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Show XVI. How Does Fertilizer Affect Plant Growth? You will need two flowerpots and a mortar and pestle. Try to get pots that hold about a quart. You will also need soil of low fertility; seeds of tomatoes, beans, corn, or wheat; nitrogen and phosphorus fertilizers; and clean sand. Get the soil from an eroded bank or from an eroded field that has been farmed heavily. Or dig into the soil a foot or more and get subsoil. For the nitrogen fertilizer use ammonium nitrate or ammonium sulfate. And for the phosphorus fertilizer use superphosphate. Grind the fertilizer materials in the mortar until they are very fine. Place about 1 pint of soil on a sheet of paper. Add to this soil one- half as much nitrogen fertilizer as can be heaped on a dime. Then add the same amount of phosphorus fertilizer. Mix these fertilizers thoroughly with the soil. If the pots hold a quart, then put a pint of the original soil in one pot and finish filling it with the fertilized soil. Fill the other pot with unfertilized soil. Plant a few of the same kind of seeds in each pot and cover with about one- fourth inch of sand to prevent soil crusting. When the plants are well established, thin them to the same number in each pot. Watch their growth for several weeks. INTERPRETATION Soil fertility is a major factor in soil conservation. By adding fertilizer and lime when needed to keep soils highly productive, we not only help conserve the soils themselves, but aid conservation in general. Dr. Emil Truog, famous soil scientist of the University of Wisconsin, points out several ways in which high soil fertility aids conservation. 1. High soil fertility produces a heavier plant growth that protects soil from washing and blowing. Land that is protected with a good cover of grass or trees does not wash. In fact, under such conditions, soil is being formed faster than it is eroded away. Where native vegetation is removed and cultivated crops are grown, leaving the land bare part or all of the time, erosion takes place. Keeping the land covered as much as possible is one of the best ways to prevent erosion but plants will not grow abundantly unless the soil is fertile. 2. Heavy plant growth, resulting from high fertility uses more water than the growth on poor soil. This leaves room for the sou to hold more water from each rain, thereby reducing runoff. All growing plants remove tremendous amounts of water from the soil and then allow it to escape as vapor through tiny holes in the leaves. The higher the fertility of the soil the larger the amount of water put to good use and the greater the crop yields. 3. Fertile soil takes in water from rainfall readily, thus reducing the amount that runs off. Well- managed soils develop a granular structure in which the finer particles join together and form granules or crumbs. These crumbs vary in size up to that of buckshot. Each is made up of hundreds of small particles, some so small they cannot be seen even with a microscope. Clay soils especially must be granulated to take in water readily. Generally, the most practical way to increase granulation is to provide organic matter. Farmers can provide organic matter by growing legumes, such as clovers, alfalfa, peas, and beans. These plants take nitrogen from the air and store it in tneir roots, stems, and leaves. When the plants are plowed into the soil, they supply organic matter. Of course, other plant materials add organic matter too. An abundant growth of weeds, if Elowed under, adds organic matter. But legumes ave the advantage of adding extra nitrogen taken from the air. The supply of nitrogen in the air over 1 acre would be worth more than $ 5 million, if it were transformed into nitrogen fertilizer. Soils high in fertility can produce larger amounts of plant growth and, hence, larger amounts of organic matter to add to the soil. Even the roots and residue left on the soil after harvesting a heavy crop of legume hay add organic matter. And when the nay is fed to livestock, more organic matter is returned in the form of manure. 4. Higher soil fertility increases crop yields on the more level fields, thus reducing the need for growing row crops on sloping fields where water erosion takes its heaviest toll. Sloping fields that erode easily can be kept in grass and trees. By improving the fertility of soils, needed agri- 21 |