Soil types and properties

Soil formation is greatly controlled by five major factors which are (i) Climate  (ii)Parent materials  (iii) topography  (iv) biotic (living organism)  (v) time

1. Climate: Elements of climate such as rainfall, temperature, wind and pressure are all very important in soil formation.
2. Temperature: The alternating heating and cooling of rocks result in the continual expansion and contraction which eventually result in cracks in the rocks and its consequent breakdown into small pieces to form the soil. Temperature affects the rate of chemical weathering of rocks
3. Rainfall: The action of running water from rainfall causes the gradual wearing away of rocks during erosion of form soil. Rainfall provides water for hydrolysis.  Also, rain drops may break down some parent rocks to form soil.

Rainfall enhances vegetative growth of plants whose roots cause further breakdown of rocks, while the rain water transports rock particles after disintegration.

• Wind: High-wind velocity in deserts carry with it other tiny rocks which collide with one another or other rocks, resulting in the breaking of rocks into tiny pieces to form soil. Wind also removes weathered materials, thereby, exposing parent materials to further breakdown.

1. Pressure: High pressure on a hanging rock may cause such rock to fall down and break into tiny pieces, resulting in the formation of soil.

2. Parent material: Parent materials constitute the major materials from which soil is formed. They are igneous, sedimentary and metamorphic rocks.  Parent materials determine the chemical composition of the soil that is formed.  It also contains different minerals which account for differences in the fertility of the soil formed from each of the different types.  Parent materials determine the physical characteristics of the soil. Hardness of parent material affects the rate of soil formation.
3. Topography: The shape of the ground in relation to the underlying rock of the earth’s surface is known as topography. Topography affects the rate of run-off and erosion.  Steep slopes encourage erosion and retard soil-formation.  Soil-formation is faster in the valley than on slopes.  Steepness of the slope affects the rate of abrasion of rocks; hence, soil is formed.
4. Biotic Factors (Living Organisms): The activities of living organisms help to speed up the process of soil formation.
5. Termite, earthworm, rodent mix the mineral and organic matter together, and these results in the formation of the soil.
6. They also allow water and air into the soil which eventually react with rocks to cause their break down into soil.

• The activities of man during tillage and other farm operations indirectly help to break rocks into tiny pieces to form soil.

1. The activities of micro-organisms which promote decomposition of organic materials aid soil-formation.
2. The roots of plants penetrate rocks and break them into tiny pieces to form soil
3. They influence the organic matter content of the soil

• Organisms produce carbondioxide which forms carbonic acid with water and enhances the weathering of rocks.
• Microbes also improve the soil aeration and water percolation. This enhances chemical and physical weathering.

1. Microbes help in the decomposition of organic matter in the soil.
2. The decay of fallen leaves of trees with the aid of bacteria results in the formation of humus, and this is rich in plants food.
3. Time: Time also plays an important role in soil-formation. It takes a long time for mature soil to be formed.
4. It takes a long time for small pieces of rock to disintegrate into grains of soil.
5. It also takes a long time for plants to decay and become part of the soil

• It also takes time for rainfall to leach chlorides, sulphates and carbonates from the soil.

1. It takes a short time in the formation of immature soil.
2. Time also determines whether or not the soil is well-developed.

PROCESS OF SOIL FORMATION

The process of soil formation is called weathering. Weathering is defined as the disintegration or breakdown of rocks into tiny pieces to form soil.  In other words, weathering can also be defined as the breaking down of rock masses (rock minerals) into simpler forms through the agents of physical, chemical and biological processes.

The Processes of soil formation include:

• Physical process (2) Chemical process (3) Biological process

1. Physical process: Agents of physical weathering are temperature, ice, wind, water and pressure.
2. Temperature: The alternating heating and cooling of the rocks produce pressure within the rocks and cause them to break down into pieces.
3. Wind: As a result of the grinding of rock surfaces by solid materials carried by wind, water and moving ice (glacier), rocks break down to form soil.

