Plants obtain inorganic nutrients from air, water and soil.
Julius von Sachs developed the technique for the study of nutrients essential for plants.
Growing of plants in nutrient-rich water medium, in complete absence of soil, is called hydroponics.
Seventeen nutrients required by plants are:
• Non-mineral nutrients (three): Carbon, hydrogen and oxygen that are derived from air and water.
• Mineral nutrients (fourteen): They are obtained from soil.
On the basis of amount of essential minerals taken up by the plants, minerals have been classified as macronutrients and micronutrients.
Macronutrients include C, H, O, N, P, K, Ca, S and Mg, while micronutrientsinclude Fe, Cu, Zn, Mn, B, Mo, Cl and Ni.Some additional nutrients required by the plants are Na, Si, Co, Se, etc.
When the level of these nutrients reduces below the critical level, visible changes occur in a plant, which are called deficiency symptoms. These symptoms include chlorosis, necrosis, stunted growth, accumulation of anthocyanin, lack of new growth,etc.
The concentration of nutrients at which growth becomes retarded is termed as critical concentration.
In plants, mobile nutrients are N, P, K, and Mg, while immobile nutrients are B, Ca, Cu, Fe, Mn, Mo, S and Zn.
If any concentration of mineral ion in the tissues reduces the dry weight of the tissues by about 10%, it is considered toxic.
Mineral absorption occurs through a combination of active and passive transport.
If the absorption of nutrients occurs due to difference in their concentration without the use of energy, it is known as passive absorption. Some ions are absorbed in the apoplast of the cells passively.
Movement of ions is termed as flux. The movement into the cell is influx and the outward movement is efflux.
If the absorption of nutrients occurs by the utilisation of energy in the form of ATP, it is known as active absorption.
Movement of absorbed minerals into the plants is called translocation.Translocation of minerals takes place through xylem.
All types of nitrogen metabolism involve recycling of ammonia and its ion known as NH4+ ion.
Nitrogen Cycle is the cyclic circulation of nitrogen in nature, chemical reactions in which atmospheric nitrogen is compounded, deposited in the soil, where it is metabolised by bacteria and plants, eventually returning to the atmosphere by bacterial decomposition of organic matter.
Four processes participate in the cycling of nitrogen through the biosphere: Nitrogen fixation, ammonification, nitrification and denitrification.
Steps in Nitrogen Cycle:
Step I: Biological nitrogen fixation is performed by microbes like Azotobacter(free-living),Rhizobium(symbiotic), etc., to yield nitrates and ammonia.
Rhizobium shows symbiotic relationship with the roots of various leguminous plants such as alpha-alpha, sweet pea, etc. The most common association between themis as nodules.
Frankia produces nitrogen-fixing nodules on the roots of non-leguminous plants like Alnus.
Rhizobium and Frankia are free living in soil, but can fix atmospheric nitrogen as symbionts.
Ammonia formed by nitrogen fixation is used for synthesis of amino acids. It can occur in two ways; namely, reductive amination and transamination.
Step II: Ammonificationis performed by bacteria.
Step III: Nitrification is performed byNitrosomonas and Nitrobacter.
Step IV: Denitrificationis performed byPseudomonas and Thiobacillus.