D．

H．ave you ever noticed the colour of the water in a river or stream after a heavy rainfall? What do you think caused this change in colour? It is soil that has been washed into the river from the riverbank or from t}le nearby fields.

C．omponents of Soil

Soil is made up of a number of layers（层），each having its own distinctive colour and texture. The upper layer is known as the litter. It acts like a blanket. limiting temperature changes and reducing water loss. The topsoil layer is made up of small particles of rock mixed with rotten plant and animal matter called humus（腐殖质），which is black and gives the topsoil its dark colour. This layer is usually rich in nutrients，oxygen，and water. Below the topsoil is the subsoil，a layer that contains more stones mixed with only small amounts of organic matter. This layer is lighter in colour because of the lack of humus. Beneath the soil lies a layer of bedrock.

Soil forms from the bottom up. Over time bedrock is attacked by rain, wind，frost, and snow. It is gradually broken down into smaller particles in a process called weathering. Plants begin to grow，and rotten materials enrich the topsoil. Most of the soil in Eastern Canada. for example. Was formed from weathered rock that was exposed when the ice disappeared l2,000 years ago.

Water Beneath the Soil

Surface water collects and flows above the ground in lakes. ponds. and rivers. Once in the soil or rock，it is called groundwater. Gravity pulls groundwater through the soil in a process called percolation（渗透）. Eventually the water reaches a layer called the water table. Under this is bedrock through which water cannot percolate.

A．s water percolates downward，it dissolves organic matter and minerals from the soil and carries them to deeper layers. This causes a serious problem because plants require these nutrients for growth.

Soil pH

Soil can be acidic. neutral. or basic. The pH of the soil is determined by the nature of the rock

from which it was formed. and by the nature of the plants that grow and rot in it.

The acidity of rain and snow can lower the pH of the groundwater that enters the soil. By burning fossil fuels such as coal，oil and gasoline，humans have been contributing to higher levels of acidity in many soils. When fossil fuels are burned. gases are released into the air and then fall back to earth as acid rain. Acid soil increases出e problem of carrying nutrients to lower soil levels. As nutrients are removed，soil is less fertile. Plants grow more slowly in acidic soil，and also become easily attacked by diseases.

67. The layer of soil that provides necessary nutrients for plant growth is called.

A． 1itter B. topsoil C. humus D. subsoil