Soil: Definition, Formation, Types, Structure, Texture, Colour and Uses of Soil

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Photo of Soil: Definition, Formation, Types, Structure, Texture, Colour and Uses of Soil

Soil covers the earth surface that is the top most layer of the Earth crust on which plants grow. Soil as a natural body forms as a result of the complex interaction of parents materials, climate, relief (topography), biological organisms and time. The main qualities of soil are its texture, degree of permeability, porosity, and its period of time. Soil is mostly referred to asearthordirt. Soil is made of varying amounts of silt, sand and clay. The proportion of these components determines if a soil is sandy, loam, clay, or any combination of these. Soil texture has a number of implications for management because it affects the ability of the soil to hold water and its ability to withstand cultivation and compaction.

Formation of Soil

Formation of soil involves the changing of soil parents materials into soil through the various steps that are required in the process. It can be seen as a systematic step, which concerns itself to changing soil parent material into a new type of soil through the process of leaching, eluviation, illuviation, humification, and mineralization which depends on the climate. However, there are certain processes that are necessary for soil formation they include:

  1. Humifications– it is the breakdown of rotten organic waste to form humus, this process involves the decomposition or partial decomposition of organic matter animal and vegetation origin in the soil. This process plays an important but varying complicated role in maintaining the fertility of the soil and it is much more noticeable in wet and warm environments than it is in dry areas or cold regions.
  2. Mineralization: it is the breaking down of organic waste into its inorganic chemical components to mineral matter content. Naturally, the inorganic substance with the exemption of rocks possesses certain chemical compositions and definite physical and chemical properties. Sometimes, these chemical properties are broken down further into mineral or in organic substances. For example, a dead leaf is mineralized to form substance as carbon dioxide. The process of mineralization is quite important because it sets free nutrients that are useful for the growing plants in the soil which can be used again.
  3. Leaching: this is the loss of nutrients from the soil caused by water flowing through it, which deprives the soil of nutrients and may pollute watercourses. Leaching involves the removal of materials such as organic and mineral salt from the upper part of the soil to the lower layer of the soil by percolating rainwater. For example, when rocks are decomposed, substances such as carbonates and salt are produced.
  4. Illuviation: it is the movement of particles and chemicals from the topsoil into the subsoil. This process involves the leaching of materials from the upper layer of the soil and depositing the materials leached down to the layer down in the same soil.
  5. Eluviation: this is the process by which fine particles are removed in suspension such as clay from the soil. It is similar to leaching but the major difference is that materials removed are solid particles.

Factors that Affects Soil Formation

  1. Parent material
  2. Biological organisms
  3. Climatic weathering
  4. Relief(topography)
  5. Time
  • Parent material- this is the material from which soil is formed, the chemical composition and mineral materials are made up of quartz which give rise to poor sandy soil.
  • Climatic weathering– rocks exposed to weather are broken down to form soils. Mechanical and chemical weathering processes are involved; hence, leaching affects soil formation.
  • Relief- relief or topography affects soils formation; it is characterized by the inclination (slope),elevation, and orientation of the terrain. Relief determines the rate of precipitation orrunoffand rate of formation or erosion of the surfacesoil profile. Soils on hilltops are usually thin because of active soil erosion. The soil in the valley is thicker and heavier since the valley is more fertile for plant growth; soil in the valley also receives eroded soil from the upland.
  • Biological organisms– vegetation, animal inhabitants, and human populations can significantly affect the formation of soils. Roots open up the soil for more aeration. Plants die and add mineral to the soil. The plants bodies are changed or decomposed in soils. Soil organisms such as earthworm pass soil materials through their gullets, as the material pass through the worm; they are ground and mixed up with organic matter from the worm. This soil is deposited as cast on the surface. Humans on the other hand are also part of the biological influx that affects soil formation. Human influence can be as severe as wholesale removal or burial (by urbanization) of an entire soil profile, or it can be as subtle as a gradual modification of organic matter by agriculture or of soil structure by irrigation.
  • Time– if a piece of rock is broken down and exposed to the factors above, over a long period of time, change will be observed and the rock will gradually become soil. The states of the soil system vary with time, i.e., they are not stable some might take longer while other may take short time to break down.

