Soil formation introduction

According to geologists, the Earth was a fiery gaseous mass around 400 to 500 million years ago. After radiating heat it slowly cools further to solidify and form rock. Soil originates from these rocks. And the soil formation from the rock depends upon these controlling factors like relief, time, and climate which helps in direct formation. There are some factors like the color of rocks that do not affect soil formation but affect the appearance and texture of the soil. Below is a discussion of soil formation and these factors.

Let us know a little about what is soil. Soil is an important element among all the elements of nature and the environment. As the plants are growing on the soil they have been providing food for the animals that live in it and have played a special role in presenting a clean environment. In a word, this soil has been carrying the lifestyle of living beings for ages.

Soil refers to both surface and subsurface soil. However, in the case of soil properties, the profile of the soil is the most important. The character of the soil in all parts of the earth’s surface is not the same. So what type of soil will be formed depends on several soil-forming factors. The followings are the factors of soil formation and those that do not play a role in soil formation (1).

Factors affect soil formation

1. Climate

Climatic factors play the most important role when soil forms over parent rock over long periods of time. Rainfall and temperature play the most important role among them.

Depending of soil properties on climate can be expressed in the equation –

S= f (M, T), o, r, p, t.

Other influencing factors (o, r, p, t) are constant. Soil properties mainly depend on moisture and on temperature. Based on the balance between temperature and moisture, hydrothermal zones develop and special soil types are formed in those zones.

Two factors included in climate are rainfall and temperature which help in soil formation (2).

• Rainfall

Usually, some water evaporates from the soil, the rest seeps into the soil. Consolidation and leaching depend on the amount and intensity of rainfall, evaporation, the texture of the soil, the slope of the land, the type of vegetation, temperature, etc. Rainfall and soil water have further influence on soil depth, minerals, and other chemical elements.

Rain causes chemical reactions in minerals and affects soil formation directly and indirectly. The leaching of minerals depends on the amount of rain. In tropical humid and temperate humid climates, heavy rains lead to more leaching. The leaching of soluble minerals like Ca, Na, and K from the soil, makes the soil more acidic.

In dry desert regions, because there is less precipitation, there are more minerals, although they are of no use to the plants. In case of low rainfall, the soluble metal elements Na are easily washed away in the soil, but K, Mg, Ca, etc. accumulate in the A and B layers of the soil. It increases soil fertility.

In less humid areas, organic matter decomposes to form humus and chernozem soils. Water is essential for the decomposition of organic matter and the activity of microbes in the soil (3) & (1).

• Temperature

Differences in temperature affect soil formation. According to Van Hoff’s formula, for every 10°C increase in temperature, the rate of chemical reaction doubles, that is, the chemical reaction doubles in soil formation. Chemical reactions slow down from the equator towards the desert. But along with temperature, the role of rainfall or water is also important.

Generally, as the temperature increases, the amount of organic matter and nitrogen in the soil decreases. Because the humus becomes mineralized. Some properties of soil are controlled by temperature. For example, in high temperatures, weathering works up to a depth of about 30 to 40 meters, if the weathering increases, the depth of the soil also increases.

As temperature increases in humid regions, the silica-sesquioxide ratio decreases.

Again, with increasing temperature, if there is more rain, the oxidation process works and the color of the soil is red or yellow. If the temperature is low, the decomposition of organic matter slows down, resulting in more organic matter and more nitrogen.

If the temperature is low, the formation of clay minerals is relatively less (1).

• Wind

Wind often plays a role in soil formation. The effect of wind is seen in the accumulation of sandy soil in desert or coastal areas. Loess soils have deposited wind over a large area of the Huang ho river valley in china (3).

• Biota

Biota refers to plants, animals, and humans. Plants and animals have considerable influence in determining the origin and properties of soil.

1. Effects of plants

Tropical rainforests produce a large amount of organic matter, but the overall amount is low because of the high rate of decomposition of organic matter. Because the rate of decomposition is lower in temperate grasslands, the amount of organic matter is higher than in tropical regions.

The hardness of the soil depends a lot on the plants. Large tree roots break down rock and turn it into the soil through bio-mechanical weathering. After the plant remains are added to the soil, they decompose in various biochemical processes to form organic humus.

However, the amount of humus is higher in grassland than in the forest. In plains areas, there are fewer trees (pines, firs, etc.) that produce less humus, but it is more acidic. The physical and chemical type of humus also creates variation in plants.

