21. Soils

                                   Soils
There are many different kinds of soils. Different soils have different types of rock and minerals in them. Some soils have more water in them than others. Some soils might have more plant and animal material in them, too.
Different kinds of soils are found in different parts of the world. There are several kinds of soils found in the United States. In some areas, the soil has a lot of clay. Other soils are very sandy. Loam is a kind of soil that has a good mixture of clay and sand.
In some places, soil layers are very thick. Lots of plants grow in places with a thick soil layer. In dry and windy places soil layers are much thinner. Layers of soil on mountains are thin because gravity pulls the soil downhill.
The type of soil in a particular place affects what kinds of plants can grow there.

Useful Words:
1. clay 黏土，泥土
2. loam 肥土
3. layer 层
4. gravity 地心引力，重力
5. downhill 往下