SS2 Agric. Science - Soil: Meaning, Types, Composition, and Properties of Soil 2

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 (SiO2) and have diameters ranging from 0.02 mm to 2.0 mm.

Properties of Sandy Soil

Sandy soil is coarse-grained and gritty
It is loose with large pore spaces
It absorbs and loses water easily.
It is not sticky when wet. Hence, it cannot form a cast or ribbon.
It is well aerated, with a low water-holding capacity.
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 supports leaching. Hence, it is low in plant nutrients.
It does not support waterlogging and erosion
Sandy soil is low in plant nutrients. Hence, it does not support crop cultivation.
Sandy soil has a grey or brownish colour.

Methods of Improving Sandy Soil

Sandy soil can be improved through the following agricultural practices:

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

Economic Importance of Sandy Soil

It is good for the cultivation of a few crops such as cotton, groundnut, cassava, e.t.c
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 the clay particles 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.

Clay particles are fine, powdery, and smooth when dry.
The particles are sticky and mouldy when wet.
The particles are tightly bound together with little pore (air) spaces
The structure is granular and does not lose water easily.
It is poorly aerated with high water holding capacity.
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.
It can easily form a ribbon or cast when moulded.
It supports waterlogging and erosion
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 a 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

Loamy soil is moist, loose, with moderately sized pore spaces.
The structure breaks easily when wet and is friable when dry.
It has a non-powdery and non-sticky texture.
It can easily be worked on or cultivated.
It contains lots of organic matter (humus).
It does not support erosion and waterlogging.
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.
It is dark-brown or black in colour

Importance/Effects of Organic Matter on Agriculture

It is very rich in plant nutrients.
It is the habitat of many soil microorganisms.
It prevents leaching in soil where it is present in an adequate amount.
It also prevents soil erosion and evaporation of soil water
It allows for good drainage and holds water in the soil for plant use.
It improves the structure of the soil by binding the particles of coarse-texture soil together.
It increases the water-holding capacity of the soil
It moderates that soil temperature which stimulates good development and growth of roots.
It has a 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:

Physical properties of the Soil: these properties include soil texture, soil structure, soil temperature, porosity, soil colour, and water holding/retaining capacity.
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 a 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 the soil profile are:

The A-Horizon, also called the topsoil, represents the surface layer of the soil profile. This horizon contains more organic matter than other horizons. The A horizon, the topsoil, is the most weathered and leached of all the soil horizons. Most food crops, especially shallow-rooted crops, derive their nutrients from this topsoil, e.g, vegetables, legumes. E.tc
B Horizon: The B-horizon, also called the subsoil, is the next horizon immediately after the topsoil. 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.
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.
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 the soil profile includes:

Level of Soil fertility: The Soil profile determines the level of soil fertility. A thick topsoil represents ha igh level of soil fertility.
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, etc. are grown in the top soil, while deep-rooted crops are grown where the sub-soil is thick.
Penetration of roots: A loosely packed sub-soil allows for easy penetration of the roots of crops.
Level of drainage and aeration: A loosely packed sub-soil also allows for easy drainage and aeration.
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 names 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:

By feeling: Take a little sample of soil and rub it between the forefinger and the thumb. A sharp feel represents the presence of sand, while a smooth or powdery feel represents the presence of clay.
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 that can pass through a particular mesh belong to the corresponding grade of soil.
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.
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:

It is useful in the evaluation of soil’s ability to supply mineral nutrients.
It supports soil microorganisms essential for plant growth.
It determines the type of crop to grow on the land.
It enables the farmer to know the type of soil in his farmland.
It determines the relative proportion of air and water in the soil.
It determines the movement of air and water.
It helps to determine tillage practice to be adopted by the farmer.

Soil structure

Soil structure refers to how the different particles of the soil are packed or arranged. It also refers to the shape and arrangement of primary particles forming 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.

Planting cover crops
Mulching
Application of manure (green manure and lime).
Avoidance of over-grazing and erosion.
Avoidance of cleaning with machines.

Types of Soil Structure

The different types of soil structure are:

Single-grained Structure: In this structure, the primary particles exist in single form and are not cemented together. It is found in sandy soil.
Crumb Structure: There exist large gravels or stones embedded within the primary particles, which are cemented together. It is found in the topsoil
Plate-like Structure: The primary soil particles are arranged horizontally and flat, resembling plates or leaflets on top of each other It is commonly found in the sub-soil.
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 topsoil.
Prismatic Structures: This could be columnar or prismatic. They are just like spheroidal structures 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 subsoil.
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 subsoil.

Importance of soil structure

It determines the level of fertility of the soil
A good soil structure supports aeration
It also prevents erosion and waterlogging.
A good soil structure promotes the activities of soil micro-organisms.
A good soil structure has a good water-retaining capacity.
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.

Soil temperature affects the formation of soil organic matter.
Soil temperature affects the decomposition of soil organic matter.
It determines seed germination and not development
It also affects the population of soil microbes.

Soil pH

The word 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 hydrogen ions (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:

Leaching: The washing down of plant nutrients below the soil beyond the reach of the plant roots, leaving behind hydrogen ions.
Use of acid fertilizers: The use of acid fertilizers like ammonium sulphate and ammonium nitrate can easily cause acidity in the soil.
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.
Nutrient uptake by plants: The absorption of soluble minerals by plants results in the accumulation of hydrogen ions, which cause soil acidity.
Presence of sulphur in the soil: sulphur undergoes oxidation and dissolution to form a 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:

Limestone (calcium carbonate) – CaCo₃
Slaked lime – Ca (OH)₂
Quick lime – CaO.
Calcium bicarbonate – Ca(HCO₃)₂
Wood ash (calcium phosphosilicate – Ca₄ (PO4)₂ SiO₄
Dolomite or Calcite – Ca (HCO₃)₂
Basic slag – Ca₃ (OHSiO₃)₂
Gypsum – CaSO₄ 2H₂O
Organic manure can also be applied to the soil to remove soil acidity.

Questions:

What is soil
State five Characteristics of sandy soil
What is soil structure
What is soil acidity
State five causes of soil acidity