Soils
Senior 1 · Geography
Soils
Lesson 3 of 4
24 min
After a heavy storm in the long rains, puddles lie for days on the rice fields in the valley bottom, while an hour after the rain the sandy path up the hill is dry. The difference is not the amount of rain: both got the same storm. The difference is in the soil, and you can measure it with three funnels and a jug of water.
Definition
Porosity: the amount of empty space (pores) between the particles of a soil or rock, given as a percentage of its total volume. The pores hold air and water. Example: a soil with a porosity of 40% has 40 cm³ of pore space in every 100 cm³.
Definition
Permeability: how easily water can pass through a soil or rock. It depends on the size of the pores and how well they are connected, not just on how many pores there are. Example: sand is highly permeable; clay is almost impermeable.
Definition
Infiltration: the soaking of water from the surface down into the soil. When rain falls faster than the soil can take it in, the extra water runs off over the surface, causing erosion and floods.
Aim: to compare the permeability of sand, loam and clay.
You need: three funnels, filter paper, three measuring cylinders, a stopwatch, a jug of water, and equal amounts (for example 200 g) of dry sand, loam and clay soil.
Method:
Fair test: only the type of soil changes. Keep the same mass of soil, the same volume of water, the same funnels and the same time.
Figure 1. Apparatus for comparing how fast water drains through three soils. The readings are the class results used in the table below.
| Soil | Time to first drop (s) | Water collected after 10 min (cm³) | Water kept in the soil (cm³) |
|---|---|---|---|
| Sand | 8 | 70 | 30 |
| Loam | 35 | 45 | 55 |
| Clay | 210 | 10 | 90* |
*Much of the "kept" water in the clay funnel was still lying on top, not soaked in. These are typical class results; your numbers will differ, but the order should be the same.
Worked example 1: analysing the results
(a) What percentage of the water drained through the sand in 10 minutes?
(b) What was the average drainage rate for the loam, in cm³ per minute?
(c) How many times faster did water drain through sand than clay? times.
Conclusion: sand is the most permeable, clay the least, and loam lies between.
Figure 2. Why clay can hold MORE water than sand but let LESS through.
Surprisingly, clay usually has a higher porosity than sand: its millions of tiny pores add up to more empty space. But the pores are so small that water clings to the flakes and can hardly move, so clay has a low permeability. Sand has fewer, larger, well-connected pores, so water rushes through. Loam, with a mixture of pore sizes and a crumb structure, drains well yet keeps enough water for plants.
Worked example 2: measuring porosity
A student fills a beaker with 200 cm³ of dry sand, then slowly pours in water until it just reaches the top of the sand. It takes 76 cm³ of water. Find the porosity.
The water filled every pore, so its volume equals the pore space.
Warning
High porosity does not mean high permeability. Clay can hold more water than sand but lets far less through. In an exam, always say which property you mean.
Important
Case study: permeability and Rwanda's land
Porosity and Permeability, by GeoScience Videos (6.5 min). Watch the bench-top demonstration and note the difference between the water a sediment holds and the water it lets through.
Your school plans to pave the whole playground with concrete. Using the ideas of permeability and infiltration, explain what might happen to rainwater after a storm and suggest two ways to reduce the problem.
Tip
Porosity = how much it HOLDS; permeability = how fast it FLOWS. A sponge full of tiny holes (clay) holds a lot but drips slowly; a basket of stones (sand) holds little but drains at once.
Practice
Remember
Sources and credits
Written by Ganzaa: the lesson text, experiment, apparatus and pore diagrams, results table, worked examples and practice. The results are typical classroom values, not a published data set.
