O Level Notes : Geography - Ecosystems - Soil Characteristics

Sandy soils have high proportion of sand and little clay. they are well drained and light coloured. water holding capacity is low. Clay soils are made up of over 25% clay. They have high water holding capacity because of high nutrient content. The soil particles are well bound together hence clay soils tend to be poorly drained. They also are also dark coloured because of high organic content. Silt soils are made up of medium size particles which contain organic matter. Loam is a mixture of sand, silt and clay. It is suitable for growth of various types of crops.

Soil texture

Refers to the coarseness and fineness of the soil. This coarseness or fineness is determined by the proportion of clay, sand and silt in the soil. Some textures are coarse while others are fine.

Soil Texture Triangle

The percentage of clay is read on the left hand side of the triangle, the percentage of silt is read on the right hand side and the percentage of sand at the bottom. For example, a soil containing 20% clay, 40% sand and 40% silt is loam. It has the texture of loam soil. The different soils combine to form certain soils as shown in fig 3.7. The percentage of the soils combining together determine the resultant soil type. Some soils are formed due to the combination of two or more soils hence their characteristics are derived of the soils combined to form them.

Soil texture tests

The texture of the soil can be tested using a cheap and simple way. To test for soil texture, follow the next steps:

• Take soil samples from different points. The samples are taken by digging using a shovel or trowel. Identify points from where to take sample by sampling.
• Put the soil drawn into well labelled polythene bags. The labels should show points from where the samples were taken.

• Take the samples to the Geography laboratory.
• At the laboratory take the soil samples and make them moist. Make sure the soil is moist and not wet.
• Rub the moist soil between the forefinger and the thumb by firmly squeezing the moist soil in your hand. 

Clay soil: due to its texture it feels sticky and you woul mould it into a shape. Such soils retain moisture and nutrients. However, they are poorly drained and can hinder penetration of roots when dry. 

Sandy soil: this soil will feel gritty and fall apart. It has poor water holding capacity. Roots will penetrate easily but need constant application of water. 

Loamy soil: this soil will mould  it into shape, but crumbles when slightly poked. It is a combination of clay, sand and silt. 

After completing the tests record the results for use. Farmers are interested in the soil texture information since certain textures support certain types of crops.

Soil structure

Soil structure refers to the way soil particles are arranged. Soil particles are arranged into units called peds. These peds appear in different forms giving different structures such as:

1. Granular

The soil particles are small and rounded. They are in the form of small grains which are not bound together. This structure is well- aerated and is ideal for productive crop farming.

2. Platy

Granular soil structure

The soil particles are not like grains. They are horizontal and flat. The peds are plate- like. Such soils are vulnerable to water logging.

3. Blocky

The soil peds are block- like. The peds are irregular in shape and such soil structure is not suitable for root development. It cannot support productive agriculture.

Blocky soil structure

4. Prismatic and columnar

Prismatic soil structure has peds resembling prisms in shape. The peds of the soil are long and vertical. They are bounded by flat to slightly rounded vertical faces. The soils are well drained but impede lateral development of root systems.

 Prismatic soil structure

Soil colour

The colour of the soil is determined by the parent rock and organic content. A soil formed from a dark coloured parent rock will take the colour of the parent rock while light coloured parent rocks produces light coloured soils. Black soils tend to contain high quantities of organic matter. The colour is also determined by the minerals found in the soil. Soils containing more iron tend to be yellow, red or orange. Soils containing calcium and silica are light grey to white in colour.

  Red coloured soil

Red coloured soils contain more iron. While dark coloured soils (fig 3.13) contain more organic matter and is fertile for crop production.

 Black soil containing organic matter

Soil chemical composition 

Soils contain different levels of certain chemicals. Soil pH is a good measure of such chemicals contained in the soil. It shows acidity and alkalinity of the soil and it controls nutrients' chemical form. It is measured using the soil pH scale which ranges from 1 to 14. Measurements below 7 indicates that the soil is acidic and measurements above 7 indicates that the soil is alkaline and a pH scale of exactly 7 shows that the soil is neutral. 

Measuring soil pH

Soil pH can be measured using an electronic pH meter or using the litmus paper.

Electronic pH meter

Digital pH test meters 

To use these electronic pH testing meters follow the next steps:

1. Dig a small hole on the ground with a trowel or a spade.
2. Fill the hole with distilled water. Distilled water has neutral pH. Fill the hole with the distilled water until the soil is muddy.
3. Insert the meter's test probe into the muddy soil for 60minutes.
4. Read the pH level on the meter's results display. The meter will show the soil's accurate pH level.

The litmus paper

To measure soil pH the litmus paper can also be used. The next steps should be followed:

1. Put soil samples into a test tube.
2. Mix the soil with distilled water. Make sure the soil is muddy.
3. Insert the litmus paper into the test tube with muddy soil.

The soil pH scale

Task: Testing for Soil pH

-Take soil samples from different parts of your school garden.

- Test the soil pH using the litmus paper.

- How is such information helpful to farmers?

NB: pH stands for potential hydrogen. A pH value below 7 indicates that the soil is acidic and 1 is the most acidic since it ranches from 1 to 14. The values above 7 indicates alkalinity which increases as the number increases, 14 being the most alkaline. Seven in the middle point between 1 and 7, it indicates neutrality in terms of acidic and neutrality.

Table 3. 2: Soil depth

Soil depth is an important characteristic of soil as it determines plant growth. Steep slopes usually have thin or shallow soils because of rapid wearing of the top soil whereas gentle sloping slopes attract soils from the steep segments of the slopes hence they grow deeper soils. The depth of the soils affect agriculture. On shallow  soils  farmers  have  to  apply  more  irrigation  water  and  to  irrigate frequently because plant roots do not go deeper on shallow soils.

Here is what we discussed in this topic

• Minerals, air, organic matter and water are the 4 major soil components.
• Microorganisms are also a soil component.
• Parent rock, organisms, climate, relief and time are the factors which affect soil formation.
• Leaching, cheluviation, eluviation and gleization are some of the tropical soil formation processes.

• Soil profiles  show  different  soil  horizons  which  are  identified  by differences in soil colour, mineral composition and stage of soil development.
• Soil characteristics  are  determined  by  texture,  structure,  chemical composition and depth.

• Soil pH, water holding capacity and texture can be measured scientifically.