsh and peat soils and occur under conditions when percolation is restricted and certain horizons become saturated with water. These two conditions result in the development of anaerobic conditions, as well as beginning of more chemical reduction processes.

Factors Affecting Soil Formation:

These are outlined as under:

(1) Climate, (CI.)

(2) Biosphere or Organism (O)

(3) Parent material (P)

(4) Relief (r)

(5) Time (t)

All these factors are interrelated and complementary to one another. Some of them may be more influential under a particular set of conditions in determining the nature of soil while the others may be less influencing.

Any soil property(s) is a collective function or the effects of all pedogenic factors (Jenny, 1991):

S = f (cl, o, r, p.t.) where f = function dependent upon a set of conditions.

Joffe (1949) creates the following two categories in the soil forming factors:

(i) Active factors:

These include agents which supply energy for soil formation. Climate and biosphere are the two important active factors.

(ii) Passive factors:

These include the sources of soil forming mass and the conditions affecting it. Parent material, relief and time are the three passive factors.

1. Climate:

It shows the following effects on pedogenesis:

(i) Heavy rainfall and high relative humidity favour the soil formation or pedogenesis. Low rainfall and low relative humidity show retarding effects on the process.

(ii) Severe erosion of materials by wind also affects the process adversely.

(iii) Horizons tend to develop faster under cool and humid forest conditions.

2. Biological factors influencing the pedogenesis.

Biological agents affect the soil formation in a number of ways:

(i) Large number of burrowing animals upturns the deep soils and thus disturb the sod profile development.

(ii) Man’s activities, such as manuring, ploughing, drainage, irrigation, cropping system, reclamation also affect the pedogenesis to a great extent.

(iii) Addition of organic residues and mixing them with minerals promote the differentiation of soil profile.

(iv) Vegetation exerts its main influence on pedogenesis through the amount and nature of organic matter which it adds to the soil and by checking the soil erosion.

3. Nature of soil forming parental material mass.

This affects pedogenesis in the following ways:

(i) High percentage of clay in the weathered materials has retarding effect on pedogenesis.

(ii) Resistant parental materials, such as granite if present in parent material mass, do not promote the soil formation.

(iii) Excessive sandyness of parent materials reduces the rate of soil formation.

(iv) Materials with low silt and clay percentages do not differentiate into soil profile quickly.

(v) Heavy accumulation of soil materials also has retarding effects.

(vi) High lime content in the parent material checks profile differentiation.

Time:

Length of time required for soil formation depends upon many interrelated factors, such as climate, biosphere, nature of parent materials, etc.

Relief:

Some of the important effects of relief in the pedogenesis are as follows:

Topography determines the drainage condition and groundwater level in the soil. High water table shows influence in the horizon differentiation. Very steep slopes do not promote pedogenesis because of the fact that deposition of parent materials is affected by downwardly flowing water. If anyhow soil is formed, water content in such a soil is very poor.

Soil materials on moderate slopes promote deeper profile development and more luxuriant vegetation than in soil on steep slopes.

In soil survey Manual of U.S. Department of Agriculture (USDA) the interrelationship of pedogenic factors has been concluded as follows:

“Soil characteristics in any one place result from the combined influence of climate and living matter, upon the parent rock materials, as conditioned by relief (topography) over periods of time including the effects of the cultural environment and man’s use of soil.”