Soil is a soft layer that covers the earth’s surface. It is created by the mixing of minerals and organic compounds resulting from natural and chemical processes such as erosion and weathering of rocks. The type of soil varies in different parts of the world, depending on various factors. These factors are called soil-forming factors. Overall, the formation of soil is a complex process that involves geological, biological, and climatic factors, and its study is crucial for understanding the ecology and sustainability of our planet.
Weathering of rocks occurs through physical, chemical, and biological processes in various natural environments. This results in the formation of a thin friable covering layer parallel to the ground surface rich in various minerals and organic matter, which supports plant growth, called soil.
Soil formation is influenced by four major factors identified by Russian geologist Vasily Dokuchaev – climate, biotic factors, parent rock, and time. Another geologist, Jenny, identified five factors namely climate, organic factors, relief, parent rocks, and time.
Both sets of factors are crucial to understanding soil formation. He formulates an equation for soil formation that is S= f (Cl, O, R, P, T), where S= Soil, = factors (climate, organic factors, relief, parent rocks, and time) (1).
Soil forming factors
Soils are formed as a result of the action of several inactive and active factors. Inactive factors are parent rock, topography, and time. Active factors are temperature, rainfall, and biota. The effect of all these factors on soil formation is discussed below.
Climate and biota are active factors in soil formation. They provide energy for soil formation and actively modify the regolith.
The climate is the most important factor in soil formation. Rainfall and temperature these two elements of climate directly participate in soil formation.
Role of rainfall in soil formation
- The properties of soil are controlled by the amount and intensity of rainfall. High rainfall increases the number of hydrogen ions in the soil. As a result, the pH level decreases and the soil becomes acidic.
- The abundance of vegetation is observed in areas with high rainfall. As a result, the amount of nitrogen in the soil increases due to the accumulation of more organic matter.
- In areas of high rainfall, calcium carbonate is converted into calcium bicarbonate by carbonation and shifted deep into the soil.
- High rainfall in humid tropical regions accelerates chemical weathering and increases the amount of silt in the soil.
- In humid-tropical climate regions, potassium, sodium, calcium, magnesium, etc. are removed from the upper layer of the soil to the lower layer by the process of eluviation.
- Pedalfer soils are formed depending on the ratio of precipitation and evaporation where free calcium is removed in the leaching process leaving only aluminum and iron oxides. Examples of these soils are tundra soil, podzol soil, and laterite soil.
- When precipitation is less than evaporation, calcium carbonate and salts rise up from the bottom of the soil by capillary action and accumulate in the upper layers of the soil. Thus pedocal soil is created. Chestnut, chernozem soils belong to this category (2).
Role of temperature in soil formation
- According to scientist Vandoff, for every 10°C rise in temperature, the rate of chemical reactions doubles. Therefore, the rate of soil formation is faster in warm tropical and equatorial regions than in cold desert regions.
- As the temperature increases, the effectiveness of the weathering increases. This results in the formation of thick regolith depending on the soil.
- Soils in warm climatic regions have less organic matter and less nitrogen.
- When the temperature is high, the amount of clay minerals like montmorillonite, kaolinite, etc. in the soil is high.
- Soils in deserts and semi-deserts are dry and saline due to high temperatures (3) & (1).
Soils are formed by the direct influence of flora and fauna such as
- The dead remains of animals and plants are decomposed and incorporated into the soil as humus, thereby increasing the nutrient value of the soil and making the soil fertile. Increasing the amount of humus in the soil increases the water-holding capacity of the soil.
- Burrowing animals such as earthworms, rabbits, ants, mice, etc. mix between the upper and lower layers of the soil. This facilitates aeration and loosens the soil.
- The presence of earthworms fertilizes the soil as earthworm excrement is rich in nitrogen and calcium.
- Plant roots enter the rock and help move to water and air.
- Decomposing leaves, flowers, fruits, etc. of plants increases the amount of organic matter in the soil.
- Plant remains are broken down by microbes to form minerals and these determine soil characteristics (2) & (4).
Inactive or passive factors
All those factors which are not so active like climate and biota, but indirectly participate in soil formation, are called passive factors. For example- topography, parent rock, and time. Their role in soil formation is discussed below (1).
1. Role of topography
Surface elevation, relief, and slope, directly and indirectly, affect soil formation. It may be mentioned here that the soil formed according to the topography is called soil catena due to the difference in topography in the regions with similar source rock and climate. The influence of topography on soil formation is discussed below
- The slopes of hill areas are very steep. Erosion is high due to the steep land slope, resulting in shallow and thin-layered soil in this region.
- Where the land slope is moderate, the soil depth is moderate and in the plains where the land slope is low, sufficient rainfall increases the infiltration process in the soil. The leaching process removes the solutes from the upper layer resulting in deeper soil.
- The southern slopes of the highlands in the northern hemisphere are brighter and warmer than the northern slopes. This results in podzol soils on the northern slopes and alkaline soils on the southern slopes (1).
2. Role of parent rock
Through weathering, erosion, and deposition, parent rock changes to form soil. The physical and chemical properties of the parent rock significantly affect the soil. For example
- Sandy soils are formed from granite and rhyolite rocks. The water holding capacity is low due to the high sand content in sandy soils.
- Clay soil is formed from basalt rock. These soils are mildly alkaline and have higher water-holding capacity than sandy soils.
- If the parent rock is peridotite and serpentine, the soil is alkaline.
- Soils formed from schist rocks have more mud particles. An abundance of aluminum and magnesium can be observed in this soil.
- If the parent rock contains more quartz, the soil is gray or light white in color.
- If there is an excess of iron and manganese in the parent rock, the soil will have a red color.
- The soil is firm and structured when the parent rock has a high lime content.
- If the amount of lime in the parent rock is high, the soil is hard (1) & (4).
3. Role of time
- It takes thousands of years for a transformed soil to form in a natural environment. According to soil scientists, it takes hundreds of years for a layer of soil just two cm thick to form. So the process of soil formation is very slow.
- The amount of soil that forms over a long period of time has its own characteristics. E.g. – laterite, podzol, charnozem, etc. Each layer is well formed in modified soil.
- In a short period of time, soil layers cannot develop well. As a result, immature soil is created.
- The time of soil formation depends on the hardness of the parent rock. e.g, Shale rocks form soils quickly but hard igneous rocks form very slowly.
- Soil formation occurs faster in permeable materials like sandstone than in impermeable materials.
- Soil formation on heavy basalt takes much longer.
- Soil is mature when formed over a long period of time. In course of time young soil is converted into mature soil and mature soil is eroded (2) & (1).
1. Which of the following is not one of the five primary soil-forming factors?
Human activity is not one of the five primary soil-forming factors. It is not a direct factor of soil formation.
2. What are the five soil forming factors?
Climate (rainfall, temperature), biota (organism), parent rock, topography, and time are five soil-forming factors.
3. What are the soil forming factors?
Some elements play an active and some passive role in soil formation and they are collectively called soil-forming factors. Climate, biota (organism), parent rock, topography, and time are five soil-forming factors.
4. Which is not a main soil-forming factors?
Human activity is not a main soil-forming factor.
5. What are the 5 soil forming factors?
There are five soil-forming factors are
- Climate (rainfall, temperature
- Parent rock
- Biota (organisms)
1. Savindra Singh. Environmental geography. Prayag pustak bhawan, Allahabad. Chapter 7: Soil system. Page no: 78- 98.
Written By: Manisha Bharati