Initially, all over the planet, small pieces of rock broke off mountains and fell, breaking into smaller pieces as they went, so that at the bottom of mountains, heaps of rocks, stones and dust formed. Plants were able to take root in this, and when these plants died and decayed they added to the pockets of soil. Eventually, much of the planet was covered with soil, its exact composition determined partly by the type of rock in the area and by the local weather conditions.
So, all soil is composed of small pieces of the local rock and decayed vegetable or animal matter.
The rock particles are classified on the basis of their size:
1. Clay is the smallest particles of broken rock in soil, less than .0002mm in diameter, so it is a fine dust. When wet, the individual particles stick together to form a solid mass. When they dry, they can bake to a hard crust. Clay holds water which does not drain away.
2. Silt is slightly larger pieces of rock than clay. It is also soft and smooth, with individual pieces close together. It too holds a lot of water, but the slightly larger particles make it a little better at draining than clay. Silt is often found in river estauries because the fine particles are washed downstream and deposited when the water flows more slowly.
3. Sand is small pieces of rock (2mm to .05mm diameter) such as quartz or sandstone. Sand particles are large enough to allow water to drain easily, but they do not hold water and are easily blown around when dry.
4. Stones, rocks and boulders are larger pieces of rock which are too big to form part of the soil but are found in many gardens. Under the surface layer of soil, they can help drainage.
