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Show cultural crops can be grown on fewer acres, thus releasing more land for livestock, forestry, wildlife, andrecreation. Most plants need about 10 chemical elements. Of these, 4 are usually in short supply and 1 or 2 are scarce enough to require some attention. The 4 major elements are calcium, nitrogen, phosphorus, and potassium. Some of the others known as trace elements, such as cobalt, iodine, or magnesium, may be deficient in some special cases. Nitrogen stimulates the growth of leaves and stems and gives the plant a dark green color. As already pointed out, nitrogen can be taken from the air by legumes. But legumes need calcium. This can be added in the form of ordinary ground limestone. Nitrogen can also be added in the form of ammonium nitrate or ammonium sulfate if it is needed quickly. Phosphorus is essential to the development of plant seeds as well as other parts of the plant. It can be added in the form of superphosphate- a fertilizer made from ground rocK phosphate treated with acids. It also can be added in the Locate a nearby robin's nest. Then find a spot where you can watch it without disturbing the robins. After you are sure the eggs have hatched, watch the nest for an hour at the same time each day and count the number of trips the parent birds make to the nest. Do this for several days and average the number of trips they make in an hour. You can be fairly sure that on each trip they make the birds are carrying at least one insect of some kind. Actually, they may be carrying several. After you have made the count for several days and have averaged the number of trips they make per hour, multiply this figure by the number of hours of daylight. This will give you an estimate of the total number of insects this pair of robins destroys in a day. form of bone meal or can be obtained from some of the byproducts of steel manufacture. Potassium is needed by plants to grow strong stems. Potassium fertilizers are made from potash salts obtained from potash mines, and from other sources. Nitrogen, phosphorus, and potassium are so commonly used in commercial fertilizers that their percentage is always noted on the fertilizer bag in the same order. For example, a 3- 12- 12 fertilizer is one that contains 3 percent nitrogen, 12 percent phosphorus, and 12 percent potassium. Since very few soils contain the right balance of all the elements needed for any one plant, some kind of fertilizer is usually needed. A soil may be high in nitrogen and potassium but low in phosphorus. Since crop yields are limited by the element most deficient, the elements that are deficient should be added. Soils can be tested to determine what elements they need. When the needed fertilizers are added, a good crop can be expected, if there is enough rainfall and other conditions are favorable. INTERPRETATION Most forms of wildlife help farmers and ranchers produce more and better crops by checking insects, weeds, and other pests. Wildlife is equally helpful to the orchardist, gardener, and city dweller. The number of beneficial forms of wildlife a well- managed farm supports is surprising. For example, on a 100- acre farm in the Eastern United States, we might find the following: Several million beneficial insects such as lady- beetles, aphis lions, and syrphus flies which feed on plant lice; chalcid and tachinid flies which feed on many kinds of insects; and assassin bugs, robber flies, and nabids, which capture and feed on other insects. More than Jfi kinds of beneficial birds and about 450 individuals, of which about 80 would be in the fence rows, 180 in the woods, 90 in the pasture, 90 in the meadow, 10 in the small grain, and 5 in the corn. These include many small birds such as sparrows, warblers, and vireos. More than a thousand beneficial small mammals, principally short- tailed shrews. About 40 percent of these would be in the meadow, 30 percent in the pasture, 20 percent in the woods, and the other 10 percent in the fence rows, grain, and corn. The food habits of birds make them especially valuable to agriculture. Because birds have higher body temperatures, more rapid digestion, and greater energy than most other animals, they require more food. Nestling birds make extremely rapid growth, requiring huge amounts of food. XVII. Observe How Birds Help To Destroy Insects 22 |