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Show V. How Does Organic Matter Help Soil Structure? Take two wide- mouthed glass jars. Make two small baskets or wire racks of ^- inch hardware screen. For each rack you will need a piece of screen about 3 by 10 inches. Bend the wire, as shown in the illustrations, so that it extends, basketlike, down into the jars. Collect lumps of soil ( not sandy) just under the sod from ( 1) a natural sod fence row or park and ( 2) a cultivated field that has been farmed heavily and where the soil is light in color. These lumps should be about twice the size of an egg. Fill the jars with water within an inch of the top. Place the lumps of soil in the baskets and lower them gently into the jars. Watch closely and make notes of what happens. 7/ 1683 INTERPRETATION Why does the soil from the heavily cultivated field fall apart and drop to the bottom of the jar while the other one holds its shape and clings together? The answer is largely the difference in the amount of organic matter and the effect it has on the soil. Organic matter has a marked effect on both the physical and chemical properties of soils. It helps soil hold water and, therefore, decreases the amount of water that runs off. It improves aeration, especially on the finer textured soils. And it makes the soil easier to work- improves soil tilth, as farmers would say. While these are all related, improving soil tilth is the one most clearly illustrated in this simple activity. Organic matter improves tilth of soils- makes the soil crumbly- causing the individual soil particles to stick together tightly in granules. These granules act, in effect, like much larger particles, in letting water and air move through the soil more readily. The large granules tend to stick together, too, because of the binding effect of the decomposed organic matter, or humus, and because of tiny roots under sod layers. Since organic matter reduces water- runoff losses, damage by water erosion is greatly reduced. When raindrops strike a bare soil with little organic matter, like the soil samples from the heavily cultivated field, or even when water runs over this bare soil it breaks down and washes away readily. 81,492 These two samples of soil were taken only 25 feet apart. The one on the left from a cultivated field; the one on the right from an undisturbed fence row. Tests show the crumbly soil takes in water 20 times faster than the other. 7 |