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Show soil as they are deposited in some flood plain or at the bottom of a lake. When you put the limestone in the vinegar, you were duplicating in a small way what plants do. Plant roots take in oxygen from the soil air and give off carbon dioxide gas. This gas is one of the important end products in the decay of organic matter. Carbon dioxide gas dissolves in the soil moisture, forming weak carbonic acid. This acid reacts just as the acetic acid in vinegar did with limestone rock and will decompose limestone and marble. The dissolving effect of this carbonated water is several times that of pure water. Since the lime in limestone is soluble, it gradually washes away leaving only the other materials as soil. It takes 40 to 50 feet of limestone to make only a few inches of soil. There are other physical and chemical factors that also aid in soil formation. For example, wind blows small rock particles against larger ones, wearing both down. As explained later, even plant and animal life play an important role in soil formation. INTERPRETATION Soil is formed from rocks very very slowly. When you rub two rocks together, small particles rub off. It takes a long time to accumulate even a spoonful. When large sheets of ice ( glaciers) moved over the land thousands of years ago, they ground rocks together, rubbing off tremendous quantities of rock particles of all sizes. Much of the north central United States is made up of soils that were formed by the action of these glaciers. Changes in temperature also help to make soil. The sun warms the rocks during the day. At night the rocks cool. The expansion and contraction chips off particles of rock just as you saw when you dropped the hot limestone into cold water. Freezing water expands with tremendous force. Water that finds its way into cracks in the rocks freezes and breaks the rocks into smaller and smaller pieces. Most of the soils we see today developed from rock material that was moved by water or wind either after this " weathering process" or while it was going on. Soils are also formed as rocks are rolled along by streams. Note how smooth the pebbles are you see on beaches and along streams. They have been rubbed together until the rough parts are knocked off. These rubbed- off particles make up This dark, deep layer of topsoil is the product of centuries of weathering, accumulation of plant and animal remains, and the work of many living organisms. The less fertile subsoil. which is lighter in color, has little or no organic matter. 3 |