Plants need food for their growth and development like all other living things Human beings and animals depend on plants for their food but plants can make their own food from natural raw materials. Plants live, grow and reproduce by taking up water and mineral substances from the soil, carbon dioxide from the air and energy from the sun to form plant tissues.
Sixteen elements have been found to be indispensable for plant growth, development and reproduction. These essential elements are referred to as “nutrients”. An elements to be called as an ‘essential nutrient’ should meet the following three criteria.
• The plant is unable to complete the vegetative or reproductive stage of its life without that element.
• The need of such a nutrient is specific and its deficiency symptoms can be corrected by supplying the same elements.
• The nutrient plays a direct role in plants active (metabolic) processes and meets its nutritional needs. Of the 16 essential elements, carbon, hydrogen and oxygen are obtained from the carbon dioxide in the atmosphere and water. They make up 90-95 per cent of the dry matter of all plants. Nitrogen, phosphorus, potassium, calcium, magnesium, sulphur, zinc. copper, iron, manganese, molybdenum, boron and chloride are obtained from the soil through the root system and they make up the 5-10 per cent of the plant. In addition, certain plant species have been shown to benefit from the presence of cobalt, sodium, silicon and possibly vanadium but these do not rank as essential nutrients.
NEED FOR FERTILIZERS
The growth and yield of crops depend on the ability of soil to supply plant nutrients. Nutrients are lost from the soil-plant system in a number ways. Removal of harvested products i.e. grain and straw takes with them the considerable quantity of all nutrients. It is, therefore, necessary to apply nutrients through mineral fertilizers or organic sources to the soil to reap the full potential of various crops. The types and amounts of fertilizer to be applied depend on the crop to be grown and nutrient supplying power of the soil.
Fertilizers are needed for the following three purposes.
1) To supply the components to the soil necessary for the plant growth and development
2) To provide an additional food to the soil so that plants grow healthier and produce better yield.
3) To maintain the pH of the soil in the range of 7 to 8 a favorable condition for the plant growth.