Chapter 6 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

S3.11.6 Chapter 6

Caring for soil

Why do farmers worry when soil washes away?

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

  • Explain how trees and grass stop soil washing away
  • Describe how compost and changing crops keep soil healthy

Soil is the thin skin of land where plants grow. A thin layer of it can take hundreds of years to form. But one storm can wash it away. So soil needs care.

Draw it first

Draw rain on a bare slope, then on grass

  1. A hillside with rain falling, drawn twice in the same place. First the slope is bare and rain washes soil down it, shown by a line of dark grains and an arrow. Then the slope is covered with grass, and roots reach down into the soil, so no grains travel. Labels: rain, soil washed away, rain, grass, roots.rainsoil washed away

    1 Draw a slope with no plants. Draw rain falling and soil grains running down the slope with the water.

  2. A hillside with rain falling, drawn twice in the same place. First the slope is bare and rain washes soil down it, shown by a line of dark grains and an arrow. Then the slope is covered with grass, and roots reach down into the soil, so no grains travel. Labels: rain, soil washed away, rain, grass, roots.raingrassroots

    2 Draw the slope again with grass. Draw the roots reaching down into the soil. The grains stay where they are.

On bare soil, rain runs downhill and carries soil with it. This is erosion. Wind can carry dry, bare soil too.

Grass and trees protect soil. Their roots hold the soil together like a net. Their leaves slow the rain, so the drops do not hit the soil so hard.

See it three ways

As a picture

A bar chart of soil lost in one storm, in kg. The bare strip of hill lost 40 kg and the grass strip lost 4 kg. The scale runs from 0 to 40 kg. Labels: bare, grass, kg lost.010203040baregrasskg lost

In words

A hill with grass loses far less soil than a bare hill in the same storm. These numbers are an example.

In symbols

Bare strip: 40 kg lost. Grass strip: 4 kg lost. The bare strip lost 10 times as much.

Try it

Wash bare soil and grass

You need: two trays, bare soil, a square of grass with its roots, cut by a grown up with a spade, two clear jars, a jug of water.

Be safe: Do this outside or over a sink. Wash your hands after you touch soil.

  1. Put bare soil in one tray. Put the square of grass in the other tray.
  2. Tilt both trays the same way. Stand a jar at the low end of each.
  3. Pour the same amount of water slowly over each tray, like rain.
  4. Look at the water that runs into each jar.

What you will see: The water from the bare soil is brown and muddy, because it carries soil. The water from the grass is nearly clear. The roots hold the soil.

If you cannot try it, this is what happens.
Two boxes showing the water that ran off. The box under bare soil is full of dark grains and is marked muddy. The box under grass is plain and is marked clear. Labels: muddy, bare soil, clear, grass.muddybare soilcleargrass

Plants make their own food in their leaves, using light. But they take water and minerals from the soil, so soil can wear out. We can give some back. Compost is made from old plant scraps, such as leaves and vegetable peels, left to rot. Worms and tiny living things turn them into dark, crumbly humus.

A cut through a compost heap in three layers. At the top are dry leaves. Under them are plant scraps. At the bottom is old compost, dark and crumbly. Labels: dry leaves, plant scraps, old compost.dry leavesplant scrapsold compost
The oldest scraps at the bottom have turned to compost.

Humus holds water like a sponge. As it rots, it gives plants minerals. Adding compost makes soil richer. Soil that has what plants need is fertile.

Suppose a farmer grows the same crop in a field every year. That crop uses up the same minerals each time. So farmers change the crop from year to year. This is crop rotation.

A table of three fields, A, B and C, over three years. In year 1 field A has maize, B has beans and C has wheat. In year 2 field A has beans, B has wheat and C has maize. In year 3 field A has wheat, B has maize and C has beans. Each field has beans once in three years. Labels: year, A, B, C, 1, 2, 3, maize, beans, wheat.yearABC1maizebeanswheat2beanswheatmaize3wheatmaizebeans
Each field grows a different crop every year.

Beans, peas and lentils have small lumps on their roots. Tiny living things in the lumps take nitrogen from the air. Plants use some of it. When the roots and leaves rot, the rest stays in the soil. Nitrogen is a mineral that plants need to grow. So a field of beans adds nitrogen to the soil.

The root of a bean plant in soil. Four small round lumps sit on the side roots. An arrow runs from a lump out into the soil, named nitrogen. Labels: stem, roots, root lumps, nitrogen.nitrogenstemrootsroot lumps
Roots of beans and peas carry small lumps. Tiny living things in the lumps take nitrogen from the air.

A wrong idea many people have

Soil lasts for ever, so we can use it any way we like.

Why it feels right: There seems to be a lot of it, and it looks the same from year to year.

The test that breaks it: In an example storm, a bare strip of hill lost 40 kg of soil. A grass strip lost 4 kg. A thin layer of soil can take hundreds of years to form.

What is true: Soil forms very slowly and can be lost quickly. Plants, compost and changing crops help us keep it.

Key words

compost
old plant scraps rotted into dark, crumbly humus.
fertile
having what plants need to grow well.
crop rotation
growing a different crop in a field each year.
nitrogen
a mineral that plants need to grow, which beans add to the soil.

