These consist of elements such as phosphorus, potassium, sulphur, calcium, magnesium, iron, chlorine, manganese, boron, zinc, copper and molybdenum, which are needed for the normal growth and development of plants. However, plants do not require all these mineral elements in equal quantities; some are needed in fairly large quantities whereas some others are needed only in minute amounts. Depending on these criteria, mineral elements, which are required by plants, can be classified into two groups: macronutrients and micronutrients. Macronutrients, consisting of such elements as phosphorus, potassium, sulphur, calcium and magnesium must be present in plant tissue in concentrations of at least 1 milligram per gram of dry matter. Micronutrients on the other hand, are needed in concentrations of 100 micrograms or less per gram of dry matter. The major micronutrients, needed by plants are iron, chlorine, manganese, boron, zinc, copper and molybdenum. However, this distinction of mineral elements into macro and micronutrients may not be always important as all these are essential to the life of the plant. There may also be other elements essential to some plants and perhaps, to all.
Source of Essential Elements for Plants
Carbon and oxygen enter the biosphere through the process of photosynthesis of plants and some Monera and Protists. Both the elements are derived from atmosphere as carbon dioxide. The principal source of hydrogen is water. Plants take up water from the soil and the process of photosynthesis incorporates the hydrogen into the plant body and thereby to the entire living world. Nitrogen exists as virtually an inert gas N2 (Dinitrogen). About four fifth of the atmosphere is nitrogen only. Dinitrogen (N2) is first processed (fixed and oxidised) by some highly specialized bacteria in the soil. This results in products such as nitrates that can be taken up by plants.
The mineral elements required by higher plants come from the soil or the water in which the plants grow. Soils differ so greatly that it is difficult to generalise. However, all soils can be seen to consist of two or more recognizable, horizontal layers called horizons, one lying on top of the other. Generally, mineral nutrients contained in the top horizon are leached (dissolved in rain or irrigation water) and carried to deeper horizons. Leaching and removal of crops, which contain mineral nutrients obtained from the soil during growth, deprive the soil of its mineral content. Often it becomes necessary to add fertilizers to agricultural soils. If too much of any element is removed, crop yields will be reduced. Organic fertilisers contain materials that improve the physical properties of the soil, providing air pockets for root growth. Inorganic fertilisers, on the other hand, cause rapid increases in soil nutrients and can be formulated to meet the requirements of a particular soil and a particular crop, more precisely.
Carbon, oxygen and hydrogen are constituents of all organic molecules. Nitrogen is an essential constituent of proteins, amino acids and nucleic acids.