How does soil form?

1. Long ago this hillside was bare rock. No soil, no plants. So where did all the soil under our feet come from? It was made, very slowly, right here.
2. Rainwater seeps into cracks. When it freezes it swells and pushes the crack wider, so pieces break off. Heat, cold and rain keep at it until the rock crumbles into bits.
3. Tough living things arrive first: crusty lichens and soft moss. They cling to the rock and help break it down. Then small plants take root, and their roots pry the cracks open.
4. Plants drop leaves, and plants and animals die. Fungi and tiny bacteria rot them into a dark, crumbly material called humus. Humus holds water and feeds new plants.
5. Earthworms, beetles and ants pull dead leaves underground and mix them with the rock crumbs. Their tunnels let air and water reach the roots. Now it is real topsoil.
6. After hundreds and thousands of years the soil has layers: leaf litter, dark topsoil, paler subsoil, broken rock and solid bedrock. Deep soil can hold up a whole forest.

This hole shows the soil from top to bottom. Tap each layer and each helper.

• Leaf litter. The top layer is dead leaves, twigs and other bits that are just starting to rot. It protects the soil like a blanket.
• Topsoil. Topsoil is dark because it is full of humus, the rotted remains of plants and animals. Most roots and most soil life are here.
• Subsoil. Subsoil has less humus, so it is paler and harder. Water carries clay and rusty iron down from above and leaves them here.
• Broken rock. This is rock that has cracked and crumbled but has not turned into soil yet. It is what the soil above was made from.
• Bedrock. Bedrock is the solid rock underneath everything. Cracks in its top are where new soil begins.
• Earthworm. Earthworms pull leaves down, eat them with soil, and leave crumbly droppings called casts. Their tunnels let in air and water.
• Roots and fungi. Roots hold soil in place, split rock as they grow and take up water. Thin white threads of fungi help them reach farther.
• Air and water spaces. Soil is full of tiny gaps called pores. Some hold water, some hold air. Roots and soil animals need both.

Switch the rain, the plants and the worms on and off, and slide the years. What does bare rock need to become deep, dark soil?

Experiment: What does rock need, besides rain and time, to become dark, rich soil?

1. Turn on the rain only and slide the years all the way up. Look at the color of the ground.
2. Now turn on the plants and the worms too.
3. Compare: which ground has a dark top layer?

What we found: Rain and time alone only make crumbled rock: pale grit with nothing to feed plants. It takes living things to make real soil. Plants add dead leaves that rot into dark humus, and worms mix the humus into the crumbs.

Turn on JavaScript to try it yourself.

Each tap is 200 years of work by one helper. Build 12 centimeters of dark topsoil before the time runs out.

Scoop up a handful of soil and you are holding four things at once. About half of it is tiny bits of rock: sand, silt and clay. A small part is humus, the dark remains of dead plants and animals. The rest is gaps filled with water and air. All four matter, because roots need food, water and air together.

The rocky part comes from weathering. Rain soaks into cracks. When the water freezes it swells and pushes the crack open. Hot days and cold nights make rock swell and shrink until it flakes. Rainwater is also a tiny bit sour, so it slowly eats away at some rocks. After a very long time, solid rock becomes grit.

Grit is not soil yet. Living things finish the job. Plants drop leaves, fungi and bacteria rot them into humus, and worms, ants and beetles stir the humus into the grit. Their tunnels let rain soak in instead of running off.

Soil forms so slowly that people treat it as precious. A layer as thick as your finger can take hundreds of years to make, but heavy rain can wash it off a bare field in one storm. That is why farmers keep plants growing on the land, and why roots, leaf litter and gentle digging protect the ground.

Key words

Weathering

The slow breaking and crumbling of rock by water, ice, heat and living things.

Humus

Dark, crumbly material left when dead plants and animals rot.

Topsoil

The dark upper layer of soil, rich in humus, where most roots and soil life are found.

Bedrock

The solid rock that lies under the soil.

Horizon

One layer of soil. Each layer has its own color and feel.

Erosion

Soil or rock being carried away by water, wind or ice.

Check your thinking

1. What first breaks solid rock into small pieces?

   Show the answer

   Weathering by water, ice and heat. Water freezing in cracks, and heating and cooling, crack rock into bits. That is weathering.

2. Why is topsoil darker than the layers below it?

   Show the answer

   It is full of humus from rotted plants and animals. Humus is dark, and most of it is made and mixed in near the surface, where the leaves fall.

3. A hillside has rain and lots of time, but nothing ever lives on it. What will you find there?

   Show the answer

   Crumbled rock with no dark topsoil. Rain and time make grit. Without plants there are no dead leaves, so no humus forms.

4. Why do farmers try to keep plants growing on a field, even in winter?

   Show the answer

   Roots and leaves stop rain from washing the soil away. Soil takes hundreds of years to make, but one storm can wash it off bare ground. Plant cover holds it.

5. In which horizon do clay and iron oxides washed down from above tend to collect?

   Show the answer

   The B horizon (subsoil). Water leaches fine clay and iron from the A horizon and deposits them in the B horizon, the zone of accumulation.

6. Which list goes from the top of the soil to the bottom?

   Show the answer

   Leaf litter, topsoil, subsoil, broken rock, bedrock. Leaves land on top and rot into the topsoil. Below it are the subsoil, the broken rock and the solid bedrock.