• Ice: The conversion of water inside cracks in rocks into ice results in increase in volume. This increase in volumes results in more pressure being exerted on the rock walls which eventually break into smaller pieces.

1. Water: Running water carries some fragments of rocks along its course and these rub against the surface of rocks in the river bed, thus breaking off small pieces of rocks.
2. Chemical Process: Agents of Chemical weathering include solution, carbonation, hydration, hydrolysis and oxidation.
3. Solution: This occurs when water dissolves and soluble minerals present in the rock are carried from place to place while water flows.
4. Carbonation: Atmospheric carbon dioxide mixes with rain water to form weak carbonic acid. This acid dissolves rocks, resulting in their breakdown.

• Hydration: This is the attachment of water with rock minerals. This results in chemical alteration of the minerals, e.g the conversion of iron II rocks to hydrated rocks.  Hematite rock is also changed to liminite.

1. Hydrolysis: This is the reaction of water with rock minerals to produce a rock, entirely different from the original one. For example olivine rock is changed to serpentine.
2. Oxidation: This is the reaction of rocks with oxygen from the atmosphere. This reaction eventually weakens and breaks down the rock to form soil.

3. Biological Process: This involves the activities of plants and animals in the breaking down to rocks to form soil.
4. It is caused by the action of animals like earthworms, termites and other soil organisms.
5. Movements of these organisms cause small fragments of rocks to disintegrate.

• Earthworms and termites burrow into the rocks and break off fragments of rocks.

1. The roots of growing plants penetrate rocks through crevices, exerting pressures which split some rocks.
2. The activities of man during farm operations such as ploughing and harrowing also break down rocks into tiny pieces.

COMPOSTION (COMPONENTS OF THE SOIL)

The soil is made up of five components which are (i) In-organic (mineral) matter          (ii) Organic water (iii)soil water  (iv) soil air  (v)living organisms.

Mineral or inorganic matter, organic matter, water and air are collectively referred to as physical components of the soil while living organisms are refered to as biological components of the soil.

1. Mineral or Inorganic Matter: The mineral matter represents small rock fragments of the soil. It forms the bulk of about 45% of total volume of the soil.  It consists of gravel, stones, sand, silt and clay

Importance/Effects of Mineral Matter on Agriculture

1. It forms the solid part of the soil and provides support for plants.
2. Mineral matter is the main source of plant nutrients such as nitrogen, calcium, magnesium, iron, etc.
3. It represents the home or habitat of all soil living organisms.
4. It holds water and air for both plants and animals activities.
5. Mineral matter has moderating effects on soil temperature.
6. It also affects soil porosity.
7. Organic matter: The organic matter represents the remains of the decomposition of plants and animals. It is about 5% of the total volume of the soil.  Leaves, roots of plants, the residue of crops, animal dung e.tc, when deposited on the soil, decay to form a dark colour on the upper part of the soil known as organic matter or humus.

Importance /Effects of Organic Matter on Agriculture

1. It is very rich in plant nutrients.
2. It is the habitat of many soil micro-organisms

• It prevents leaching in soil where it is present in adequate amount.

1. It also prevents soil erosion and evaporation of soil water.
2. It allows for good drainage and hold water in the soil for plant use.
3. It improves the structure of the soil by binding the particles of coarse-texture soil together.

• It increase water-holding capacity of the soil
• It moderates the soil temperature which stimulates good development and growth of roots.

1. It has buffering effect that moderates pH values.
2. It improves soil cation exchange capacity
3. It improves soil aeration.

• It is responsible for the loose, friable condition of loamy soil.

3. Soil water: Soil water refers to the water in the soil which is usually obtained either from rain or irrigation. Water represents 25% of the total volume of the soil.  It is usually found in the soil within the pore spaces.  When water is too much in a soil (covering the soil surface), the soil is said to be waterlogged.  A waterlogged soil can, however, be improved by drainage to make such soil more productive.  On the other hand, a situation where there is lack of water in the soil for a very long time to the extent that plants cannot absorb water even when supplied again, results in a condition called permanent wilting point.  The plant at this stage can die.

