Soil: Concept, Composition and Density

After reading this article you will learn about the Soil:- 1. Concept of Soil 2. Definition of Soil 3. Components 4. Physical Properties.

Concept of Soil:

The soil is the covering on the solid crust of the earth’s land mass. Soil is made up of broken down rock material of varying degree of fineness and changed in varying degrees from the parent rocks by the action of different agencies, such that the growth of vegetation is made possible (Fig. 1.1).

There are two basic concepts of soil. The first pedology considers soil as a natural entity, a biochemically weathered and synthesised product of nature. Certain aspects, such as the origin of the soil, its classification and its description, are involved in pedology.

The second edaphology conceives of the soil as a natural habitat for plant. Edaphology is the study of the soil from the standpoint of higher plants. It considers the various properties of soils as they relate to plant growth and production.

Definition of Soil:

Soil may be defined as:

1. “A dynamic natural body on the surface of the earth in which plants grow, composed of mineral and organic materials and living forms.” -Buckman and Brady

2. “Soil is a natural body developed by natural forces acting on natural materials. It is usually differentiated into horizons from mineral and organic constituents of variable depth which differ from the parent material below in morphology, physical properties and constitutions, chemical properties and composition and biological characteristics.”—Joffe and Marbut

3. “Soil is the more or less loose and crumbly part of the outer earth crust in which, by means of their roots, plants may or do find foothold and nourishment as well as all other conditions essential to their growth.” —Hilgard

4. “Soil is the uppermost weathered layer of the soil earth’s crust ; it consists of rocks that have been reduced to small fragments and have been more or less changed chemically together with the remains of plants and animals that live on it and in it.” — Raman

Components of Soil:

The soil consists of four major components i.e., mineral matter, organic matter, soil air and soil water.

Volumetric composition of mineral (inorganic) soil is:

1. Mineral matter 45%

2. Organic matter 5%

3. Soil water 25%

4. Soil air 25%

Macro organisms like rodents, insects and worms and microorganisms like bacteria, fungi and algae live in soil in large numbers. There is a three phase system in which the mineral and organic matter form the solid phase, the water containing salts and some gases in solution the liquid phase, and the various gases the gaseous phase. Each phase contains a number of constituents which make the whole system highly complex. On account of the changes continually taking place in the soil, the system is never in equilibrium.

1. Mineral matter:

The minerals are extremely variable in size. Some are as large as the smaller rock fragments; others, such as colloidal clay particles, are so small that they cannot be seen without the aid of an electron microscope. Quartz and some other primary minerals (biotitic, muscovite etc.) have persisted with little change in composition from the original rock. In general, the primary minerals dominate the coarser fractions of soil. Secondary minerals (silicate clay, limonite, haematite etc.) are prominent in the fine materials, especiallyin clays. Clearly, mineral particle size does much effect on the properties of soil.

2. Organic matter:

Soil organic matter represents partially decayed and partially synthesised plant and animal residues. Such material is continually being broken down by the action of soil microorganism. Consequently, organic matter is a transitory soil constituent and renewed constantly by the addition of plant residues.

Organic matter influences soil properties and consequently on plant growth. Organic matter functions as granulator mineral particles. It is responsible for the loose, easily managed condition of productive soils. Organic matter improves the physical condition of soils, it also increases water-holding capacity. It is major source of plant nutrients i.e., nitrogen, phosphorus and sulphur. Finally, organic matter is the main source of energy for soil microorganisms.

3. Soil water:

Soil water is the major component of the soil in relation to the plant growth. The water is held within the soil pores. If the moisture content of a soil is optimum for plant growth, plants can readily absorb the soil water. Not all the water, soils can hold is available to plants. Much of water remains in the soil as thin film.

Soil water dissolves salts and makes up the soil solution, which is important as a medium for supplying nutrients to growing plants. There is an exchange of nutrients between the soil solids and the soil solution and then between the soil solution and plants.