Types of Soil

  1. Loamy soil
  2. Sandy soil
  3. Clay soil
  4. Silt
  5. Laterite

The main types of soil according to texture are clay soil; silt, sand and gravel, but most soil are mixtures of two of these or more. A good example of this is loamy soil which combines both sandy and clayey soil. Most of these soils contain hard layers of mineral deposits called hard pans which impede drainage. Hard pans occur mainly in humid climatic regions. The main forms of soil in the world are-

Loamy Soil: A loamy soil is formed from the combination of sand and clay with a high proportion `of organic matter. This type of soil is very fertile, this has a dark colour with medium-sized grains of sand, which crumbles easily and is very fertile, hence, it is often extensively cultivated or used for agricultural purposes.

Characteristics of Loamy Soil

  • Loamy soil contains a lot of organic matter
  • It is loose and moist in nature
  • Loamy soil is well aerated and can hold water
  • Loamy soil does not support erosion and water logging

Sandy Soil: A soil is said to be sandy when the proportion of sand particles in the sample soil are made up of over 50% (0.02- 2mm) coarse grain. Sandy soil is usually not good for farming because it is poor in plant nutrients. This soil drains easily and it is naturally low in plant nutrients through leaching, and it is often called light soil, as it is easy to work and also hungry soil since it needs fertilizer, sandy soil feels gritty when touched.

Characteristics of sandy soil

  • Sandy soil drains easily and is naturally low in plant nutrients through leaching.
  • Sandy soil is coarse and gritty
  • It is not sticky when wet.
  • Sandy soil has high percolation while capillarity is very low
  • Sandy soil is well aerated and cannot hold water
  • This soil has large pore spaces.

Clay Soil: A soil is said to be clayey if the proportion of clay in a sample of the soil is high (soil with more than 35% clay size material). Clay soil is fine-grained and holds much water and it has the capacity of attracting and retaining plant nutrients.

Characteristics of clay soil

  • Clay soil is sticky when wet and can hold more water than most other types of soil.
  • Clay soil is lightly packed with little pore spaces.
  • It is poorly aerated
  • Percolation is low but capillarity is high.
  • They lie wet in the winter, and are liable to poaching.
  • They are slow to warm in springtime. In long periods of dry weather, clay soils become hard and wide cracks may form.

Silt: these consist of very small quartz particles, they are micro sand grains but have more surface area than sand.

Laterite: this type of soil is leached and it is very poor for agricultural purposes.

Relative proportion of soil particles
Relative proportion of soil particles


Composition of soil

Soil is made up of five (5) distinct components namely

  1. Organic soil
  2. Inorganic soil
  3. Soil organisms
  4. Soil water and
  5. Soil air

Organic Soil

This soil is made up of remains of decayed plants and animals that form humus soil and provide essential nutrients for plants. Organic matter in the soil is good for promoting aeration, drainage and soil moisture retention in the soil. The organic matter also reduces erosion and supplies plant nutrients such as sulfur, phosphorous and nitrogen to the plant’s root zone.

Inorganic Soil

This is a substance which does not come from an animal or a plant, this type of soil forms from the greatest proportion of the solid materials which provide support and nutrient to plants.

Soil Organisms

Soil organisms include earthworm, ants, termites, moles, gophers, millipedes, tenebrionid beetles, rodents and bacterial which are useful to plants and animals. Many times, it may cause diseases to these plants animals.

Soil Water

Soil water contains about 25% of the total volume of soil which can be found between the pore spaces of the soil. The significant role of soil water is that it helps to dissolve plant nutrients and aids plant growth.

Soil Air

It is the air content of the soil. It contains the same gases as the atmosphere, but in different amounts, because it is modified by the constituent parts of the soil. Soil air contains almost the same proportional volume as that of soil water and it can be located between the soil particles. Soil air is important because it aids the respiration of soil organisms.

