Soils from rocks
If you buried a rock in the soil and left it for a year, it would look the same when you dug it up. If you could leave it for another hundred years, it would still look the same. After a few thousand years the rock would be smaller. In time, all you would find would be grains of sand and particles of silt. These changes take place because of the way a rock is made.

A rock is made from minerals. There are many different kinds of mineral. One important mineral found in rocks is called silica. It is mixed with other minerals in the rock. A rock in the soil is attacked by water. Many of the minerals in the rock eventually dissolve in the water and flow away through the soil. Silica is unusual. It is not attacked by water and remains behind to form particles of sand and silt.

Silica is also found in the smallest particles in the soil. They are the clay particles. A clay particle is up to a thousand times smaller than a grain of sand. The tiny size of the clay particles gives the clay special properties. When a piece of clay is wet, it becomes sticky and swells up. As the clay dries, it shrinks and becomes hard. Clay also has another important property. It can catch and hold minerals that dissolve in the water.

There is another part to the soil that is not made from rock. It is made by living things, particularly plants. When a plant dies, its roots, stem and leaves rot away and form a black substance in the soil called humus. This substance sticks the sand grains, silt and clay particles together to make lumps called soil crumbs. A soil crumb can be up to three millimetres across. If you want to see soil crumbs, pull up a grass plant, such as a weed, in a flowerbed. Rub its roots in your hand and the soil crumbs will fall out.

How long does it take to form soil?
Some soils are made when sand and silt are carried down river and then dumped. These soils can build up quickly. It may only take fifty years to make a layer of soil thirty centimetres thick. Where a soil forms directly from the rock underneath, it takes much longer. It can take a hundred years for one centimetre of soil to form. In some places it can take a thousand years to make a soil one centimetre deep.

Are all soils the same colour?
No. The colour of the soil varies from place to place. In Britain many soils tend to be brown. On the prairies of North America the soil is black. In Spain and Greece you can see red soils. In Morocco you can find yellow soil.

What gives the soil its colour?
One of the minerals in the soil contains iron. It forms a coating on the soil particles and makes them red or yellow. This mineral also forms tiny particles that mix with clay. A second substance, which colours soil, is humus. It may be black or brown. The colour of a soil is due to the way the coloured substances in it are mixed. For example, the brown soils of Britain are due to the large amount of humus in the soil. In the past most of the land here was covered in forest and this led to the building-up of humus in the soil. Red soils are produced in areas where there are large amounts of iron minerals in the soil and little humus because few plants have grown in the soil in the past.

What makes the plants rot in the soil?
There are billions of tiny living things in the soil called bacteria. There are also fungi such as moulds. The bacteria and fungi feed on the dead plants and break them down into humus.

What happens to dead animals?
The bacteria and fungi also feed on them and the animal remains form part of the humus. Even the wastes from animals, such as cowpats, are rotted down to form humus. As there are many more plants than animals, most of the humus is made from plant remains.

Why is the subsoil so different from the topsoil?
There are spaces between the particles in the topsoil that let in air. Bacteria and fungi need air to rot down plants. When it rains, tiny particles from the topsoil are carried into the subsoil where they block up the spaces. This prevents plant remains from being washed into the subsoil and closes down air spaces so bacteria and fungi cannot live there. The looser material in the topsoil allows animals, such as worms, to burrow in it and mix it up. The material in the subsoil is more tightly packed so burrowing animals cannot live there and mix it up.