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Show II. Are All Soil Particles the Same Size? Fill a fruit jar about two- thirds full of water. Pour in soil until the jar is almost full. Replace the cover or put one hand tightly over the top of the jar and shake it vigorously. Then put the jar on the table and let the soil settle. Allow plenty of time because the very small particles will be slow in settling. Then hold a card or heavy piece of paper against the side of the jar and draw a diagram showing the different layers. Label each layer ( clay, silt, sand). Do this with several soils taken from different places and compare the diagrams or compare the jars directly. To demonstrate that coarse soils have pore spaces that can be filled with finer soils, try the following: You will need a half- pint fruit jar, some marbles, and some sand. Put the sand in a measuring glass so that you can check how much you use. Fill the fruit jar about two- thirds full of marbles. Then add sand. Tap the jar on the table a few times as you add the sand to be sure that all the pores are filled. Stop when the sand completely covers the marbles. Check how much sand you used. INTERPRETATION Soil particles vary greatly in size. The largest particles settle to the bottom first. The fine particles settle slowly; some, in fact, are suspended indefinitely. Soil scientists classify soil particles into sand, silt, and clay. Starting with the finest, clay particles are smaller than 0.002 millimeters in diameter. Some are so small that ordinary microscopes do not show them. Silt particles are from 0.002 to 0.05 millimeters in diameter. Sand ranges from 0.05 to 2.0 millimeters. Particles larger than 2.0 millimeters are called gravel or stones. Most soils, as found in nature, contain a mixture of sand, silt, and clay in different proportions. Size of soil particles is important. The amount of open space between the particles has a lot to do with how easily water moves through a soil and how much water it will hold. Too much clay, in proportion to silt and sand, causes a soil to take in water very slowly. Such a soil also gives up its water to plants slowly. These soils are sticky when wet. Loam and silt loam refer to soils that have a favorable proportion of sand, silt, and clay. A silt loam, for example, contains no more than 50 percent sand nor more than 27 percent clay. The rest, naturally, is silt. Size of soil particles is important for other reasons, too. It affects the ease of working the soil, what crops can be grown, and the efficiency of certain fertilizers. The marbles- and- sand demonstration shows that the pore space is larger in coarse soils and that the spaces may be filled partly with finer soil particles. This makes a more dense soil. Sandy soils that have no fine clay or silt particles filling the pore space cannot hold as much moisture since there is less surface area for the water to cling to and the pores are so large that the weight of the water causes much of it to run down and out of the soil. For this reason, medium and coarse sandy soils low in clay, are known as droughty soils- crops cannot live long in them without very frequent rains. When fine soil particles fill the large pore spaces, the soil can hold more water for plants because there is more surface area for water to cling to. And since the size of the pores is reduced, the weight of the water is less and it doesn't run out of the soil so readily. 4 |