plant growth cannot continue if there is not a supply of minerals in a soil. the materials which are available for this purpose can be divided into two groups: the bulky, organic materials which are called manures, and the more concentrated, inorganic chemical substances which are called fertilizers . Farmyard manure, or dung, consists of a mixture of litter, solid excreta and urine. It contains three most important substances for plant materials - nitrogen, phosphate and potash. Manure is added to the soil for several reasons. It improves the physical condition of the soil. It also keeps up the level of humus in the soil, and maintains the best conditions for the activities of soil organisms. Finally, it makes up for the plant nutrients which have been removed by crops or lost by leaching and soil erosion. Another kind of manure is green manure. This includes leguminous crops which grow quickly such as clover and lucerne. Such crops supply additional nitrogen as well as organic matter. A leguminous crop which is ploughed under will add as much nitrogen to the soil per acre as 3 to 10 tons of farmyard manure. Fertilizers are usually classified according to the particular food element which forms their main constituent. So, they may be grouped as nitrogenous fertilizers, phosphatic fertilizers, potassic fertilizers and so on. The most commonly used fertilizer which contains nitrogen is ammoniumsulphate, which is made from ammonia and sulphuric acid, and which contains 21% nitrogen. This element encourages rapid vegetative growth and gives plants a healthy green colour. Another valuable nitrogenous fertilizer is urea, which is made from ammonia and carbon dioxide, and contains 46% nitrogen. The most widely used phosphatic fertilizer, superphosphate, is made by treating mineral phosphate with sulphuric acid. Phosphorous stimulates the formation of a plant's roots, and promotes fruit and seed production. Tropical soils are very often poor in this element. Finally, wherever high crop yields are expected, potash is used together with nitrogen and phosphorous. Potassium makes the plant tissues stronger. This helps the plant to withstand mechanical damage such as broken branches and torn leaves. In this way the entry of disease bearing agents, or pathogens, such as bacteria and fungi, is prevented. Potassium is important for all plants but particularly so for those that produce oil and starch or sugars. Oil palm and tapioca plants require potassium in large amounts. It is usually supplied in the form of muriate of potash (potassium chloride), which contains 50 to 60% potassium oxide (K20) and sulphate of potash (potassium sulphate). All plants are affected by thedegree of acidity or alkalinity of the soil. The less the nutrient supply, the more acid the soil becomes. Because mineral salts are basic, an acid soil has a low base content. Acidity makes some elements unavailable to plants. If a soil is very acid, with a pH value of less than 5,0, lime can he added to correct this acidity. The main constituent of lime is calcium, an impoitant plant food. The presence of lime helps to make essential elements of plant food more easily available to plants. Nitrogen, phosphorous and potassium are more easily available in a well-limed soil than in an acid soil.
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