Rocks and soil
Recognise that soils are made from rocks and organic matter, and that different soils have different properties
What a learner can do afterwards
- State that soil is a mixture of broken-down rock particles and decomposed organic matter (humus)
- Compare at least two types of soil (sandy, clay, loam) and describe their different properties
- Explain that soil formation takes a very long time as rocks gradually break down
1 · Read
You have compared rocks by their properties. Soil contains mineral particles from broken-down rock. It also contains organic matter from once-living things, plus air, water and living organisms. Humus is well-decayed organic matter. Soil is a mixture, not just crushed rock.
Weathering breaks down rock. Water can enter cracks and freeze, widening them. Growing roots can widen cracks too. Small rock particles can mix with decaying material as soil develops. This takes a long time. A thin soil layer can take hundreds of years or longer to develop.
A taller test bar means more water drained through in the same time. Sandy soil usually drains quickly: its larger particles leave larger spaces. Clay has smaller particles and small spaces that hold water, so it usually drains more slowly.
Sandy soil feels gritty. Wet clay can feel smooth and sticky. Loam mixes sand, silt and clay particles. Silt is smaller than sand but larger than clay. Loam often suits garden plants because it holds some water while letting extra water drain. It can contain humus too.
Soil contains mineral particles, organic matter, air, water and life. It develops slowly. Soil types differ in particle sizes and drainage.
All five readings written for this dataset.
More to read
Soil below the surface
Soil can have layers with different colours and textures. A layer near the surface may contain more decayed plant material. A deeper layer may contain more rock fragments.
The layers vary between places. They do not always have the same thickness or ingredients. Scientists study these differences to learn about a soil's history.
Spaces between soil particles
The spaces between soil particles can contain air or water. Their size affects drainage.
Sandy soil usually has larger spaces, so water passes through quickly. Clay usually has smaller spaces that hold water longer. Particle size is not the only influence. The way particles fit together and the organic matter present also affect drainage.
Comparing soil samples
A fair drainage comparison uses equal amounts of soil and water, matching containers and the same waiting time. Otherwise, a result might reflect the setup rather than the soil.
Measure the water that passes through each sample. More collected water means more drained through during that time. The result describes those samples, not every soil of that type.
Earthworms in their soil home
Earthworms eat organic material and mix soil as they move. Their tunnels can provide paths for air and water.
You can observe a worm without collecting it. Ask an adult before looking under a loose stone or leaf. Leave the worm in its home and gently replace its cover. Wash your hands after touching soil.
Rock particles are one ingredient
Weathering can break rock into smaller particles and change some minerals. These materials can become part of soil.
Crushing a stone gives you rock pieces, not a complete soil mixture. Soil also contains organic matter, water, air and living organisms. Learning about soil means looking at all of these ingredients.
2 · Watch
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