Essential Plant Nutrients for Horticultural Crops
According to-the present state of our knowledge, sixteen-elements are considered essential for growth of fruit plants. These are Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorus, Potash, Calcium, Magnesium, Sulphur, Zinc, Manganese, Copper, Iroa, Boron, Molybdenum and Chlorine. New refinements in experimental; techniques may add in the future more elements to this list of essential elements of the sixteen essential elements, Carbon, Hydrogen and Oxygen are obtained from air and water. Carbon makes up the great bulk of the plant and is obtained from carbon dioxide in the air. Plants synthesis carbohydrates by the process, of photosynthesis. Hydrogen is derived mainly from the breakdown of water. Oxygen is obtained from air as well as water. Lack of oxygen in the soil may result in injury to .the roots, particularly if soils are full of water, but there is no lack of oxygen in the tree for direct nutrient use as long as moisture is present
The rest of the thirteen element/s are obtained from mineral arid organic components of the soil. Based on/Jie quantities required for optimum plant development, these elements are grouped into major and minor or macro and micro-nutrients. The role of these essential nutrient elements in the growth and fruiting of plants is as follows:
Nitrogen is an essential constituent of proteins and chlorophyll and is present in many other compounds of great physiological importance in plant metabolism such as nuclectides, phosphatides, alkaloids, enzymes, hormones, vitamins etc. It increases chlorophyll content imparting dark green colour to foliage and promotes rapid early growth. The nitrogen supply governs to a considerable degree, utilization of potassium, phosphorus and other elements.
Phosphorus is a constituent of nucleic acid phytin and phospholipids. It is essential in laying down the primordial for the reproductive parts of the plant. Phosphorus is also an essential constituent of a majority of enzymes which are of great importance in carbohydrate metabolism, fat metabolism and also in respiration. It is closely related to cell multiplication and development. Phosphorus stimulates root growth, flowering and aids in fruiting.
Unlike all other major nutrients, potassium does not enter into the composition of any of the important plant constituents such as fat protein chlorophyll and carbohydrates concerned in plant metabolism. As such, its role in difficult to determine. It occurs in a state of solution in the cell sap. It imparts increased vigour and disease resistance to plants. It regulates utilization of available water in the plant. It is essential in the formation and transfer of starches and sugars. Thus, potassium is required in large quantities of banana. It accelerates enzymic action, helps in the formation of protein and chlorophyll and improves keeping quality of fruits. With citrus fruits, however, an excess of potash has a bad effect on quality.
Calcium promotes root development and growth, influences the Water economy of the plant and many physiological processes. The effects of calcium are, however, antagonistic to those of potassium. Therefore,, an optimum ratio of potassium to calcium is of great importance for a favorable water balance. Calcium improves intake of other plant nutrients, such as nitrogen, iron, boron, zinc, copper and manganese by correcting soil pH.
It is a constituent of chlorophyll and plays a part in the production of carbohydrates, proteins fats and vitamins and in certain catalytic reactions in the enzyme systems. It acts as a carrier of phosphorus in the plant and promotes the formation of fats. It helps in the translocation of starches and regulates the uptake of other nutrients.
It is not a constituent of chlorophyll but helps in its formation and encourages vegetative growth and root development. It is an essential constituent of many proteins and enzymes and essential oils.
It is primarily concerned with-the-uptake and efficient use of calcium in the plant. It helps in absorption of nitrogen. It is associated with cell division, flowering and fruiting processes, pollen germination, metabolism of carbohydrates, nitrogen, water and pectin substances, absorption of salt and hormone movement in the plant. It is necessary in the translocation of sugar in the plant.
The function of manganese is regarded as being closely associated with that of iron. It also supports the movement of iron in the plant. It helps in chlorophyll formation, acts as a catalyst in oxidation – reduction reactions in plants. As a constituent of chlorophyll, it helps in respiration and in protein synthesis. A good managanese supply sometimes helps in counteracting the bad effect of poor aeration.
Though it is not a constituent of chlorophyll it helps in its formation. It is very immobile element within the plant. It helps in absorption of other elements. As a constituent of enzyme systems which bring about oxidation – reduction reactions in the plant, it regulates respiration, photosynthesis and reduction of nitrates and sulphates. These reactions are essential for plant development and reproduction. It is also essential for the synthesis of proteins contained in the chloroplast.
It is a constituent of several enzyme systems and also influences the formation of some growth hormones in the plant. It regulates utilization of water in the plant.
It acts in enzyme systems which bring about oxidation – reduction reactions, especially the reduction of nitrates to ammonia prior to amino acid and protein synthesis in the cells of the plant.
Copper has an important function in root metabolism as well as in utilization of ammonical nitrogen by plants. It acts as "Electron Carrier" in enzymes which bring about oxidation – reduction reactions in plants. It helps in utilization of iron in chlorophyll synthesis. It regulates respiration in the plant.
Chlorine has been proved to be an essential plant nutrient in 1954 by T.C. Broyer and his associates at the University of California (U.S.A). The need for chlorine for proper plant development has been established for sugar – beer carrot cabbage lettuce, barley, wheat, cotton and clover. The exact role which chlorine plays in plant nutrition has not yet been clearly defined.