Types of Soil Water

There are four major types of soil water.  These are:

1. Hygroscopic water: This water is tightly held by the soil particles such that it is never available to the
2. Field capacity: This is the type of water left in the soil after excess water has been drained off, following heavy rainfall. This water is available to the plants.
3. Capailary water: This is the water which rises above the water table in the soil and it is held in the fine and medium pores of soil particles by surface tension. Capillary water is easily available to plants.
4. Gravitational water: this is the water which can drain from the soil under the influence of gravity. It is available to plants but it is often pulled down beyond the reach of the roots.

Importance/Effects of Soil water on Agriculture:

1. Water is an important agent of weathering of rocks in the soil.
2. It helps to dissolve plant nutrients into solution form which can easily be absorbed by plant roots.

• Water is an essential raw material for photosynthesis.

1. Hydrolysis of may food nutrients and enzymatic activities are promoted by the presence of water.
2. Loss of water from plant through a process called transpiration is helpful in the cooling of the plants.
3. Presence of water aids easy tillage of the soil and also helps to improve the soil structure.

• It also promotes the activities of soil organisms.
• It is needed for the germination of seeds.

1. It provides the medium for soil reaction.
2. It aids the turgidity of cells.
3. It protects plants from injurious effects of high temperature.

4. Soil air: This refers to the gasses present in the soil pores found between the soil particles. The amount of soil air varies, depending on the amount of soil water, the sizes of the pore spaces, the type of soil and the amount of living organisms in the soil.  The percentage of air is about 25% of the total volume of the soil.  The ability of air to circulate freely in the soil is called aeration.

Importance /Effects of Soil Air on Agriculture

1. Soil air, especially oxygen, is necessary for the growth and development of plants.
2. Oxygen in the soil promotes easy germination of seeds.

• Soil organisms require oxygen for respiration

1. Excess of carbon dioxide in soil when combined with water can cause acidity and aid weathering of rocks.
2. Some plant-disease organisms such as fusarium which causes damping-off are favored by poor aeration.
3. It is needed in soil reactions, particularly carbon and nitrogen cycles.

5. Living Organisms: These refer to plant and animals which inhabit the soil. They range from microscopic organisms to bigger organisms.  Some are beneficial while others are harmful to crops and livestock.  The most commonly found groups of soil organisms include bacteria, fungi, virus, nematodes, insects (
   g, termite, soldier ants), millipede, centipede, earthworm, snails, reptiles, mammals (e.g. rats and rodents)

Importance/Effects of Living Organisms on Agriculture

Soil organisms are very useful in many ways, especially in soil-formation and improving the soil for the growth of crops.  Their effects are:

1. Soil organisms improve soil structure and granulation
2. They also improve the aeration of the soil.

• They help to decompose organic materials in the soil to form humus

1. They improve soil water percolation or drainage.
2. They also increase the collocidal properties of the soil
3. Some organisms like bacteria help to fix nutrients into the soil.

• Some soil micro-organisms produce acidic materials which help to break down rocks.
• Soil organisms enhance the cation exchange capacity of the soil.

1. Through burrowing, they leave holes for crop root penetration.
2. They increase the mineral or nutrient status of the soil.
3. They stabilize soil pH through the increase in soil organic matter and buffering.

SOIL TYPES AND THEIR PROPERTIES

Definition of Soil: Soil is defined as the uppermost layer of the earth’s crust which provides support and nutrients for plant growth.

There are three main types of soil.  These are sandy soil, clay soil, and loamy soil.

Sandy Soil:  Soil is said to be sandy if  the proportion of sand particles in a sample of the soil is very high.  The particles are mainly quartz (SiOz) and having a size of 0.02mmm to 2.0mm in diameter.

Properties of Sandy Soil

1. Sandy soil is coarse, grained and gritty
2. It is loose with large pore spaces

• It absorbs and loses water easily.

1. It is not sticky, when wet. Hence, it cannot form a cast or ribbon.
2. It is well aerated with low water holding capacity.
3. Percolation in sandy soil is high but capillarity is low.

