Fertilizers are chemicals used to improve the fertility of the soil and increase crop productivity. ‘Green Revolution’ has come about because of the use of chemical fertilizers. They brought multiplication in crop productivity to several folds. Chemical fertilizer industry needs large scale use of fossil fuels. Large scale use of chemical fertilizers is resulting into several evil effects such as the loss of soil fertility, environmental pollution, persistence of fertilizer residues in the produce. Keeping this in view, the attention is now being given on the use of biofertilizers to improve the agricultural production.
The fertilizers are classified into three categories—1. Organic fertilizers 2. Inorganic fertilizers and 3. Biofertilizers.
1. Organic fertilizers : Farm-yard manure, compost, green manure and vermicompost are the main organic fertilizers. The use of this type of manures has been a very old practice. The role of farm-yard manure in improving the fertility of soil and enhancing the crop yield is well known and has been quite good.
Decomposition of animal excreta, crop and plant residues, domestic wastes results in the production of compost. It has helped a lot in increasing the fertility of the soil.
The use of green parts of the plants directly in the fields has been in practice for many years. There are two methods in the application of green manure to agricultural fields. In one method nitrogen fixing plant such as Crotalaria juncea, Phaseolus trilobus, Phaseolus mungo, Sesbania sesban etc. are raised in fields like rice fields. When these plants reach a certain height, the field is flooded with water and thoroughly ploughed. After some time, rice transplantation is made. In another method branches from trees like Gliricidium maculata, Sesbania, Derris (Pongamia) are cut and directly put into the fields slightly under soil.
The role of earthworms in protecting the soil fertility and in improving the physical features of the soil is well-known to the farmers. With the help of earthworms, organic wastes can be converted into highly beneficial manure. The practice of earthworm rearing is referred to as ‘vermiculture’ and the manure produced in this method is known as ‘vermicompost’. Vermicompost contains 18% carbon, 2-3%, nitrogen, 1-2% phosphorus and 1.5% patassium. It increases the yield of crops and fertility of soil.
2. Inorganic fertilizers : You have already studied that macro and micro elements which are required for plant growth and good yield. The source for 13 essential elements as the minerals are present in the soil. Among these mineral elements nitrogen, phosphorus and potassium are more important. The use of nitrogen, phosphorus and potassium fertilizers is in great demand to enhance the yields of agricultural crops. Important forms of NPK (Nitrogen, Phosphorus, Potassium) fertilizers are given below:
(A) Nitrogen fertilizers
(i) Urea (NH2.CO.NH2) : Urea is an important organic form of nitrogen fertilizer. In this fertilizer nitrogen is present in the form of amide. The nitrogen content is 46%. This is very useful for increasing crop yield.
(ii) Ammonium sulphate [(NH4)2 SO2)] : This contains 21% nitrogen (as ammonia) and 24% sulphur (as sulphate). This nitrogen fertilizer in the form of ammonium sulphate is very useful.
(iii) Calcium ammonium nitrate : This contain 26% nitrogen in the form of nitrate and ammonia in equal proportions. It increases the crop yield.
(B) Phosphorus fertilizers : The grade of phosphate fertilizers is expressed as a percentage of phosphorus pentoxide (P2O5) equivalent.
(i) Superphosphate : 16 to 20 percent as available P2O5 percent. Most commonly used phosphorus fertilizer in India.
(ii) Rock phosphate : 25 to 35 as available P2O5 percent. Effectiveness depends on the degree of fineness and soil.
(iii) Bone-meal : 17 to 30 as available P2 O5 percent. It is available in two forms, raw bone-meal and steamed bone-meal. The more finely powderd it is, the quicker its P2 O5 becomes available in the soil and increases fertility.
(iv) DAP (Diammonium phosphate): In this, 40% phosphorus and 18% nitrogen are present.
(C) Potassium fertilizers : The quality or grade of potassium fertilizers is expressed as a percentage of potassium oxide (K2O) equivalent.
(i) Murate of potash (Potassium chloride) : It contains 50 to 63 percent K2O, the whole of which is readily available. It is the most commonly used potassic fertilizer and highly effective in increasing the crop productivity.
(ii) Sulphate of potash : It contains 49 to 52 percent K2O. It dissolves readily in water and becomes available to the crop almost immediately.
It addition of NPK fertilizers, the use of micronutrients is on the rise for enhancing crop productivity. Several formulations containing one or more combinations of microelements are now available in the market. Particularly the use of zinc is being encouraged in agricultural fields in our country to increase the yield of rice.
3. Biofertilizers : As the name indicates, ‘biofertilizers’ denote the nutrient input for plant growth which is of biological origin. Biofertilizers comprise a variety of living microorganisms. Nitrogen fixation by Rhizobium in the root nodules of legumes is well-known. Employment of legume crops in mixed cropping and crop rotation for soil enriching has been in practice for several centuries. In addition to Rhizobium, several other microorganisms also have the capability of fixing atmospheric nitrogen which enhances fertility of the soil. Some of the biofertilizers are given below:
(A) Rhizobium inoculants : Rhizobium inoculants are being increasingly used to increase the productivty of legume crops. Rhizobium inoculants have now become industrial products in several countries and are being produced on large scale. Several Rhizobium strains are now available in the market. Rhizobium inoculants are applied to the seeds with gum or carboxy methyl cellulose and the seeds are then sown in the field. The plants which develop from such seeds possess larger number of root nodules with better growth and yield.
(B) Cyanobacteria : You have studied that blue green algae such as Anabaena and Nostoc either independently or in symbiotic association to fix atmospheric nitrogen. In Japan blue green algae are used in rice fields for increasing the yield manifold. In our country also the use of blue green algae is increasing every year. Farmers can develop cyanobacterial inoculants in their fields by adopting simple procedures. These inoculants develop as dense mats. These mats are to be cut into small fragments or blocks and can be introduced into rice fields. 10 kilograms per hectare provide very good yield.
(C) Azospirillum : In maize, sorghum, wheat, barley and finger millet, Azospirillum lives as an endosymbiont in cortical cells and protoxylem vessels. Besides the ability to fix atmospheric nitrogem, Azospirillum also secretes certain growth promoting substances. Experiments have shown that the yield in case of sorghum and pearl millet has been increased by employing Azospirillum.
(D) Azolla : Azolla is a water fern. In leaves, the blue green algae Anabaena azolla lives as an endosymbiont and fixes nitrogen. The importance of Azolla as biofertilizer was first experimentally shown in Vietnam in 1957. Azolla adds 30-40 kg. of nitrogen per hectare. By using Azolla as biofertilizer in rice fields, yields can be improved many times.
(E) VAM : Vesicular arbuscular mycorrhizae (VAM) are endogenous mycorrhizae. VAM fungi increase the phosphate absorption by the roots. Experiments have shown the yield of potato, wheat, maize, soyabean and red gram can be increased by the employment of VAM fungi.
Importance of biofertilizers : The use of biofertilizers provides several advantages :
(i) By the use of biofertilizers, the use of harmful chemical fertilizers can be curtailed substantially.
(ii) Biofertilizers do not posses environmental pollution problem. They are eco-friendly in nature.
(iii) Biofertilizers are cheap and available in plenty. By following some simple procedures, even the farmers can also develop biofertilizers on their own.
(iv) Ne energy input in the form of fossil fuels is required in the production of biofertilizers.
(v) Crop produced does not contain any toxins.
(vi) The production cost of biofertilizers is cheaper than chemical fertilizers.
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