Does Soil Contains Small Amounts of Air and Water (Moisture) in it ? Explain

The concept of pH, a measure of acidity or alkalinity, is a key aspect of Chemistry Topics, particularly in acid-base reactions.

How will you Prove that Soil Contains Air in it?

Air is present in the spaces between the soil particles. This air provides the oxygen required for respiration by the roots of plants and other organisms (which live in soil). The presence of air in the soil can be shown by performing a simple activity as follows.

Activity 2

We take some dry soil in a beaker. Add water to the beaker and stir the soil and water with a glass rod for a while. We will see the bubbles coming out of the soil (see Figure).

These bubbles are of the air which was present in spaces between the soil particles. Actually, when we stir the soil with water, then water enters the spaces between the soil particles and expels the air present there. This expelled air is seen in the form of bubbles coming out from the soil. So, this activity shows that soil contains air.

Sand particles are quite large. Sand particles cannot fit closely together, so there are large spaces between sand particles. The large spaces between sand particles are filled with air. Due to this, sandy soil provides much more air to the plant roots. On the other hand, clay particles (being much smaller) pack tightly together leaving little space for air. So, clayey soil provides much less air to the plant roots which grow in it.

Soil Contains Water (Or Moisture)

Soil always contains some water in it which is called ‘soil moisture’. Water is usually present as a thin film around the soil particles. It is absorbed by the roots of the plants.

The capacity of a soil to hold water is important for the growth of various crops. Even a dry looking soil has some water (or moisture) in it. The presence of water (or moisture) in the soil can be shown by performing an activity as follows.

Activity 3

We take some dry soil in a hard glass beaker and cover it with a lid (such as a watch glass). Heat the beaker by using a burner. We will see that tiny drops of water are formed on the inner side of the lid (see Figure).

This shows that even the soil which appears to be dry to us, contains some water. Actually, there is an invisible film of water around the soil particles. When soil is heated, then water present in it evaporates forming hot water vapour (or steam).

This hot water vapour rises, gets cooled and condenses on the cold, inner surface of the lid to form drops of liquid water (see Figure).

The presence of water (or moisture) in the soil leads to an interesting phenomenon which can be easily observed by us. If we pass through a farmland during a hot summer day, we will see that the air above the soil is shimmering (shining with a slightly shaking light).

This happens as follows : The soil in the farmland contains some water. On a hot summer day, this water of soil evaporates to form water vapour. The water vapour coming out of the soil reflects the sunlight irregularly due to which the air above the soil seems to shimmer.

To Find the Percentage of Water (or Moisture) in a Soil

We will now describe an activity to find out the water content (or moisture content) in a given sample of soil. It is found in terms of percentage’ which means ‘water present in 100 grams of a soil’. This can be done as follows.

Activity 4

1. Take some soil and weigh it on a balance. This will give us the original mass of soil taken.
2. Place this soil on a newspaper and keep it in bright sunshine to dry. Allow the soil sample to dry for about two hours. After drying, weigh the soil again on a balance. This will give us the mass of dry soil.
3. Subtract the mass of dry soil from the original mass of soil. The difference in the mass of soil’before and after drying’ will give us the mass of water (or moisture) present in the original mass of soil taken.
4. The percentage of water (or moisture) in the given sample of soil can then be calculated by using the
   formula :

Percentage of water in soil = \(\frac{\text { Mass of water in soil }}{\text { Original mass of soil }}\) × 100

The calculation of water content in a given sample of soil will become clear from the following example.

Example Problem.
200 grams of a soil is taken and dried completely in bright sunshine. The mass of dried soil is found to be 170 grams. Calculate the percentage of water (or moisture) present in the given sample of the soil.
Solution:
In this case :
Original mass of soil = 200 g (g = gram)
Mass of dried soil = 170 g
So, Mass of water in soil = 200 – 170
= 30 g
We know that:
Percentage of water in soil = \(\frac{\text { Mass of water in soil }}{\text { Original mass of soil }}\) × 100
= \(\frac{30 \mathrm{~g}}{200 \mathrm{~g}}\) × 100
= \(\frac{30}{2}\)
= 15 per cent
Thus, the given sample of soil contains 15 per cent water in it. We can also say that the given sample of soil contains 15 per cent moisture in it. By saying that a soil contains 15 per cent water, we mean that 100 grams of this soil contains 15 grams of water.