• Sandy soil heats up easily during the day and cools down quickly during the night
• It support leaching. Hence, it is low in plant nutrients.

1. It does not support water logging and erosion
2. Sandy soil is low in plant nutrient. Hence, it does not support crop cultivation.
3. Sandy soil has grey or brownish colour.

Methods of Improving Sandy Soil

Sandy soil can be improved throughout the following agricultural practices:

1. Planting cover crop: Cover crops help to provide shade, prevent erosion and add more nutrients to the soil.
2. Application of compost manure: Compost manure helps to bind the sand particles together and also add humus (nutrients) to the soil.
3. Application of farm yard manure: this will also improve the structure of the soil as well as add nutrients to the soil.
4. Mulching the soil: Muching sandy soil prevents water loss through evaporation and nutrient loss by water erosion
5. Avoidance of bush burning: Bush burning promotes soil erosion, kills soil organism and removes organic matter which can contribute to the fertility of the soil.

Economic importance of Sandy Soil

1. It is good for the cultivation of few crops such as cotton, groundnut, cassava, e.t.c
2. It is also useful in building construction, especially its combination with cement and water in block-moulding.

Clay soil

Definition: Soil is said to be clayey if the proportion of clay in a sample of the soil is very high.  The relative size of clay particle is less than 0.002mm in diameter.  It is a heavy type of soil because it is difficult to work on or cultivate.

Properties of Clay soil.

1. Clay particles are fine, powdery and smooth when dry.
2. The particles are sticky and mouldy when wet.

• The particles are tightly bound together with little pore (air) spaces

1. The structure is granular and does not lose water easily.
2. It is poorly aerated with high water holding capacity.
3. Percolation in clay soil is low but capillarity is high.

• It does not support leaching. Hence, it contains plant nutrients.
• It is hard when dry and sticky when wet.

1. It can easily form a robbor or cast when moulded.
2. It supports water-logging and erosion
3. It has a grey or brownish colour

Note: Clay soil can be improved through (i) liming (ii) addition of organic manure.

Loamy Soil

Definition: Loamy soil is a mixture of sand and clay particles with high proportion of organic matter.  Loamy soil is more fertile than either clay or sandy soil.  If a type of soil is described as sandy loam, it means that the proportion of sand is high and if it is clay loam, it shows that the proportion of clay is high, while that of sand is low.

Properties of Loamy soil

1. Loamy soil is moist, loose with moderate sized pore spaces.
2. The structure breaks easily when wet and friable when dry.

• It has non-powdery and non-sticky texture.

1. It can easily be worked on or cultivated.
2. It contains lots of organic matter (humus).
3. It does not support erosion and water logging.

• It is well aerated and it can hold water.
• It contains plant nutrients, hence, it is the best soil for the cultivation of crops, e.g, maize, rice, cowpea, cassava, yam, tomatoes, okra, millet, pepper.

1. It is dark-brown or black in colour

Importance/Effects of Organic Matter on Agriculture

1. It is very rich in plant nutrients.
2. It is the habitat of may soil micro-organisms.

• It prevents leaching in soil where it is present in adequate amount.

1. It also prevents soil erosion and evaporation of soil water
2. It allows for good drainage and holds water in the soil for plant use.
3. It improves the structure of the soil by binding the particles of coarse-texture soil together.

• It increases water-holding capacity of the soil
• It moderates that soil temperature which stimulates good development and growth of roots.

1. It has buffering effect that moderates pH values.

Properties of the Soil

Properties of the soil include soil texture, soil structure, soil temperature, porosity, soil colour, water-holding/retaining capacity and soil pH.

The properties of the soil are grouped into two:

1. Physical properties of the Soil: these properties include soil texture, soil structure, soil temperature, porosity, soil colour and water holding/retaining capacity.
2. Chemical properties of the soil: These include the soil pH and cation exchange capacity.

SOIL PROFILE

Soil profile is defined as the vertical section of the soil, showing series of horizontal layers of different types of soil. These horizontal layers are called horizons.

