The passive soil forming factors are those which represent the source of soil forming mass and conditions affecting it. These provide a base on which the active soil forming factors work or act for the development of soil.
Parent Material: It is that mass (consolidated material) from which the soil has formed.
Two groups of parent material
i) Sedentary: Formed in original place. It is the residual parent material. The parent material differ as widely as the rocks
ii) Transported: The parent material transported from their place of origin. They are named according to the main force responsible for the transport and redeposition.
a) By gravity – Colluvial
b) By water – Alluvial, Marine, Locustrine
c) By ice – Glacial
d) By wind – Eolian
Colluvium: It is the poorly sorted materials near the base of strong slopes transported by the action of gravity.
Alluvium: The material transported and deposited by water is, found along major stream courses at the bottom of slopes of mountains and along small streams flowing out of drainage basins.
Locustrine: Consists of materials that have settled out of the quiet water of lakes.
Moraine: Consists of all the materials picked up, mixed, disintegrated, transported and deposited through the action of glacial ice or of water resulting primarily from melting of glaciers.
Loess or Aeolian: These are the wind blown materials.
When the texture is silty – loess; when it is sand – Eolian.
The soils developed on such transported parent materials bear the name of the parent material; viz. Alluvial soils from alluvium, Colluvial soils from Colluvium etc. In the initial stages, however, the soil properties are mainly determined by the kind of parent material.
Endodynamomorphic soils: With advanced development and excessive leaching, the influence of parent material on soil characteristics gradually diminishes. There are soils wherein the composition of parent material subdues the effects of climate and vegetation. These soils are temporary and persist only until the chemical decomposition becomes active under the influence of climate and vegetation.
Ectodynamomorphic soils: Development of normal profile under the influence of climate and vegetation.
Soil properties as influenced by parent material: Different parent materials affect profile development and produce different soils, especially in the initial stages.
Acid igneous rocks (like granite, rhyolite) produce light-textured soils (Alfisols).
Basic igneous rocks (basalt), alluvium or Colluvium derived from limestone or basalt, produce fine-textured cracking-clay soils (Vertisols).
Basic alluvium or Aeolian materials produce fine to coarse-textured soils (Entisols or Inceptisols).
The nature of the elements released during the decaying of rocks has a specific role in soil formation. (e.g.) Si and Al form the skeleton for the production of secondary clay minerals.
Iron and manganese are important for imparting red colour to soils and for oxidation and reduction phenomena.
Sodium and potassium are important dispersing agents for day and humus colloids.
Calcium and magnesium have a flocculating effect and result in favorable and stable soil structure for plant growth.
2. Relief or Topography: The relief and topography sometimes are used as synonymous terms. They denote the configuration of the land surface. The topography refers to the differences in elevation of the land surface on a broad scale.
The prominent types of topography designations, as given in FAO Guidelines (1990) are:
| Sr. No | Land surface | with slopes of |
1 | Flat to Almost flat | 0 – 2 % |
2 | Gently undulating | 2 – 5 % |
3 | Undulating | 5 – 10 % |
4 | Rolling | 10 – 15 % |
5 | Hilly | 15 –3 0 % |
6 | Steeply dissect | >30% with moderate range of |
7 | Mountainous | > 30% with great range of |
Soil formation on flat to almost flat position: On level topographic positions, almost the entire water received through rain percolates through the soil. Under such conditions, the soils formed may be considered as representative of the regional climate. They have normal solum with distinct horizons. But vast and monotonous level land with little gradient often has impaired drainage conditions.
Soil formation on undulating topography: The soils on steep slopes are generally shallow, stony and have weakly- developed profiles with less distinct horizonation. It is due to accelerated erosion, which removes surface material before it has the time to develop. Reduced percolation of water through soil is because of surface runoff, and lack of water for the growth of plants, which are responsible for checking of erosion and promote soil formation.
Soil formation in depression: The depression areas in semi-arid and sub humid regions reflect more moist conditions than actually observed on level topographic positions due to the additional water received as runoff. Such conditions (as in the Tarai region of the Uttar Pradesh) favour more vegetative growth and slower rate of decay of organic remains. This results in the formation of comparatively dark- coloured soils rich in organic matter (Mollisols).
Soil formation and Exposure/ Aspect: Topography affects soil formation by affecting temperature and vegetative growth through slope exposures (aspect}. The southern exposures (facing the sun) are warmer and subject to marked fluctuations in temperature and moisture. The northern exposures, on the other hand are cooler and more humid. The eastern and western exposures occupy intermediate position in this respect.
3. Time: Soil formation is a very slow process requiring thousands of years to develop a mature pedon. The period taken by a given soil from the stage of weathered rock (i.e. regolith) up to the stage of maturity is considered as time. The matured soils mean the soils with fully developed horizons (A, B, C). It takes hundreds of years to develop an inch of soil. The time that nature devotes to the formation of soils is termed as Pedological Time.
It has been observed that rocks and minerals disintegrate and/or decompose at different rates; the coarse particles of limestone are more resistant to disintegration than those of sandstone. However, in general, limestone decomposes more readily than sandstone (by chemical weathering).
Weathering stages in soil formation | ||
Sr. No | Stages | Characteristic |
1 | Initial | Un weathered parent material |
2 | Juvenile | Weathering started but much of the original |
3 | Virile | Easily weather able minerals fairly decomposed; clay content increased, slowly weather able minerals still appreciable |
4 | Senile | Decomposition reaches at a final stage; only |
5 | Final | Soil development completed under prevailing |
The soil properties also change with time, for instance nitrogen and organic matter contents increase with time provided the soil temperature is not high.
CaCO3 content may decrease or even lost with time provided the climatic conditions are not arid
In humid regions, the H+ concentration increases with time because of chemical weathering.
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