The earth beneath our feet

Yasmin Jayathirtha

After looking at experiments that we can do with ‘earth in a bottle’ it is time to come down to earth and consider what we can learn about the soil in our neighbourhood. We have learnt that environmental factors decide the type of soil present in a place and deforestation and runoff can degrade the quality of soil and this has an effect on the vegetation that can grow there. A lot can be learnt by observation, but some very simple experiments can illustrate the ideas effectively.

Soil is that part of the earth where life flourishes and is divided into top and sub soil. Soil is made up of six main components: rock particles, soil water, organic matter (humus), air, lime and mineral salts. It is the combination of these that makes up the quality of the soil. We can look at each of these factors in turn.

Equipment:

• Gas jars with lids
• Test tubes
• Beakers
• Crucibles – steel katoris can be used
• Funnels and filter paper
• Measuring cylinders
• Tin cans
• Concentrated hydrochloric acid
• pH papers/universal indicator solution
• Access to a balance
• Access to an oven
• Desiccator – a simple one can be made by taking a stainless steel box with a tight fitting lid. Use silica gel desiccant as the drying agent

Soil particles: The size of the particles vary from place to place. Very degraded slopes will have large particles, while tank and pond bottoms will have very fine silt that has been washed down.

Collect samples of soil from various places. Get a sample of sand for comparison.

Half fill a gas jar or test tube with the soil sample. Add water till it is about three-quarters or more full. Notice the air bubbling out. Cover and shake. Let it stand. The soil settles down with the heaviest particles at the bottom and humus floating on top. It is a very graphic illustration of soil types.

Soil Water: Weigh an empty crucible or katori, and half fill it with soil. Weigh the crucible and contents. Keep in an oven at 100º C for at least half an hour. Remove and cool in the desiccator. Reweigh after cooling. Repeat till there is no further change. The difference in weight gives the water present in the soil and we can calculate the percentage of water present. This experiment can be done simultaneously with different samples. We can repeat this experiment after a rain and one or two days later.

Humus: After the experiment on determining the water content, heat the soil sample strongly on a bunsen flame, stirring once in a while. The organic content burns away. Cool in the desiccator and weigh after cooling. If you use katoris, they will become discoloured, but can be kept for lab use. Repeat with different samples.

Lime: Take a small amount of soil in a test tube. Add a few drops of concentrated hydrochloric acid to it. Does it fizz? If it does, the soil contains lime.

The pH: Take a sample of soil in a test tube, add distilled water, shake and allow to settle. Dip a piece of pH paper in the water and check the value. Is the soil acid or alkaline? Where does the most acidic soil come from? What grows in each type of soil?

Air: Take a 100 cm³ measuring cylinder. Fill the cylinder with soil till the 50 cm³ mark, tapping the cylinder to make sure the soil is settled. Add water carefully till the 100 cm³ mark. Stir the soil with a glass rod till all the air has bubbled out. What is the new water level now? We can assume that the difference between the two levels is the amount air in the soil sample. Of course, we have to be careful to make sure that the soil is well settled but not packed down.

Water holding capacity: Soils differ in their capacity to ‘drain’. Some soils (sandy) let water run through while others (clay) will hardly let water penetrate. Mulching with organic material is said to improve both the drainage and the water holding capacity. Take 100 cm³ measuring cylinders, put funnels on top. Put filter paper and a measured amount of soil in each funnel. Tap the funnel to settle the soil. Pour 30 cm³ of water into each funnel and note the volume of water drained out in a measured time. Take the fastest draining and the slowest draining soil and mulch with organic matter – it can be dried leaves, manure. Repeat the experiment and see if mulching makes a difference to drainage.

Good soil is one that is neither too acidic or basic, retains water and air, has organic content and drains well. All these qualities are related to each other and usually depend on the amount of organic matter in the soil. That said, life is inventive and plants and other organisms have evolved to live in soil that is not ideal – from the cacti in the deserts to the carnivorous plants in acidic boggy soils.

Funny molecules

☺ Two sodium atoms were walking down the street, when suddenly, one of them started rummaging in his pocket. “I have lost an electron.” “Are you sure?” says the other. “Yes, I am positive.”

☺ What was the raven looking for in the chemistry lab? A Chromate (crow mate).

☺ All sugars have to have a name ending in -ose like glucose, lactose… A very famous biochemist Albert Sentz-Georgi discovered a new sugar. Not knowing its exact composition, he wanted to name it godnose.

☺ Why did the white bear dissolve in water, but not the brown one? The white one was polar.

☺ The neutron walked into his friend’s bar and ordered a drink. When he asked the bartender “what do I owe you” The bartender replied “for you, no charge”.

The author works with Centre for Learning, Bengaluru. She can be reached at yasmin.cfl@gmail.com.