Physical Properties of Soil

Texture:Soil texture refers to the relative proportions of particles of various sizes such as sand, silt and clay in the soil- the texture of the soil is determine by water absorption, water storage,aeration and itsability to supply nutrient.

Soil texture diagram
Soil texture diagram


Soil structure-Soil structure can be defined in terms of grade, class and type of aggregates. Grade of structure could range from structure less, weak, moderate to strong. Class of structure on the hand describes the average size of individual aggregates (very fine to very coarse particles) while typedescribes the form or shape of the aggregate (Granular, blocky, platy, prismatic). The various class divisions are described as very fine or very thin, fine or thin, medium, coarse or thick and very coarse or very thick. However, soil structure is determined by its physical, chemical and biological characteristics. A good soil structure is important, as it can affect the availability of air, water and nutrients for plant growth. Agricultural practices can significantly influence soil structure. Poor soil structure can greatly reduce plant growth, making it difficult for plants to obtain water, air and nutrients and also impeding seedling emergence due to surface crusting.

The structure and texture of soil affects the soil’s ability to hold or drain water and withstand cultivation and compaction by machinery and stock. Sandy soils for example have low ability to retain water and are easily damaged. While heavy clay soils are very dense, and do not drain water very well and have small pore spaces. So also, sand is weakly structured because the sand grains are only weakly bonded together. Very heavy dispersive clay, which sets hard into large sheets when dry, has a massive structure. Most soil types fall in between these two structures.

Soil colour- Soil colour gives an indication of the various processes going-on in the soil as well as the type of minerals in the soil. For example,

  • Red soil indicates that there is abundance of iron oxide under oxidized conditions (well drainage) in the soil.
  • Dark colour indicates the accumulation of highly decayed organic matter.
  • Yellow colour is due to hydrated iron oxides and hydroxide in the soil
  • Black nodules are due to manganese oxides
  • Mottling and gleying are associated with poor drainage and/or high water table.
  • Abundant pale yellow mottles coupled with very low pH are indicative of possible acid sulphate soils. Colours of soil matrix and mottles are indicative of the water and drainage conditions in the soil and hence suitability of the soil for aquaculture.

Porosity– porosity reflects the capacity of soil to hold air and water, the porosity of a soil horizon increases as its texture becomes finer.

Permeability- describes the ease of transport of fluids and their dissolved components, permeability of soil decreases as the average pore size becomes smaller. Permeability also varies from one layer to another. Pore size, texture, structure and the presence of impervious layers such as clay pan determines the permeability of a soil. For example, clayey soils with platy structures have very low permeability.

Consistency– it is the resistance of a soil to deformation or rupture and it is determined by the cohesive and adhesive properties of the soil mass. Consistence is a term used to designate the manifestation of the cohesive and adhesive properties of soil at different moisture contents. A knowledge of the consistence of the soil is important in tillage operations, traffic and pond constructions. Consistence gives also an indication of the soil texture.

Consistency of soil is described for three moisture levels:

  • Consistency of wet soil: Non-sticky, slightly sticky, sticky, very sticky; non-plastic, slightly plastic, plastic and very plastic.
  • Consistency of moist soil: loose, very friable, friable; firm, very firm and extremely firm.
  • Consistency of dry soil: loose, soft, slightly hard, hard, very hard and extremely hard.

Uses of Soil

  1. Soil provides the only medium through which agriculture is practiced to provide food for man.
  2. Soil is necessary for growing food, supporting livestock and supporting other natural resources such as the forests, and supplying groundwater.
  3. Soil serves as a source of all minerals used by man.
  4. Soil provides plant with all the needed nutrients for their continuous existence.
  5. Soil acts as a reservoir for air and water needed by soil living organisms to survive.
  6. All vegetation types form their basis on soils and without soil, plants cannot get support and nutrients for growth.
  7. Soil provides the habitat for most organisms especially bacteria, earthworm, termite, millipede, moles. etc.
  8. All forms of construction works are carried out on the soil.