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Download The role of calcium and magnesium in agriculture
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Soil The nature of soil Soil generally has a loose texture and consists of solid mineral and organic matter, water and air spaces. The mineral portion of the soil is formed from parent rocks by the weathering action of physical, chemical and biological processes. The organic portion of soil consists of the remains of plant biomass in various stages of decay. Typical soils support high populations of bacteria, fungi and animals such as earthworms. Soils exhibit distinctive layers with increasing depth. The layers are called horizons. The top layer, typically several cm’s in thickness, is the A horizon or topsoil. It is the layer of maximum biological activity and contains most of the soil organic matter. Metal ions and clay particles in the A-horizon are subject to considerable leaching. The B-horizon, or subsoil, receives materials such as organic matter, salts, or clay particles leached from the topsoil. The C-horizon is composed of weathered parent rocks from which the soil originated. The soil minerals, reflecting the composition of the crustal rocks, are largely silicates. The role of calcium and magnesium in agriculture For healthy plants, an adequate supply of calcium compounds in the soil is required. This is because calcium is an essential constituent of plants. Not only are they a principle factor in controlling the pH of the soil but also they affect the plants ability to absorb nutrients through the roots. Magnesium is essential for plant growth. All green plants contain chlorophyll, which is essential for photosynthesis, and chlorophyll contains magnesium. The greater part of the magnesium in plants is present as Mg 2+ ions, associated with both organic and inorganic anions. Amongst other functions, these ions are necessary for the functioning of several plant enzymes. The pH of the soil influences the concentration of plant nutrients in the soil solution and hence their availability. Example: A soil with a pH of about 5 has a higher concentration of plant nutrients, such as aluminium and manganese, compared to a soil of pH 7. Tea is a well-known example of a crop, which thrives, in very acid soils and it contains far more aluminium than most plants. Calcium in the soil is mostly in the exchangeable form, with some reserve as calcium carbonate. Ca-soil + 2NH4+ (aq) → (NH4)2-soil + Ca2+ (aq) The soil is made up both of inorganic and organic materials. An important part consists of clay minerals, which carry a negative charge; the humus derived from the organic matter is similarly charged. These charges are balanced by cations in the soil, mainly the H+, Ca2+, Mg2+, K+ and Na+ ions. Rainwater, which contains carbonic acid, H2CO3, leads, as it percolates through the soil, to the replacement of cations such as Ca2+ by H+. Fertilizers such as ammonium sulphate (NH4)2SO4, are also involved in the cation exchange. The NH4+ ions held in the soil are converted first to NO2- and then to NO3- as the result of bacterial action. Hydrogen ions are produced simultaneously to balance these anions and these replace ammonium ions in the exchange complex, making the soil more acidic (the more exchangeable H+ the more acid the soil) Exchangeable calcium and other ions are removed from the soil by growing crops. Except in chalk soils the weathering of mineral fragments in the soil is generally not enough to replace the calcium lost in these various ways, and the deficiency has to be made good by the addition of either calcium hydroxide (slaked lime) or calcium carbonate (ground limestone) Like calcium, magnesium is removed from the soil by both rain and crops. In most medium and heavy soils the weathering of soil minerals restores the loss; in many sandy soils it may be necessary to add magnesium carbonate. As grazing animals obtain their food from grass it is necessary for the grass to provide all the nutrients that they require. Cattle and sheep feeding on lush spring pastures are liable to suffer from hypomagnesaemia (commonly called grass staggers) if the grass has an inadequate level of magnesium – animal feed is sometimes supplemented with magnesium carbonate powder.