Percentage Composition Of Soil Constituents

Back to: BIOLOGY NEW LOWER SECONDARY CURRICULUM BOOK 2

Activity 1.4(a): Determing the percentage of air in the soil

The learners should remember the constituents of soil studied in the section 1.2. Guide them to find the percentage of air, water and humus in the soil through carrying out the following activities.

Key question: What is the percentage of air in the soil? What you need

• measuring cylinder(2)
• dry soil sample
• water              
• glass rod

What to do

1. Put about 50cm³ of dry soil in a
   measuring cylinder and tap the cylinder with a finger to compact or level out the soil.
2. Pour 50cm³ of water in another
   measuring cylinder.
3. Add the 50 cm³ of water onto the soil in the measuring cylinder. Take the reading of the mixture. (Carefully observe what happens as you pour the water onto the soil).

Stir the mixture and leave it to settle until no more bubbles appear.

1. Read and record the final volume of the
   mixture in the measuring cylinder.
2. Calculate the volume of air in the soil
   sample.

Volume of air = (Volume of soil + volume of water added) – (final volume of mixture in measuring cylinder)

% of air = volume of air  x 100

        volume of soil

• Repeat the procedure (1-6) using soil samples obtained from two different locations around your school.
• Make a conclusion on the soil sample collected.
• Present your findings to the rest of the class.

Exercise

Lufafa, a Senior two student, carried out an investigation to determine the percentage of air in a given soil sample and obtained the following values: Volume of soil used = 40cm³

Volume of water added to soil = 50cm³ Final volume of water + soil after mixing = 80cm³

• Determine the volume of air in the soil sample.
• Calculate the percentage of air in the soil sample.

Suggested responses

1. 10cm³
2. 25cm³

Activity 1.4 (b): Determing the percentage of water the soil

                 Duration: 80 Minutes

Key question: What is the percentage of water in the soil? What you need

• evaporating dish or tin lid
• fresh soil               
• weighing scale
• Bunsen burner/stove

What to do

1. In groups, weigh a clean evaporating
   dish and record its mass.
2. Fill the evaporating dish with fresh soil and
   record the mass of the evaporating dish
   containing soil.
3. Dry the soil by heating it gently over
   a Bunsen burner flame or a stove for
   about 20 minutes.
4. Allow the soil to cool and weigh it again.

weighing scale

  dry soil sample being                                          reweighed

Figure 1.8: Steps for determining the percentage of water in soil sample

• Continue heating and weighing until
  you obtain a constant mass.
• Reweigh the evaporating dish containing
  soil and record its mass after heating.
• Obtain the mass of water.

Mass of water = (mass of evaporating dish containing soil before heating) –(mass of evaporating dish containing soil after heating)

8.    Determine the percentage of water in
the soil sample.

Percentage of water Mass of water

Original mass of soil sample

To get the original mass of the soil sample, we subtract the mass of the clean evaporating dish without soil from the mass of the evaporating dish with soil before heating.

• Repeat the procedure (2-8) using soil samples from two different locations around your school.
• Make a conclusion on the percentage of water in the different samples. What do the results tell you about type of soil samples obtained.
• Present your findings to the rest of the class.

Suggested responses The values obtained in experiments 1.4 (a), (b) and (c) vary from one individual to another, but an exercise follows immediately to cater for the learners’ practice and understanding of the given formulae.