Chapter 5 of 8

All 8 chapters
  1. 1Three kinds of rock, readunderstood
  2. 2Inside the Earth, readunderstood
  3. 3Weathering and erosion, readunderstood
  4. 4The rock cycle, readunderstood
  5. 5Soil, readunderstood
  6. 6Caring for soil, readunderstood
  7. 7Fossils, readunderstood
  8. 8Reading Earth's history, readunderstood

S3.11.5 Chapter 5

Soil

What is soil made of?

Your guess

Pick one before you read on. It is fine to be wrong.

Your guess

  • You picked A. Read on. At the end, the chapter comes back to your guess.

  • You picked B. Read on. At the end, the chapter comes back to your guess.

  • You picked C. Read on. At the end, the chapter comes back to your guess.

By the end you can

  • Separate soil into layers by shaking it in a jar of water
  • Compare how fast water drains through sandy soil and clay soil

Take a handful of soil and look at it closely. What is in it? Soil is the thin top layer of the land. It is not just crushed rock. It is a mix of four things.

Soil has tiny grains of rock. It has humus, which is the rotted bits of dead plants and animals. It has water. And it has air in the gaps between the grains.

Dig down and you can see that soil has layers. The top layer is dark, with the most humus. Under it the soil has less humus and more rock. Far down there is usually solid rock.

A cut through the ground in four layers. The top is topsoil, dark with humus. Under it is subsoil, with more rock grains. Under that is broken rock, and at the bottom is solid bedrock. Labels: topsoil, subsoil, broken rock, bedrock.topsoilsubsoilbroken rockbedrock
Soil comes from rock that weathering broke up, and from humus.

The rock grains in soil come in different sizes. Sand grains can be seen, and they feel gritty. Silt grains are smaller and feel like flour. Clay grains are the smallest, too small to see one by one. Clay feels sticky when it is wet.

Draw it first

Draw a jar of soil, shaken and settled

  1. A jar of soil and water drawn twice in the same place, with a ruler in cm. First the jar has just been shaken, with grains of every size mixed through the water. Then it has settled in layers: small stones at the bottom, then sand, silt and clay, then water with bits of dead plant floating on top. Labels: soil and water, small stones, sand, silt, clay, water, humus.cm024681012soil and water

    1 Draw a jar with soil and water in it. Just after shaking, the grains and water are mixed through.

  2. A jar of soil and water drawn twice in the same place, with a ruler in cm. First the jar has just been shaken, with grains of every size mixed through the water. Then it has settled in layers: small stones at the bottom, then sand, silt and clay, then water with bits of dead plant floating on top. Labels: soil and water, small stones, sand, silt, clay, water, humus.cm024681012small stonessandsiltclaywaterhumus

    2 Draw it again after it has stood still. The biggest grains have sunk to the bottom, and the finest are at the top. Draw bits of dead plant floating in the water.

Try it

Shake soil in a jar

You need: a clear plastic jar with a lid, a few spoons of soil, water, a ruler.

Be safe: Use a plastic jar, never glass. Close the lid tight. Wash your hands after you touch soil.

  1. Fill the jar about one third with soil. Pour in water until the jar is nearly full. Close the lid tight.
  2. Shake the jar hard for one minute. Stand it somewhere that nobody will knock it.
  3. Look at the jar after one minute, after one hour and after one day.
  4. Measure each layer with the ruler and write down how many cm thick it is.

What you will see: The biggest grains sink first and the finest settle last. So the layers go small stones, sand, silt and clay from the bottom. Bits of dead plant float. The water may stay cloudy for a day, because clay is so fine.

If you cannot try it, this is what happens.
A jar of soil after one day, with a ruler in cm beside it. From the bottom, the small stones are 1 cm thick, the sand 4 cm, the silt 2 cm and the clay 1 cm. Water 3 cm deep is on top, with bits of dead plant floating in it. Labels: small stones, sand, silt, clay, water, humus.cm024681012small stonessandsiltclaywaterhumus

Why do the layers go in that order? A big grain is heavier than a small one. It sinks fast through the water. A tiny clay grain sinks very slowly. The humus is light, so it floats.

A table of grain sizes in mm. Stones are over 2 mm. Sand is 0.06 to 2 mm. Silt is 0.002 to 0.06 mm. Clay is under 0.002 mm. Labels: grain, size, stones, over 2 mm, sand, 0.06 to 2 mm, silt, 0.002 to 0.06 mm, clay, under 0.002 mm.grainsizestonesover 2 mmsand0.06 to 2 mmsilt0.002 to 0.06 mmclayunder 0.002 mm
Grain sizes get smaller from stones to clay.