4. Soil air:

A part of the soil volume that is not occupied by soil particles known as pore space, is filed partly with soil water and partly with soil air. As the pare space is occupied by both water and air, volume of air varies inversely with that of water. As the moisture content of the soil increases, the air content, decreases and vice-versa.

The soil air contains a number of gases viz., nitrogen, oxygen, carbon dioxide and water vapours. Soil air differs from the atmosphere in several respects. First, soil air contains much greater proportion of carbon dioxide and a lesser amount of oxygen than atmospheric air. Second, soil air has a higher moisture content than the atmosphere.

The content and composition of soil air is determined to a large degree by the soil-water relationships. Following a rain, large pores are the first vacated by the soil water, followed by medium-sized pores as water is removed by evaporation and plant utilisation. Thus, the soil air ordinarily occupies the large pores.

As soil further dries up, medium size pore spaces occupy the soil air. Soil air has marked effect on the plant and their root growth. It has also effect on soil microorganism, plant nutrients formation and their availability.

Physical Properties of Soils:

Physical properties (mechanical behaviour) of a soil greatly influences its use and behaviour towards plant growth. The plant support, root penetration, drainage, aeration, retention of moisture, and plant nutrients are linked with the physical conditions of the soil.

Physical properties also influence the chemical and biological behaviour of all soils. The physical properties of a soil depend on the amount, size, shape, arrangement and mineral composition of its particles. The physical properties also depend on organic matter content and pore spaces.

Following are the important physical properties of soils:

1. Soil density

2. Soil porosity

3. Soil consistence

4. Soil colour

1. Density of Soil:

Density represents weight (mass) per unit volume of a substance.

Density =Mass/Volume

Soil density is expressed in two well-accepted concepts—particle density and bulk density.

Particle density:

The weight per unit volume of the solid portion of soil is called particle density. Generally, particle density of normal soils is 2.65 grams per cubic centimetre. The particle density is higher if large amount of heavy minerals such as magnetite, limonite and hematite are present in the soil. With increase in organic matter of the soil the particle density decreases. Particle density is also termed as true density.

Bulk density:

The oven dry weight of a unit volume of soil inclusive of pore spaces is called bulk density. The bulk density of a soil is always smaller than its particle density. The bulk density of sandy soil is about 1.6 gram/cu. cm., whereas that of organic matter is about 0.5. Bulk density normally decreases as mineral soils become finer in texture.

The bulk density varies indirectly with the total pore space present in the soil and give a good estimate of the porosity of soil. Bulk density is of greater importance than particle density in understanding the physical behaviour of soils. Generally, soils with low bulk densities have favourable physical conditions.

2. Porosity of Soil:

Pore Space:

The volume of soil mass that is not occupied by soil particles is known as pore space. The pore space is usually occupied by air and water. In the pore spaces, the plant roots grow and exist. It directly controls the amounts of water and air in the soil and thus indirectly controls plant growth and crop production. The size and shape of pores and pore spaces vary considerably.

In general, two sizes are recognised:

(i) Macro or non-capillary pores:

These are the large pores, allow readily movement of air and water and do not hold much water under normal condition. Sands and sandy soils have a large number of macro pores.

(ii) Micro or capillary pores:

In contrast, in the micro pores, movement of air and water is restricted to some extent. Clay and clayey soils have a greater number of micro pores. Size of individual pores, rather than total pore space, in a soil is more significant in its plant growth relationship. For ideal conditions of aeration, permeability, drainage and water retention, a soil should have about an equal amount of macro and micro pores.

Soil Porosity:

Soil porosity is the percentage pore space. Porosity refers to that percentage of soil volume which is occupied by pore spaces.

It can be calculated by the formula:

For example, a soil having bulk density of 1.3 and particle density of 2.65 has the following percentage pore space.

Factors affecting Soil Porosity:

Following factors affect the percentage of pore spaces:

1. Soil texture:

In sandy soils, pores are quite large, thus, total pore space is less. In fine-textured soils, there is possibility of more granulation and more total pore space because there are pores (macro) between individual particles and within granules (micro pores).

