Elements mainly derived from soil in inorganic form are known as mineral elements. The three main sources of nutrients for plants are air, water, and soil. Elements obtained from air are known as non-mineral elements, such as carbon, oxygen, and hydrogen. Irrigation water is also a source of mineral elements from dissolved salts, mainly NaCl, Na2SO4, NaHCO3, MgSO4, CaSO4, CaCl2, KCl, and K2SO4. The third environmental source of nutrition for autotrophic plants is soil. Figure 2.1 shows the relative distribution of various elements in the earth’s crust, most of which are required for plant growth. Of these, oxygen constitutes about 46.5%, copper being 0.01%, and zinc, nickel, and selenium are present in traces. The relative percentage of some of the well-known macronutrients required for plant growth (calcium, potassium, magnesium, and phosphorus) is in the range of 3.6–0.12%. Phosphorus is the least abundant among the four elements. Among macroelements, nitrogen is the most abundant (44%) followed by potassium, calcium, magnesium, phosphorus, and sulfur. Iron is the most abundant (51%) microelement followed by manganese, boron, zinc, and copper. Mineral elements are essentially used in the synthesis of a variety of important organic compounds. They also play a variety of roles as ions or as components of inorganic compounds (Table 2.1). Mineral nutrients continually cycle through all living organisms. However, they enter biosphere primarily through the roots. Elemental composition of plants reflects the composition of soil. In soil, more than 60 elements are present, but not all are absorbed by the plants. At the same time, many of the ions absorbed by the plants remain in ionic state inside the cells for an indefinite period. Many of these ions are incorporated either into the structure of more complex molecules, such as storage proteins, calcium oxalate, glycosides, etc., or into the protoplasm or cell walls. Some mineral elements are utilized in one organ of a plant and get subsequently released by disintegration of cellular constituents. These are further translocated to other organs of the plant for reutilization. Redistribution of minerals which have accumulated in cells but have not been actually utilized is also common in plants. Plants have the ability to accumulate essential elements and exclude nonessential elements. Some minerals are required in minute quantities, e.g., plant requires 60 million times less molybdenum than hydrogen (Box 2.1). Such minute quantities could not be detected by the crude methods which were used earlier. In addition to seven ash elements (the incombustible fraction of plant tissue), all elements which are required in large quantity (except for iron) are now known as macronutrients. There is another group of minerals, the micronutrients, needed in only minute quantities. It includes molybdenum, copper, zinc, manganese, boron, chlorine, and nickel.

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