The grains also decide how well water can drain, which means run down through the soil. Sandy soil has big grains with big gaps between them. Water drains through it fast.

Clay soil has tiny grains packed tight, with tiny gaps. Water drains through it slowly, so clay soil stays wet and sticky.

See it three ways

As a picture

Two measuring cylinders side by side, with a scale in mL. After 100 mL of water was poured on each soil, the cylinder under sandy soil holds 70 mL. The cylinder under clay soil holds 20 mL. Labels: sandy soil, clay soil, 0, 20, 40, 60, 80, 100, mL.020406080100mLsandy soil020406080100mLclay soil

In words

Water runs through sandy soil fast and through clay soil slowly. The soil with the bigger gaps drains faster.

In symbols

One test, for example: 100 mL poured on each soil. Sandy soil let 70 mL through in a minute. Clay soil let 20 mL through. Your numbers will differ.

Try it

Compare how soils drain

You need: two plastic cups, two jars, sandy soil and clay soil, water, a measuring jug, a clock.

Be safe: A grown up must make the holes and put the nail away. Wash your hands after you touch soil.

  1. Ask a grown up to poke three small holes in the bottom of each cup with a nail.
  2. Fill one cup half full of sandy soil and the other half full of clay soil. Stand each cup on top of a jar.
  3. Pour 100 mL of water into each cup at the same time. Wait one minute.
  4. Read how much water has dripped into each jar.

What you will see: More water comes through the sandy soil than through the clay soil. The clay holds the water back.

If you cannot try it, this is what happens.
Two cups of soil with small holes in the base, each dripping into a jar. The left cup holds sandy soil, shown with dots, and the jar under it holds a lot of water. The right cup holds clay soil, shown with crossed lines, and its jar holds only a little. Labels: sandy soil, clay soil.sandy soilclay soil

A wrong idea many people have

Soil is just crushed rock.

Why it feels right: Soil is brown and gritty, like ground-up stone.

The test that breaks it: Shake soil in a jar of water. Bits of dead plant float on top. Crushed rock alone would all sink. Plants grow poorly in pure sand.

What is true: Soil is rock grains mixed with humus, water and air. The humus holds water, and as it rots it gives plants the minerals they need.

Key words

soil
the thin top layer of land, made of rock grains, humus, water and air.
humus
the rotted bits of dead plants and animals in soil.
sand
rock grains you can see, between 0.06 and 2 mm.
silt
grains smaller than sand that feel like flour.
clay
the smallest grains, which feel sticky when wet.
drain
to run down through soil.

Back to your guess

The answer is A: Soil is bits of rock mixed with humus, water and air.

Soil is more than crushed rock. It holds humus from dead plants and animals, and it holds water and air. Its grains can be big or tiny.

Check yourself

Try each one before you look. A wrong answer tells you where to go next.

  1. You shake soil in water and let it settle. Which grains sink to the bottom first?

    Tap your answer.

    1. Not this one. Tiny grains are light and sink very slowly. They settle last, at the top of the soil layers. Go to S3.11.5. Try another.

    2. Not this one. Humus is light. It floats on the water. Go to S3.11.5. Try another.

    3. Yes, that is right. Big grains are heavier and sink fast. Fine grains sink slowly. That is why the layers go from big at the bottom to fine at the top.

    See the answer

    The biggest grains, such as small stones and sand

    Big grains are heavier and sink fast. Fine grains sink slowly. That is why the layers go from big at the bottom to fine at the top.

    • If you chose A: Tiny grains are light and sink very slowly. They settle last, at the top of the soil layers. Go to S3.11.5.
    • If you chose B: Humus is light. It floats on the water. Go to S3.11.5.

  2. Which layer in the jar is made of the finest grains?

    Tap your answer.

    1. Not this one. Sand grains are bigger than silt and clay grains. They sink earlier, so they are lower in the jar. Go to S3.11.5. Try another.

    2. Not this one. Small stones are the biggest grains in the jar. They sit at the bottom. Go to S3.11.5. Try another.

    3. Yes, that is right. Clay grains are smaller than 0.002 mm, so they settle last and form the top layer of soil.

    See the answer

    Clay

    Clay grains are smaller than 0.002 mm, so they settle last and form the top layer of soil.