Back to your guess

The answer is B: Soil takes hundreds of years to form, so lost soil cannot be replaced soon.

A thin layer of soil can take hundreds of years to form. Plants, compost and changing crops help to keep it.

Check yourself

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

  1. Why does grass help to stop soil washing away?

    Tap your answer.

    1. Not this one. Weight is not the reason. The roots hold the soil together like a net, and the leaves slow the rain. Go to S3.11.6. Try another.

    2. Yes, that is right. Roots hold soil together like a net. Leaves slow the falling drops so they do not knock the soil loose.

    3. Not this one. Grass does not soak up all the rain. Water still runs off, but the roots hold on to the soil. Go to S3.11.6. Try another.

    See the answer

    Its roots hold the soil, and its leaves slow the rain.

    Roots hold soil together like a net. Leaves slow the falling drops so they do not knock the soil loose.

    • If you chose A: Weight is not the reason. The roots hold the soil together like a net, and the leaves slow the rain. Go to S3.11.6.
    • If you chose C: Grass does not soak up all the rain. Water still runs off, but the roots hold on to the soil. Go to S3.11.6.

  2. What is compost?

    Tap your answer.

    1. Yes, that is right. Worms and tiny living things rot plant scraps into humus. We call the result compost.

    2. Not this one. Compost is made from plants, not rock. Crushed rock would add grains but no humus. Go to S3.11.6. Try another.

    3. Not this one. Fresh leaves only become compost after worms and tiny living things have rotted them. Go to S3.11.6. Try another.

    See the answer

    Old plant scraps that have rotted into dark, crumbly humus

    Worms and tiny living things rot plant scraps into humus. We call the result compost.

    • If you chose B: Compost is made from plants, not rock. Crushed rock would add grains but no humus. Go to S3.11.6.
    • If you chose C: Fresh leaves only become compost after worms and tiny living things have rotted them. Go to S3.11.6.

  3. What does adding compost do for soil?

    Tap your answer.

    1. Not this one. Compost is made from plants, not from rock. What it adds is humus. Go to S3.11.6. Try another.

    2. Not this one. Compost makes soil crumbly and easier to dig. It also holds water. Go to S3.11.6. Try another.

    3. Yes, that is right. Humus holds water like a sponge. As it rots, it gives plants minerals. That makes the soil fertile.

    See the answer

    It adds humus, which holds water and gives plants minerals as it rots.

    Humus holds water like a sponge. As it rots, it gives plants minerals. That makes the soil fertile.

    • If you chose A: Compost is made from plants, not from rock. What it adds is humus. Go to S3.11.6.
    • If you chose B: Compost makes soil crumbly and easier to dig. It also holds water. Go to S3.11.6.

  4. Why do farmers grow beans in a field in some years?

    Tap your answer.

    1. Not this one. It is the other way round. Beans add nitrogen. Other crops use it up. Go to S3.11.6. Try another.

    2. Yes, that is right. Tiny living things in lumps on bean roots take nitrogen from the air. They put it into the soil.

    3. Not this one. Many crops grow in soil. Beans are special because they add nitrogen, and that helps the next crop. Go to S3.11.6. Try another.

    See the answer

    Beans put nitrogen back into the soil for later crops.

    Tiny living things in lumps on bean roots take nitrogen from the air. They put it into the soil.

    • If you chose A: It is the other way round. Beans add nitrogen. Other crops use it up. Go to S3.11.6.
    • If you chose C: Many crops grow in soil. Beans are special because they add nitrogen, and that helps the next crop. Go to S3.11.6.

  5. A farmer grows the same crop in the same field every year. What may happen?

    Tap your answer.

    1. Yes, that is right. The same crop takes the same minerals from the soil year after year. Crop rotation spreads the work between different crops.

    2. Not this one. Taking the crop away takes the minerals with it. Without compost or a change of crop, the soil gets poorer. Go to S3.11.6. Try another.

    3. Not this one. Soil changes. Crops use up its minerals, so farmers change the crop or add compost. Go to S3.11.6. Try another.

    See the answer

    The crop uses up the same minerals each time, so the soil gets poorer.

    The same crop takes the same minerals from the soil year after year. Crop rotation spreads the work between different crops.

    • If you chose B: Taking the crop away takes the minerals with it. Without compost or a change of crop, the soil gets poorer. Go to S3.11.6.
    • If you chose C: Soil changes. Crops use up its minerals, so farmers change the crop or add compost. Go to S3.11.6.

  6. From an earlier chapter

    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. Sand has bigger gaps than clay. Go to S3.11.5. Try another.

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

    See the answer

    Sandy soil

    Sandy soil has big grains and big gaps, so water runs through 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. Sand has bigger gaps than clay. Go to S3.11.5.

  7. In one storm a bare strip of hill lost 40 kg of soil. A grass strip lost 4 kg. How many times as much did the bare strip lose?

    See the answer

    10 times

    40 ÷ 4 = 10. The bare strip lost 10 times as much.

Keep it

Copy this into your exercise book.

Roots hold soil and leaves slow rain. Compost adds humus. Changing crops stops soil wearing out. Beans add nitrogen.

Draw: Draw a slope with grass and roots. Beside it draw three fields with a different crop in each, and mark the beans.

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.