A soil profile in an area of the humid tropics such as the forest zones may have about four fairly distinct horizons. The first thing that is noticed in a soil profile is soil colour. The horizons show different colours, e.g., the top soil may be dark, followed by brown below.

After the colour, the next feature which can be noticed is the texture or particle sizes which increase from the top to the bottom.  These two characteristics enable the different horizons to be identified.

Horizons of Soil Profile

The four major horizons of soil profile are:

1. The A-Horizon, also called the top soil, represents the surface layer of the soil profile. This horizon contains more organic matter than other horizons. The A horizon, the top soil, is the most weathered and leached of all the soil horizons.  Most food crops, especially shallow-rooted crops derive their nutrients from this top soil, e.g, vegetables, legumes. E.tc
2. B Horizon: The B-horizon, also called the sub-soil, is the next horizon immediately after the top soil. It is rich in minerals which are carried or leached down by percolating water.  The B-horizon is suitable for the cultivation of deep-rooted crops like cocoa, rubber, orange,  oil palm e.t.c.
3. C-Horizon: The C-horizon, also called parent materials, represents the type of material from which top-soil and sub-soil are derived. It is the parent materials which are small fragments of rocks that are unweathered and found at the bottom of the soil profile.
4. D-Horizon: The D-horizon is also called the bedrock. It represents the unweathered rock materials.  This horizon is found at the bottom of the profile and they are usually of large soil particles.

Importance of Soil Profile

The suitability of a soil for agriculture is determined by looking at the soil profile.  The importance of soil profile includes:

1. Level of Soil fertility: Soil profile determines the level of soil fertility. A thick top soil represents high level of soil fertility.
2. To know the type of crop to grow: it helps the farmers to know the type of crop to grow. For example, shallow-rooted crops like cowpea, groundnut, e.t.c are grown in the top soil, while deep-rooted crops are grown where the sub-soil is thick.
3. Penetration of roots: A loosely-packed sub-soil allows for easy penetration of roots of crops.
4. Level of drainage and aeration: A loosely packed sub-soil also allows for easy drainage and aeration.
5. Easy percolation: A loosely-packed sub-soiled also ensures easy percolation of water, thereby preventing the occurrence of erosion.

SOIL TEXTURE

Definition: Soil texture refers to the relative proportion (sizes) of the various particles of the soil.  In other words, it refers to the degree of fineness or coarseness of the various soil particles.

The particles that make up a soil sample include gravel, sand, silt and clay.  Silt and clay are usually referred to as the primary particles of the soil.

The name and sizes of the various soil particles are shown in the table below.

Determination of Soil Texture

Soil texture can be determined by various methods.  These methods include:

1. By feeling : Take a little sample of soil and rub them between the fore-finger and the thumb. A sharp feel represents the presence of sand while a smooth or powdery feel represents the presence of clay.
2. By mechanical analysis through sieving: The various sizes/fractions in a sample of dried soil can be separated by putting the sample into a series of various measured mesh diameters and shaking vigorously. One starts with the sieve which has the smallest mesh diameter and progresses up the table to the sieve with the largest. The particles which can pass through a particular mesh belongs to the corresponding grade of soil.
3. By sedimentation: A sample of soil placed inside a glass jar and large volume of water is added and the mixture is vigorously shaken and allowed to settle. At the end, large particles like coarse sand and gravel settle at the bottom, while the organic materials float on top of water in the glass jar.
4. By moulding: Mix a sample of soil with little water and try to mould the mixture. If the mixture is sticky.  It shows that clay is present but if it is not sticky and cannot form a ribbon or cast, it shows that sand is present.

Importance of Soil Texture

Soil texture is very important, especially to farmers in the following ways:

1. It is useful in the evaluation of soil ability to supply mineral nutrients.
2. It supports soil micro-organisms essential for plant growth.

• It determines the type of crop to grow on the land.

1. It enables the farmer to know the type of soil in his farmland.
2. It determines the relative proportion of air and water in the soil.
3. It determines the movement of air and water.

