Soil

Soil is a vital natural resource. It is formed by weathering, the slow breaking down of rocks by wind, water and climate, mixed with humus from rotting plant and animal remains. A vertical section through the layers of soil is called the soil profile, and each layer is a horizon. The A-horizon, or topsoil, is dark, soft and rich in humus and minerals; the B-horizon has less humus and more minerals and is harder; the C-horizon is made of small lumps of rock with cracks; and below lies the bedrock. Depending on the proportion of particle sizes, soil is sandy (large particles, loose, drains fast), clayey (fine particles, holds water, sticky) or loamy (a mixture of sand, clay and silt with humus, best for most plants). The rate at which water passes through soil is its percolation rate, found as amount of water (mL) / percolation time (min). Clayey and loamy soils suit wheat and gram, clayey soil rich in humus suits paddy, and sandy loam suits lentils. Trees reduce soil erosion.

Detailed Worksheet: Soil Section A - Definitions (10 marks) 1. What is weathering? How does it help in forming soil? (2 marks) 2. What is a soil profile? What is a horizon? (2 marks) 3. What is humus? Why is it important for soil? (2 marks) 4. Define percolation rate and write its formula. (2 marks) 5. What is soil erosion? Name two agents that cause it. (2 marks) Section B - Calculations and Applications (15 marks) 6. In an experiment, 200 mL of water took 40 minutes to percolate through a soil sample. Calculate the percolation rate. Another sample allowed 300 mL to pass in 25 minutes; find its rate and identify which sample is more sandy. (3 marks) 7. 50 g of soil is placed in a funnel with filter paper and 200 mL of water is poured. 150 mL is collected below. (i) How much water did the soil absorb? (ii) Is the soil likely to be sandy or clayey? (iii) Explain. (3 marks) 8. A sample of 100 g of moist soil weighed 88 g after heating in the sun for a day. Find the mass of water lost and the percentage of moisture in the soil. (3 marks) 9. Compare sandy, clayey and loamy soils on particle size, water holding capacity and air content. (3 marks) 10. Which soil is suitable for (i) paddy (ii) wheat and gram (iii) lentils and other pulses? Give a reason for each. Name one use of clay apart from farming. (3 marks) Section C - Diagrams (10 marks) 11. Draw a labelled diagram of a soil profile showing the A-horizon, B-horizon, C-horizon and bedrock, with one feature of each. (4 marks) 12. Draw the set-up of the activity to find the percolation rate of soil using a tin can open at both ends. (3 marks) 13. Draw a diagram showing the experiment to show that soil contains water, using a test tube heated over a flame. (3 marks) Section D - Analysis and Higher-order Thinking (15 marks) 14. A potter uses clayey soil to make pots. (i) Why is clayey soil sticky? (ii) Why is it good for pots? (iii) Why is it poor for growing most plants without humus? (iv) How can it be improved for farming? (5 marks) 15. In deserts, strong winds carry away the top layer of soil. (i) Name this process. (ii) Why is it more common where there is little vegetation? (iii) Suggest two ways to prevent it. (iv) Why is losing topsoil harmful? (5 marks) 16. Plastic bags, polythene and chemicals often end up in soil. (i) How do they pollute soil? (ii) Why can polythene harm soil organisms? (iii) Suggest two steps to reduce soil pollution. (iv) Why are earthworms called friends of farmers? (5 marks) Instructions: Time allowed 2 hours. Attempt all sections. Show the formula and units in calculations of percolation rate.