Composition of Soil

Soil refers to the loose surface of the earth as identified from the original rocks and minerals. It is derived through weathering process.

So in a simply soil may be defined as the accumulation of natural bodies which has been synthesized in profile from a diversified mixture of disintegrated and weathering Minerals and decomposition of organic matter, which cover the surface of the earth and supplies when containing the optimum amount of air and water, mechanical support and substances for plants.

Composition of Soil

So soil is mainly composed of 4 things –

1. Minerals matter

2. Organic matter

3. Water

4. Air

Soils are mainly composed of mineral matter and organic matter which contains water and air and other soluble nutrients.

Soil is composed with minerals which is 45%, air which is 25%, water 25% and organic matter 5%. Composition of soil is an important subject of nutrient management. In this, soil minerals and organic matter hold the soil nutrients at one place while water is provide nutrients for plant uptake.

Composition of Soil

Simply we can say that the basic component of soil is mineral, water, air and organic matter. The ratio of these components may fluctuate on the different regions depending on numerous factors such as cultivation practice, water supply, climate condition etc. This factors may affect the ratio of composition in the soil.

The solid phase of soil component is organic matter and minerals and these are generally stable in nature. Air and water are the liquid component of soil which are the most complex properties of the soil.

All these components of soil cannot be separated with much satisfaction because they are parent very intimately mixed with each other. The mineral matter forms the bulk of soil solids and a very small amount of the soil solids occupied by organic matter.

Physically, the soil consists of stones, larger pebbles, dead plant twings, roots, leaves and other part of the plants, fine sand, silt, clay and humus.

In the organic matter portion of the soil, about half of the total organic matter comprised of the dead remains of the soil life in all stages of decomposition and the remaining half of the organic matter in the soil is alive.

The living part of the organic matter consists of plant roots, bacteria, earthworms, algae, fungi, nematodes, actinomycetes and many other living organisms.

Soil components comes from :-

1. Mineral matter :- Mineral matters are generally comes from the decomposition of rocks. It is a very slow process.

2. Organic matter :- Organic matter are obtained by the decomposition of living organisms, reduces etc.

3. Water :- water is in the soil obtained from the atmosphere and by the reaction of soil with the help of microbial activity.

4. Air :- Air in soil comes from the atmosphere and reaction of roots and microbes.

Components of soil


The size and composition of mineral matter in soils are variable due to nature of parent rock from which it has been derived. It is generally composed of very fine broken rock fragments and minerals either dominated by inorganic constituents or dorminated by distinct minerals like quartz and feldspars.

The rock fragments are disintegrated and broken portion of the massive rocks from which the regolith through weathering of soil have been formed. These materials are usually very coarse and the minerals are extremely variable in size. Some are smaller rock fragments and other are colloidal clay particles that cannot be seen without the help of an electon microscope.

In the composition of soil, primary minerals such as quartz, biotite, muscovite etc, dominate the coarser fractions of soil, on the other hand, the secondary minerals like silicate clays and hydrous oxide clays of iron and aluminum etc. are present as very finer fraction, clay in the soils.

Organic matter

Soil organic matter exists as partly decayed and partially synthesized plant and animal residues. Such organic residues are continually being broken down as a result of microbial activity in soil and due to constant changes it must be replenished to maintain soil productivity.

The organic matter content in a soil is very small and varies from only about 3 to 5 percent by weight in a top soil. To partly cocomposd plant and animals, soil organic matter contains living and dead microbial cells.

Organic matter is a store house of nutrients in soil. Besides these, organic matter is responsible for most desirable surface soil structure, promotes a greater proportion of larger pore sizes, improves water holding capacity and also aeration status of the soil.

It is a major source of nitrogen, 5-60 percent of the phosphorus and perhaps about 80 percent of the sulphur. Besides these it can also supply different trace elements like boron, molybdenum etc. in soil science which are essential for the plant growth. Organic matter is the main source of energy for soil micro-organisms.

Organic matters act as a chelate. Chelate is an organic compound that can bound to a metal by more than one bond and form a ring of cyclic structure by that bonding. Due to chelate formation between organic matter and various metals that availability of those metallic elements will be increased to the plants through increasing their mobility in soil.

Organic matter contribute to the cation exchange capacity in soil. Organic matter reduce soil erosion, shades the soil and keep the soil cooler in very hot weather and warmer in winter.

Soil water

Soil water plays a very significant role in soil science for plant growth relationships. Water is held within the soil pores with varying degrees of forces depending on the amount of water present. With the increasing amount of water in soil the forces of retention of water by the soil will be low and vice-versa.

So the movement of water in the soil is primarily influenced by characteristics of the soil. All soil water held by different forces are not always available to the plant e.g. water held by soil with a high forces of attraction are not available to the plants.

Soil water also presents along with dissolved salts and makes up the soil solution. This soil solution acts as an important medium for supplying different essential nutrient elements to growing plants through exchange phenomena between soil solid surfaces and soil solution and finally between soil solution and the plant roots.

Soil air

Air spaces or pore spaces (voids) in a soil consists of that portion of the soil volume not occupied by soil solids, either mineral or organic. Under field conditions, pore spaces are occupied at all times by air and water. The more the water, the less the room of air and vice-versa. The relative amount of ait and water in the pore space fluctuate continuously.

During rainy season, water replaces air from the soil pore spaces but as soon as water disappeared by download movement, surface evaporation and transportation etc., air gradually replaces the water as it is lost from the pore spaces.

Soil air is also one of the basic composition of soil, which contains various gases like carbon dioxide, very small amount of oxygen and nitrogen etc. Soil air differ from the atmospheric air with the relative amount of those above gases.

Generally soil air contains much more carbon dioxide and small amounts of oxygen than that of atmospheric air due to microbial respiration where large amounts of carbon dioxide releases into the soil and the oxygen is taken up by soil micro-organisms.

The dynamic nature of soil air is apparent. Soil air influence the growth of plants and crops as well as the activity of different beneficial microorganisms present in the soil.

Volume of soil components

Soil contains about 50 % solid space and 50 % pore space. The total solid space of the soil is occupied by mineral matters which is about 45% and organic matters which is about 5%. This percentage may be fluctuate under different conditions.The total pore space of the soil is occupied by air and water on the ratio of 50:50 .i.e. In this case 25 % air and 25 % water.

The proportion of air and water will vary under natural conditions depending upon the weather and environmental factors. So it must be emphasized that the above four major composition of soil exist mainly in an intimately mixed condition which encourages various reactions within and between the groups and gives optimum condition for the crop growth.


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