• It helps o determine tillage practice to be adopted by the farmer.

Soil structure

Soil structure refers to the way in which the different particles of the soil are packed or arranged.  It also refers to the shape and arrangement of primary particles form compound particles.

Soil structure has a direct effect on crop yield.  If the soil structure is good, air will circulate well while waterlogging, erosion and leaching will be reduced.  The structure of the soil can be preserved in the following ways.

1. Planting cover crops
2. Mulching

• Application of manure (green manure and lime).

1. Avoidance of over-grazing and erosion.
2. Avoidance of clean cleaning with machines.

Types of Soil Structure

The different types of soil structure are:

1. Single-grained Structure: in this structure, the primary particles exist in single form and are not cemented together. It is found in sandy soil.
2. Crumb Structure: There exist large gravels or stones embedded within the primary particles which are cemented together. It is found in the top soil
3. Plate-like Structure: The primary soil particles are arranged horizontally and flat, resembling plates or leaflets on top of each other it is common found in the sub-soil.
4. Spheroidal Structure: This is also referred to as granular structure. The particles are cemented together in a circular form with lots of air spaces.  It is commonly found in top soil.
5. Prismatic Structures: This could be columnar or prismatic. They are just like spheroidal structure with air spaces.  When the top ones are round, they are columnar, but when they are flat just like a prism, they are prismatic.  It is found in sub-soil.
6. Block-like structures: The aggregates are like blocks whose edges are irregular and may be either sharp or rounded. It is commonly found in the sub-soil.

Importance of soil structure

1. It determines the level of fertility of the soil
2. A good soil structure supports aeration

• It also prevents erosion and water togging.

1. A good soil structure promotes the activities of soil micro-organisms.
2. A good soil structure has a good water-retaining capacity.
3. It also supports the growth of crops.

Soil Temperature

Soil temperature refers to the temperature within the soil.  In other words, it is the temperature usually different from the temperature of the air above the soil.

1. Soil temperature affects the formation of soil organic matter.
2. Soil temperature affects the decomposition of soil organic matter.

• It determines seed germination and not development

1. It also affects the population of soil microbes.

Soil pH

The word of pH (pondus de Hydronium) is defined as a measure of the degree of acidity or alkalinity of the soil.  It can also be described as a measure of the concentration of hydrogen (H+) in the soil.  A higher hydrogenions (H+) concentration indicates soil acidity while a lower concentration of hydrogen ions (OH+) indicates soil alkalinity.

Causes of Soil Acidity

Soil acidity can be caused by any of the following:

1. Leaching: The washing down of plant nutrients below the soil beyond the reach of the plant roots leaving behind hydrogen ions.
2. Use of acid fertilizers: The use of acid fertilizers like ammonium sulphate and ammonium nitrate can easily cause acidity in the soil.
3. Presence of acid parent materials: The presence of acid parent materials results in the easy dissolution of the rocks, leaving behind minerals rich in hydrogen ions.
4. Nutrient uptake by plants: The absorption of soluble minerals by plants results in the accumulation of hydrogen ions which cause soil acidity.
5. Presence of sulphur in the soil: sulphur undergoes oxidation and dissolution to form weak acid in the soil.

Removal of Soil Acidity

Soil acidity can be removed by the application of liming materials which are rich in calcium.

Examples of liming materials include:

1. Limestone (calcium carbonate) – CaCo₃
2. Slaked lime – Ca (OH)₂

• Quick lime – CaO.

1. Calcium bicarbonate – Ca(HCO₃)₂
2. Wood ash (calcium phosphoilicate – Ca₄ (PO4)₂ SiO₄
3. Dolomite or Calcite – Ca (HCO₃)₂

• Basic slag – Ca₃ (OHSiO₃)₂
• Gypsum – CaSo₄ 2H₂O

1. Organic manure can also be applied to the soil to remove soil acidity.

Questions:

1. What is soil
2. State five Characteristics of sandy soil
3. What is soil structure
4. What is soil acidity
5. State five causes of soil acidity