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MAJOR PLANT FOOD ELEMENTS |
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For many years nitrogen, phosphorus, and potassium were known as major soil-derived plant food elements, and all others as minor elements. In more recent times sulphur, calcium, and magnesium have heen added to the former group by some authorities. The distinction in terms between major and minor elements is based on the quantity required by plants and not on their relative importance.
Nitrogen Popular garden manures are all rich in nitrogen, particularly bird-droppings, urine, blood-and-bone, dung of any type, urea, soot, sulphate of ammonia, and the various nitrates of which sodium nitrate is the one most commonly used. Sulphate of ammonia is very acid; nitrate of soda is alkaline. There are about seventy-five million pounds of atmospheric nitrogen above every acre of land, but it is absolutely unavailable and useless to all plants except legumes-such as the clovers, lucerne, peas, and beans. It is most likely to be deficient in light soils. Nitrogen is absorbed generally as nitrates; nitrifying soil bacteria render it available from organic matter. Nitrogen is needed by a plant before any leaf-growth is apparent; it is a key element in plant proteins. Its general effect is to produce big dark leaves and lush growth. Young plants that show yellowing of the foliage respond almost immediately to light dressings of nitrogenous manures. All forcing manures are high in nitrogen content, but must be used sparingly and in repeated applications rather than in heavy doses. Inorganic nitrogenous manures are best applied as very weak aqueous solutions. The strength should never exceed one ounce to two gallons of water, and even then should be used on wet soil and be followed by a further watering. By judicious application they can be employed to retard maturity of blooms; exhibitors sometimes use them for this purpose. Urea is becoming the most popular of this type of manure, though sulphate of ammonia and nitrate of soda are still better known. Nitrogen is needed in the breaking-down of vegetable matter in the soil. Land fed with straw, green crops, seaweed, sawdust, and the like should be given, soon afterwards, a good dressing with highly nitrogenous manure; otherwise there will be a temporary nitrogen deficiency. The nitrogen used in the process of disintegration is not lost; it is later released for plant consumption. Conversely, soils treated with nitrogenous chemicals rapidly decline in humus content, with consequent proportionate loss of fertility. Potassium Potassium is needed by plants in large quantities. It appears to increase their resistance to heat, cold, and disease, especially mildew. When deficient in potash, a plant is stunted and its leaves die, first at the tip and later along the margins. Most heavy clays contain adequate potassium and they do not allow it to leach away. Sandy soils are generally deficient. Gardens regularly dressed with vegetable matter and dung are usually well supplied with potassium. It is very soluble and is apt to wash out of organic substances if they are exposed to wet conditions before spreading. Virile plants retain their potassium, but return it to the soil as they age. When killed they release it and allow it to be washed out quickly. Potash is most abundant in wood ashes, young vegetable matter, seaweed, fresh dung, and in nitrate, sulphate, and chloride (or muriate) of potassium. Potassium sulphate is the most commonly used potassium salt. Its sulphate radical could raise the sulphur content of the soil to a level that would be toxic to soil micro-organisms and earthworms in the same manner as ammonium sulphate, but it is not used in sufficient quantity to cause any such trouble. It is to be preferred to the chloride. The nitrate is expensive, but the best of the three. It adds no toxic element, and is a strong nitrogenous food. |
