Pedogenesis can be defined as the process of soil development. Late in the 19th century, two scientists Hilgard in the United States and the Russian Dukuchaev suggested independently that pedogenesis was principally controlled by climate and vegetation. This idea was based on the observation that comparable soils developed in spatially separate areas when their climate and vegetation were similar.
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In the 1940s, Hans Jenny extended these ideas based on the observations of many subsequent studies examining the processes involving in the formation of soils. Jenny believed that the kinds of soils that develop in a particular area are largely determined by five interrelated factors: parent material; climate; living organisms; topography; and time.
(i) Bed Rock or Parent Material:
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The underlying rock containing minerals is called bed rock or parent material. The soil forming material is obtained by the weathering of this rock. Although all the three types of rocks (igneous, sedimentary and metamorphic) provide soil-forming materials, it is largely derived from the sedimentary rocks because these rocks are widely distributed on the earth’s surface. The colour, structure and mineral contents of the soil depend upon the parent material.
(ii) Climate:
The climate of an area is a major factor of soil formation. Parent material is more important for short period but climate becomes more important during longer duration. Two different parent materials may develop the same soil in one type of climate.
For example, sandstone and granite are two different rocks in Rajasthan but they have given birth to same type of sandy soil under arid climatic conditions. Similarly, same parent material may form different types of soil under different climates. For example, the crystalline granite produce laterite soil in moist climate of Rajmahal hill and clayey soil in the dry climate of Andhra Pradesh.
The availability of moisture also has an influence on soil pH and the decomposition of organic matter. At regional and local scales, climate becomes less important in soil formation, instead, pedogenesis is more influenced by other factors.
(iii) Living Organisms:
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Living organisms have a role in a number of processes involved in pedogenesis including organic matter accumulation, profile mixing and bio-geochemical nutrient. It includes vegetation, bacteria and animals.
(a) Vegetation:
It includes trees, shrubs, grasses and mosses. The decay of leaves and roots of different forms of vegetation provides humus to soil which increases its fertility. This provides life to new plants. It increases chemical action and the rate of soil formation. Humus also quickens the weathering of minerals in the process of soil formation.
(b) Bacteria:
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Bacteria acts differently in different climates and forms different types of soils. Bacteria live on humus. In humid tropical climate, bacterial activity is intense. Consumption of humus is so much that soils are left poor in it. In cold climate, bacterial activity is limited and the soils are rich in humus. Bacteria also changes nitrogen from air into a chemical form which is usable by plants.
Through litterfall and the process of decomposition, organisms add humus and nutrients, to soil which influences soil structure and fertility. That is why bacteria is termed as nitrogen-fixing agent.
(c) Animals:
Burrowing animals like ants, rodents, termites and some birds move the surface soil downwards and the sub-soil from it lower horizon upwards. The mixing of two soil layers helps in improving the texture and its aeration. Small organisms like earthworms live in millions in the soil and help in reducing its grains to small size and in mixing the mineral and the organic matter.
(iv) Topography:
Elements of topography (such as altitude, relief and slope) affect the deposition and erosion of soil and the run-off of the water. On steep slopes, there is lot of erosion by river, glacier and wind and only a thin layer of coarse soil is left. This is known as residual soil. This eroded material is deposited in areas of gentle slope and a thick layer of soil is formed.
Such soils are very fertile because they are formed from different kinds of rocks from over a wide catchment. But in certain flat areas, the soils are poorly drained and suffer from leaching (downward movement of salts), Thus, the best soils are found in areas of gentle slope with good drainage.
(v) Time:
Time is an important factor in the formation and development of the soil. The process of formation of soil is very slow. One to two centimetres thick stable soil cover may take thousands of years to form under the most favourable conditions.
The characteristics of soil undergo drastic changes with the passage of time. There are three stages of the development of soil —1. Young stage, 2. Mature stage, and 3. Old stage.
The fresh river alluvium or glacial till are soils of the young stage. They have a greater control on the parent material and have a poorly developed soil profile. Mature soil are developed over a long time.
The influence of climate and of organic matter is clearly visible on the mature soils. They have well-developed soil profile. With further lapse of time, humus is properly mixed with soil and its colour changes. The importance of parent material is almost over and the soil attains its old stage. Soils of old stage are found in the tropical lands and are one to six million years old.