Soil: sand, silt, clay and layers
Why does water vanish into sandy soil but make puddles on clay? Discover what soil is made of, the layers you would see if you dug a deep hole, and run a fair test on how fast water drains through different soils.
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A typical soil is about half solid material and half gaps (pores). In a typical soil, about 45% is minerals from broken rock, 5% is organic matter, 25% is air and 25% is water.[1][2]
The mineral part comes from rock that has been broken down by weather, water and living things. This rock is called the parent material. Soil forms over hundreds to thousands of years.[1][2]
Scientists sort the mineral grains by size. Sand grains are the biggest (2 mm down to 0.02 mm), silt is smaller, and clay particles are the tiniest, smaller than 0.002 mm.[1]
Water drains quickly through sandy soil because there are big spaces between the grains. Clay holds water well but drains very slowly, because the spaces between the particles are tiny. A soil with a good mix of sand, silt and clay is called a loam.[8][1]
If you dig a deep hole, you see layers called horizons. On top is a layer of leaves and rotting plants (O). Below is dark topsoil, where most roots grow (A). Then comes subsoil, which is harder to dig (B), and then broken-up parent rock (C), with solid bedrock underneath.[1][2]
Dead plants and animals rot into dark humus, which helps soil hold water and gives plants minerals. Fungi and bacteria do much of this recycling, and earthworms mix leaves into the soil.[2][7][6]
About 95% of our food is grown directly or indirectly in soil. But about a third of the world's soils are damaged, for example by erosion, so protecting soil matters.[3]
Try it: Fair test: which soil drains fastest?
You need
- Three clear plastic cups with a few small holes poked in the bottom (by an adult)
- Three more cups to catch the water
- Sand, clay-rich soil and garden compost (or loam)
- A measuring jug and water
- A stopwatch
- Coffee filters or kitchen paper
Safety
An adult makes the holes in the cups. Use clean bought sand and compost or soil from a clean place, never where pets go. Wear gloves if you like and wash hands afterwards. Mop up spills.
What to do
- Line each holed cup with a coffee filter and stand it on an empty cup.
- Fill each lined cup to the same level with one type of soil. Do not press it down.
- Predict: through which soil will the water drain fastest?
- Pour exactly 100 ml of water onto the first soil and start the stopwatch.
- Stop the watch when water stops dripping out, or after 5 minutes. Measure how much water came through.
- Repeat with the other soils. Keep the cups, the amount of soil and the amount of water the same.
What happens, and why?
Sand has big gaps between its grains, so water rushes through. Clay particles are tiny and pack tightly, so the gaps are tiny and the water moves very slowly; clay also holds on to more of the water. Changing only the type of soil, and keeping everything else the same, makes it a fair test.[8][1]
Play: Which soil layer?
Imagine digging a deep hole. Which horizon does each description belong to?
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Quiz
Other ages: Ages 6–7 · Ages 15–16