Grassland soil has a high ratio of carbon and nitrogen. However, the carbon-nitrogen ratio is relatively low in forest soil. The mosses, lichens, etc. that grow in the cracks of the rock, when they die, mix with the rock and increase the amount of organic matter. Plants also play a special role in preventing soil erosion through their roots.

2. Effects of animals

Earthworms, ants, weevils, prairie dogs, etc. burrow into soft rock and turn it into the soil. Moreover, they turn hard soil into soft soil. Soils rich in earthworms are rich in N and Ca and have high water-holding capacity.

3. Effects of microbes

The remains of plants and animals in the soil are decomposed mainly by aerobic bacteria and become humus. In acidic soil, fungi instead of bacteria help in decomposition. In the absence of oxygen under the soil, anaerobic bacteria completely decompose the remains and produce large amounts of humus (1) & (4).

• Topography

The depth, texture, moisture, etc. of the soil depends on the slope of the land, topography, etc. Along the slopes of the land, weathering, and erosion materials cannot accumulate, rolling down. As a result, the depth of the soil is not high, the soil is immature. Moreover, because the water does not accumulate in the sloping areas, the soil is not formed well. On the other hand, in flat land, soil storage is better and the depth is greater, and the amount of water in the soil is higher. Soil erosion rate is higher on the steep slopes of the land, and less on lower slopes. The relationship between the differences in soil slope is called Soil Catena (3).

• Time

Soil takes thousands of years to form from parent rock. If the soil elements are active for a short period of time, the soil is immature. It takes at least hundreds of years for the soil to form a record in a favorable environment. Mohr mentions the five fathers of mineral analysis. Namely

• Unweathered parent rock
• Beginning of weathering
• Chemical reactions and dissociation of minerals
• The end stage of chemical analysis.
• Completion of the soil formation process.

The variation in time depends on the variation in the interaction of other regulators. Soils are formed quickly because of rapid chemical weathering in warm-humid forests. In dry or cold Polar Regions, soil formation is slow because chemical weathering is inhibited. If the sediments accumulated along the river are not very old, the well-developed profile is not formed (2).

• Parent rock

The influence of parent rock on soil formation is-

• The physical and chemical properties of soil such as texture, structure, water-holding capacity, color, pH value, etc. are controlled by the basic elements of the soil.
• That is why different types of soil can be seen in different rocks. For example, if the amount of feldspar in granite and gneiss rock is high, clay is formed. Basalt rocks form silt, laterite, and red soil.
• Endodynamomorphic soils are formed when the original rock influence is maximum. For example lithosol, regosol, and alluvium.
• The color of the soil rich in mineral salts, limestone, and quartz is white, gray, and sometimes olive green.
• Clay soil is formed when the original rock mineral is feldspar. If there is more clay, it helps to form different types of soil structures.
• If the parent rock is peridotite and serpentine, the soil is alkaline.
• And if the amount of lime in the parent rock is high, the soil’s structure is hard (1).

Factors do not affect soil formation

• Color of rocks

The crust was originally made up of rocks. Later it is broken into small pieces by the process of rock erosion. This part of the soil is made up of sand, silt, and clay particles. Rocks are crushed to form minerals and organic compounds that mix to form soil. Soils are formed from rocks through natural and chemical changes such as erosion, weathering, mass wasting, etc. in the presence of organic matter.

Rocks mainly play a role in soil formation. But one property of rock i.e. color of rocks does not affect soil formation. The composition of the soil is not affected by the color of the rocks.

Areas where soil formation is greatest

Soil formation is greatest in plain areas with warm and moist climates.

Q&A

1. Which of the following factors does not affects soil formation?

The color of the rocks does not affect soil formation.

2. Which factor does not affect soil formation?

The factor that does not affect soil formation is the color of the rocks.

3. In which of the following areas will soil formation be greatest?

Soil formation is greatest in the plain area with high rainfall and high temperature.

4. Given enough time, what factor is typically most important in soil formation?

The climate (temperature and precipitation) factor is typically the most important in soil formation.

5. In which area will soil formation be greatest?

Flat areas with warm and wet climatic zone areas are the greatest soil formation.

Reference

1. Savindra Singh. Environmental geography. Prayag pustak bhawan, Allahabad. Chapter 7: Soil system. Page no: 78- 98.