    • If you chose A: Sand grains are bigger than silt and clay grains. They sink earlier, so they are lower in the jar. Go to S3.11.5.
    • If you chose B: Small stones are the biggest grains in the jar. They sit at the bottom. Go to S3.11.5.

  3. Bits of dead plant float on the water in the jar. What are they called?

    Tap your answer.

    1. Yes, that is right. Humus is the rotted bits of dead plants and animals. It is light enough to float.

    2. Not this one. Silt is made of tiny rock grains. They sink. The floating bits are from dead plants. Go to S3.11.5. Try another.

    3. Not this one. Sand is rock and it sinks. The floating bits are rotted plants, called humus. Go to S3.11.5. Try another.

    See the answer

    Humus

    Humus is the rotted bits of dead plants and animals. It is light enough to float.

    • If you chose B: Silt is made of tiny rock grains. They sink. The floating bits are from dead plants. Go to S3.11.5.
    • If you chose C: Sand is rock and it sinks. The floating bits are rotted plants, called humus. Go to S3.11.5.

  4. Which soil lets water drain through the fastest?

    Tap your answer.

    1. Not this one. Clay has tiny grains packed tight. The gaps are tiny, so water drains slowly. Go to S3.11.5. Try another.

    2. Not this one. The size of the gaps decides the speed. Big grains leave big gaps, and water runs through them quickly. Go to S3.11.5. Try another.

    3. Yes, that is right. Sandy soil has big grains and big gaps, so water runs through it quickly.

    See the answer

    Sandy soil

    Sandy soil has big grains and big gaps, so water runs through it quickly.

    • If you chose A: Clay has tiny grains packed tight. The gaps are tiny, so water drains slowly. Go to S3.11.5.
    • If you chose B: The size of the gaps decides the speed. Big grains leave big gaps, and water runs through them quickly. Go to S3.11.5.

  5. A drainage test pours 100 mL of water on clay soil. Only 20 mL comes through in a minute. What does this show?

    Tap your answer.

    1. Yes, that is right. Only a fifth of the water ran through. Clay has tiny grains with tiny gaps, so water moves slowly.

    2. Not this one. The numbers show the opposite. Sandy soil lets far more water through in the same time. Go to S3.11.5. Try another.

    3. Not this one. Clay grains are the smallest. Their tiny gaps are what slow the water. Go to S3.11.5. Try another.

    See the answer

    The tiny gaps hold the water back, so clay soil drains slowly.

    Only a fifth of the water ran through. Clay has tiny grains with tiny gaps, so water moves slowly.

    • If you chose B: The numbers show the opposite. Sandy soil lets far more water through in the same time. Go to S3.11.5.
    • If you chose C: Clay grains are the smallest. Their tiny gaps are what slow the water. Go to S3.11.5.

  6. From an earlier chapter

    A river drops sand in layers. The layers are buried and pressed for millions of years. What kind of rock may they become?

    Tap your answer.

    1. Not this one. Igneous rock forms when melted rock cools. Pressed layers of sand do not melt. Go to S3.11.4. Try another.

    2. Not this one. Metamorphic rock is changed by heat and pressure. Sand that is only buried and pressed becomes sedimentary rock. Go to S3.11.4. Try another.

    3. Yes, that is right. Buried, pressed and glued sediment becomes sedimentary rock, such as sandstone.

    See the answer

    Sedimentary rock

    Buried, pressed and glued sediment becomes sedimentary rock, such as sandstone.

    • If you chose A: Igneous rock forms when melted rock cools. Pressed layers of sand do not melt. Go to S3.11.4.
    • If you chose B: Metamorphic rock is changed by heat and pressure. Sand that is only buried and pressed becomes sedimentary rock. Go to S3.11.4.

  7. In 100 mL of water poured on clay soil, 20 mL came through. How much stayed in the soil?

    See the answer

    80 mL

    100 − 20 = 80. The clay held back 80 mL.

  8. The soil in a settled jar is 8 cm thick. The sand layer is 4 cm. What percent of the soil is sand?

    See the answer

    50 percent

    4 ÷ 8 = 0.5, and 0.5 as a percent is 50.

Keep it

Copy this into your exercise book.

Soil is rock grains, humus, water and air. Shaken in water, big grains sink first: stones, sand, silt, clay. Sandy soil drains fast. Clay soil drains slowly.

Draw: Draw a jar with four layers from the bottom: stones, sand, silt, clay. Add water on top with humus floating.

Reading with a child? A short page for the adult: what this chapter teaches and three questions to ask.

No internet tomorrow? Save the whole book as one file (924 KB). It opens on any phone.