SOIL PROFILE

Soil Profile

Diagram of soil profile

Soil Profile: A vertical section cut through the soil that exposes its morphology is referred to as a soil profile.

Horizons of soil profile

The four major horizons of soil profile are:

1. A-Horizon: Also called the top soil, represents the surface layer of the soil profile. This horizon contains more organic matter than other horizons.

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.

3. C-Horizon: The C-horizon, also called parent materials, represents the type of material from which top soil and sub-soil are derived.

4. D-Horizon: The D-horizon is also called the bedrock. It represents the unweathered rock materials.

Fig. 30: Soil Profile

• O Horizon
• A Horizon
• B Horizon
• C Horizon
• R Bedrock
• Solum

Importance of Soil Profile

1. The suitability of a soil for agriculture can be determined by looking at the soil profile.
2. A farmer can decide roughly what crop is best to grow.
3. Determines the level of soil fertility.
4. A loosely-packed sub-soil allows for easy penetration of roots of crops.
5. A loosely-packed sub-soil allows for easy drainage and aeration.

Soil Acidity (pH)

The word pH (Pondus de Hydronium) is a measure of the degree of alkalinity or acidity of a substance like soil. The measuring scale is graduated from 1 to 14. A soil whose pH is below 7 is said to be acidic and above 7 is alkaline. Addition of a soil being acidic is most common in Nigeria compared to the alkaline ones. An acidic soil is not usually good for cultivation of most Food crops. It is corrected by application of lime fertilizer.

Soil pH

It can also be described as a measure of the concentration of hydrogen ions (H⁺) in the soil. A higher hydrogen ion (H⁺) concentration indicates soil acidity while a lower concentration of hydroxyl ions (OH⁻) indicates soil alkalinity.

Causes of soil acidity: Soil acidity can be caused by any of the following:

(i) Leaching
(ii) Use of acid fertilizers.
(iii) Presence of acid parent materials.
(iv) Nutrient uptake by plants.
(v) Presence of sulphur 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:

(i) Limestone (calcium carbonate) — CaCO₃
(ii) Slaked lime (calcium hydroxide) — Ca(OH)₂
(iii) Quick lime (calcium oxide) — CaO.
(iv) Calcium bicarbonate — Ca(HCO₃)₂.

Effects of soil acidity

1. Low pH or high acidity causes the disintegration of clay minerals which are leached from the soil.
2. Low pH causes the accumulation of aluminium and manganese which may be toxic to plant roots.
3. Low pH reduces the population of some useful organisms such as bacteria and earthworms.

Soil Alkalinity

Causes and removal of soil alkalinity

Soil alkalinity is caused when there are excessive quantities of soluble minerals in the soil. Soil alkalinity can be removed by:

1. Application of irrigation to dissolve some of the salts.
2. Application of some acid fertilizers like ammonium sulphate.
3. Application of sulphur to the soil.

Soil water

Soil water refers to the water in the soil which is usually obtained either from rain or irrigation.

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

(i) Hygroscopic water.
(ii) Capillary water.
(iii) Field capacity.
(iv) Gravitational water.

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 be absorbed by plant roots.
3. Water is an essential raw material for photosynthesis.
4. Hydrolysis of many food nutrients and enzymatic activities are promoted by the presence of water.

Soil air

Soil air refers to the gases present in the soil pores found between the soil particles.

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.
3. Soil organisms require oxygen for respiration.
4. Excess of carbon dioxide in the soil when combined with water can cause acidity and aid weathering of rocks.

Soil living organisms

These refer to plants and animals which inhabit the soil. The most commonly found groups of soil organisms include bacteria, fungi, viruses, nematodes, insects (e.g. termite, soldier ants).

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.
3. They help to decompose organic materials in the soil to form humus.
4. They improve soil water percolation or drainage.

Soil Colour

This is the first characteristic of soil you will notice. Colours of horizons in each profile vary. Top soil is of a similar uniform colour due to the presence of humus. Sub-soils are of more sticking colours. Drainage affects the colour of soil.

Well-drained soils in West Africa are often red or reddish brown. Mottling is common in soils which are dry at certain times of the year, and submerged in water at other times.

Soil colour is altered by the amount of water present in the soil at any particular time. Some soils change colour very markedly when water is added, while others change only slightly.

When describing soil colour it is therefore always necessary to state whether the soil is wet, moist or dry.