2. Soil structure:

A soil having an aggregate structure has greater pore space than a structure less or single-grain soil. For the same size of aggregates as individual grains in a single- grain soil, and for similar mode of arrangement, a soil having aggregate structure has additional pore space between the primary particles. Granular or crumby type of structure has more porosity than plate-like.

3. Arrangement of soil particles:

When the sphere-like particles are arranged in columnar form [fig. 2.3 (c)] it gives the most open system of packing. Thus, the number of pore space will be less. When the particles are arranged in the pyramidal form [Fig. 2.3 (c)] it gives the closest system of packing. So in this system porosity would be more.

Fig. 2.3 Types of Structure

4. Organic matter:

Increase in organic matter content in the soil, increases the percentage of pore space.

5. Macro organism:

Macro organism like earthworm and insects increase the macro pores in the soil.

6. Depth of soil:

Pore space in the sub-soil has been found to decrease in comparison to soil.

7. Cropping:

Cropping tends to lower the total pore space in comparison to virgin or un-cropped soils. This reduction is associated with a decrease in organic matter content. Continuous cropping often results in a reduction of large or macro pore spaces.

3. Soil Consistence:

Soil consistence represents at varying moisture conditions, the degree and kind of cohesion and adhesion of soil material. Cohesion refers to the attraction of substances of like characteristics such as that of one water molecule for another. Adhesion is the attraction of unlike materials, for example, attraction between soil and water molecule. Consistence of soil depends on the texture, colloids, structure and especially the moisture content of soil.

The consistence of soil is generally described at three soil moisture levels:

(i) Consistence when soil is wet:

For wet soils (moisture content at field capacity), consistence is described in terms of stickiness and plasticity.

Stickiness:

The quality of adhesion to other objects is called stickiness. The degree of stickiness is identified by the following terms: non-sticky, slightly sticky, sticky and very sticky.

Plasticity:

Plasticity is the capacity to be moulded. When stress is applied, the shape changes and after removing the stress it keeps the changed shape. Terms use to describe the degree of plasticity are: non-plastic, slightly plastic, plastic and very plastic. (Table 2.9).

(ii) Consistence when soil is moist:

For slightly wet condition of soil (moisture content between air dry and field capacity), consistence is described in the following terms:

Loose: non-coherent

Very friable: coherent but very easily crushed

Friable: easily crushed

Firm: crushed under moderate pressure

Very firm: crushed only under strong pressure

Extremely firm: resists crushing between thumb and forefinger.

(iii) Consistence when soil is dry:

For dry condition of soil (air dry), consistence is characterised by rigidity and hardness.

In describing the consistence of dry soils, the following terminology is used:

Loose: non-coherent

Soft: breaks under slight pressure between thumb and forefinger to a powdery mass

Hard: breaks with difficulty under pressure

Very hard: very resistant to pressure: cannot be broken between thumb and forefinger

Extremely hard: extreme resistance to pressure; cannot be broken in the hand.

4. Soil Colour:

The colour of soil varies widely. It is an easily observable characteristic and is an important criterion in description and classification of soils. Colour of a soil is inherited from its parent rock material (termed as lithochromic); for example, red soils developed from red sand stone. Often the soil colour is a result of soil forming process and is termed as acquired or pedochromic.

Soil Colour and Composition:

(a) Black and dark grey colour:

The variations from black to dark grey colour of soil are mainly due to organic matter.

(b) Brown colour:

This is the most common soil colour and is due to a mixture of the organic matter and iron oxides.

(c) Red-yellow colour:

Red colour is associated with un-hydrated ferric oxides, whereas yellow colour indicates some degree of hydration (Table 2.10).

(d) White colour:

Silica and lime generally impart white colour.

(e) Bluish and greenish colour:

Some of the bluish and greenish colours are due to the presence of ferrous compounds. This reducing condition occur in ill-drained soil.

(f) Mottling colour:

Colour variegation or mottling in soils indicates alternating oxidising and reducing conditions due to a fluctuating water table.