Soil Notes

Introduction to Soil

Soil is an important natural resource. Plants need soil to grow. Microscopic organisms such as bacteria, fungi, microscopic algae and protozoa live in the soil. Soil beetles, soil mites, earthworms, nematodes, millipedes, centipedes, ants and termites also live in the soil.

Soil Formation

Soil is the uppermost layer of the Earth's surface. It is formed by the breakdown of rocks over thousands of years by a process called weathering.
Weathering is the process of breaking down rocks into smaller particles by the action of wind and water, change in temperature and penetrating roots of plants.
Weathering is a continuous and natural process that occurs due to heat, cold,  wind, rain and frost, and also due to biological factors like roots of plants and soil organisms.
Weathering is of three types: physical, chemical and biological.
Physical weathering (Mechanical weathering):
The breaking down of big rocks into smaller sized rock particles with the help of agents like wind, water or change in temperature is called physical weathering.
Chemical weathering:
Chemical weathering changes the chemical nature of the soil. When rain water mixed with gases like carbon dioxide, Sulphur dioxide and nitrogen oxides in the atmosphere  it forms acid rain. When acid rain falls on the surface of the Earth  it makes the soil slightly acidic.
Biological weathering:
The roots of trees and borrowing of animals inside the soil bring about this type of weathering. Roots of trees and plants grow through the rocks contribute to weathering.

Soil formation takes place in three main stages

First stage:  Parent rock or Bedrock get broken down into smaller-sized rocks by the action of wind, water or change in temperature or with the help of decaying remains of plants and animals present in the parent rock.
Second Stage: Weathering continues to form smaller rock particles. In this stage the dead plants and animals present in the soil get decayed by the action of bacteria are the microorganisms to form humus.
Third stage: In this stage, mineral and salts reach deeper into the soil with water and increase the fertility of the soil to make it suitable for growth of plants.

Factors affecting soil formation

There are many factors that affect soil formation
Climate: Climate conditions like harsh wind, rainfall and fluctuating temperature leads to the formation of cracks in the rocks.
Landscape: The landscape in the hills facilitate the running down of water,  thereby eroding the rocks with more intensity and causing them to break or crack.
Nature of the parent rock: Soil formed will have similar composition as that of the parent rock. For instance, if the parent rock has high calcium content  the soil formed will also be rich in calcium.

Soil profile

Soil profile is a vertical section of soil showing its different layer or Horizons. Soil is made up of a number of layers. Each distinct layer of soil is called a soil horizon.
Main horizons of the soil:
Horizon A (topsoil): It is the uppermost layer of soil. The soil is made up of sand, silt, clay and pebbles. It is rich in humus. It is a fertile layer of soil. Horizon A  supports plants. This layer vary from place to place. It can be loamy, clayey, silty or sandy.
Horizon B  (subsoil): Compared to Horizon A, this is a harder region, contains a large amount of pebbles, little humus, roots of trees and shrubs. It is a storehouse of minerals.
Horizon C: Below the subsoil is the infertile zone (lacking humus) that contains partially weathered rocks with a lot of cracks and crevices.
Horizon R: This layer is the lowermost layer in the soil profile and is known as the Bedrock as it contains hard rocks that provide the base to the other three layers.
Composition of Soil:
Soil has four main components:
  • Parent material (clay, sand or gravel)
  • Water
  • Air (present in the spaces between the soil particles)
  • organic matter (humus)
Along with these components soil also contains soil organisms and Minerals.
Properties of soil:
Absorption: The ability of a particular type of soil to absorb and retain water is called its water- holding capacity. The moisture (water) content of the soil depends on its water-holding capacity.
Pore space: Soil has spaces between its particles known as pore spaces. Pore space is a very important property of soil. The water and air content of the soil and the flow of water through the soil depends on the pore space of the soil.
Percolation rate: Percolation means the movement of water through the soil. Percolation rate is the rate or speed at which water flows through the pores in the soil. The soil having a higher water holding capacity will have a low percolation rate and vice versa.

Types of soil

Soils are of different type because of their water absorbing capacity. This is because of the differences in the size of particles in each type of soil.
Sandy soil:
  • Contains large coarse particles which we call send.
  • Sand grains form large spaces between the particles which makes sandy soil very porous and cannot hold water for long.
  • This soil is mainly found in places with warm climates such as deserts and marshy areas.
Clayey soil:
  • Contains fine particles made of clay.
  • The soil particles have little air spaces between them.
  • Clayey soil is rich in minerals, many of which are used by the plants.
  • Soil is very dense and less porous. As a result, clayey soil gets easily waterlogged during the raining season  reducing the availability of air between the soil particles, making the soil infertile.
  • Paddy and cotton grow well in this type of soil.
Loamy soil:
  • Contains an equal amount of sand and silt and lesser amount of clay.
  • The soil is well aerated and also contain a lot of organic matter (humus), that support a rich variety of soil organisms which keep the soil fertile.
  • It is used to grow vegetables, onion, garlic, pulses, wheat and maize.
Silty soil:
  • Silt particles are smaller than sand but much larger than the clay particle.
  • It remains moist for a longer time, silty soil does not get waterlogged easily.
  • Silty soil also has a good amount of mineral nutrients and some amount of humus.
  • It is used to grow vegetables, fruit bearing trees and pulses.
Suitability for crops
The kind of minerals present in the soil, the acidity and alkalinity of the soil and humus are the factors that play an important role in the growth of crops
The Indian Council of Agricultural Research (ICAR) has identified the following soil types:

Soil erosion

Soil erosion is the gradual removal or wearing away of the Earth's soil surface by natural agents such as running water, wind, waves and snow. It takes away the fertile topsoil, making the soil infertile and unsuitable for plant growth.
Causes of Soil Erosion:
  • The loss of protective education through deforestation, overgrazing, ploughing, over cultivation and forest fire makes the soil susceptible to being swept away by strong wind and running water.
  • Inappropriate farming techniques such as deep ploughing the land two or three times a year to produce annual crops can lead to soil erosion.
Prevention of Soil Erosion:
  • The force of wind and flowing water can be reduced by growing cover crops. A cover crop is grown to cover the land after harvesting the main crop. Cover crops prevent soil from being exposed to wind and flowing water.
  • The force of wind and water can also be reduced by growing trees and shrubs on the boundaries of fields. Rows of plants on the boundaries of field are known as shelter belts.
  • Preventing overgrazing, afforestation, step farming and constructing river embankment.

Soil pollution

Soil pollution is the presence of unwanted substances in the soil that can be harmful for all living beings.
Causes of soil pollution:
  • Solid waste (human excreta, fruits and vegetable peels, plastics, metal objects, etc.), excess use of chemicals such as fertilizers, pesticides and insecticides, acid rain are the main contributors to soil pollution.
  • Untreated sewage sludge, human excreta, livestock, etc., used as manure also contributors to soil pollution.
Prevention of soil pollution:
  • Decreasing the generation of waste.
  • Compost improves the quality of soil, loosens the soil and helps air and water to move more freely.
  • Farmers should be encouraged to use biofertilizers, biopesticides and fungicides.
  • Preventing the use of harmful chemicals.
  • Recycling and reusing waste not only helps in reducing the felling of trees but also helps in decreasing soil pollution.
  • Afforestation (planting of trees)
  • Measures also need to be taken to prevent overgrazing and over-cropping as they lead to soil erosion.
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