OCR Text |
Show IX. Compare How Much Water Different Soils Hold You will need 2 cans of equal size ( coffee cans will do); two 18- inch squares of cloth; some heavy string; a package or similar scale that weighs up to 64 ounces or 2,000 grams; and a container of water, such as a 2- or 3- gallon bucket or a 5- quart oilcan with the top cut out. Put equal volumes of soil in the two cans. Take the soil for one from a field or garden that has been cultivated for several years and that shows lack of organic matter. This sample should be hard and cloddy. Get the other from a well- managed field where grasses and legumes have been grown, or from a good pasture or similar location. This sample should be crumbly and free from clods. First allow the soils to dry. Empty the two soil samples on the cloth squares, pull the corners together, and tie with a heavy string. Weigh each sample and record the weight. Saturate each bag of soil by holding it in the water long enough to soak thoroughly. Remove the soil samples from the water and allow them to drain off the free water for a few minutes. Then weigh again and record the weights. Calculate the difference in weight. Another way to measure the water- holding capacity of soils is to use two old- fashioned lamp chimneys or cylinders as in the previous activity. Tie a cloth over the top, turn them upside down, and fill them about two- thirds full with the same two soils. Be sure the soils are equally dry. Place the chimneys in small- mouth fruit jars, as shown in the drawing. Pour a pint of water into each chimney. Then note how long it takes the water to begin to drip into the jars, how much water comes from each soil, and how long the water continues to drip. INTERPRETATION When organic matter is used up, soil packs together. Thus, a cloddy soil has fewer air spaces, its particles do not cling together in granules, and the lack of organic matter means that it weighs more than an equal volume of crumbly soil from a well- managed plot. Not only does a crumbly soil take in water faster than a cloddy one, it holds more. The thoroughly decomposed organic matter ( humus) in a crumbly soil can absorb lots of water. On a dry- weight basis, this humus has a water- holding capacity of several hundred percent and may act like a sponge. In addition to the water held by the organic matter itself is the water held in the pores between the soil particles and between the soil granules. Hundreds of very fine soil particles are glued together by the organic matter into soil granules. This increased water- holding capacity of soils high in organic matter under natural conditions makes a big difference in the intake of water. These well- managed soils can absorb most of the rain and snowmelt ( if the soil is not frozen). This means there will be less erosion. Streams will run clear. Of course, when the soil is saturated by a long period of rainfall, any additional water then runs off. But until the soil is saturated it will 11 |