In India, the availability and affordability of fossil fuel based chemical fertilizers at the farm level have been ensured only through imports and subsidies. Indiscriminate and imbalanced use of chemical fertilizers, especially urea, along with chemical pesticides and unavailability of organic manures has led to considerable reduction in soil health. Biofertilizers can act as a renewable supplement to chemical fertilizers and organic manures. They have the capacity to produce natural resistance in plants against pests and soil borne diseases, adding fertility to soil.
Nitrogen fixation by leguminous and other crops is reported to be 44 million metric tons per annum. The appropriate strain of Rhizobium can increase the crop yield up to 10-35%. Also, residual N is beneficial for the next crops grown in the same field.
It has been estimated that 40-250 kg N / hectare(ha) / year is fixed by different legume crops by the microbial activities of Rhizobium.
Biofertilizers are preparations containing beneficial micro organisms like N2 fixers, PO4 solubilizers in a viable static state intended for seed or soil application and designed to improve soil fertility.
1. They reduce the need for chemical fertilizers.
2. They provide the plant with certain vitamins, plant growth promoting substances and increase the vigour of the plant.
3. It is cheap and cost effective.
Based on the nutrients that they provide, biofertilizers are of the following types
Rhizobium – legume symbiosis is a well studied plant microbe interaction and Rhizobium is the most extensively used nitrogenous biofertilizer in India.
Rhizobium is a gram negative, non-spore forming aerobic bacillus inhabiting the soil in a free living state. The colonies of Rhizobium on YEMA (Yeast Extract Mannitol Agar) plate are gummy, pale white in colour (Figure 11.7). They can establish symbiotic relationship with leguminous plants and fix atmospheric nitrogen thereby greatly improving soil fertility.
The flowchart explaining the mass production of Rhizobium biofertilizer is given below
Carrier based Rhizobium inoculants are mixed with water to form slurry to which the seeds of plants are added (Figure 11.8). The coated seeds are dried in shade and used for sowing.
Several soil bacteria like Pseudomonas and Bacillus possess the ability to convert insoluble mineral phosphates into soluble form by secreting organic acids thereby making it available to plants.
For mass cultivation and inoculant preparation, the cultures are grown in Pikovaskaya broth for 7-18 days and mixed in suitable carrier like peat or lignite. After curing for a week, the inoculants are packed and made ready for use in a similar manner as Rhizobium inoculants.
Mycorrhiza means fungus root. It describes the symbiotic association between plant and fungus. Vesicular Arbuscular Mycorrhiza (VAM) is an endomycorrhiza which is used as a fungal biofertilizer. They mobilize the soluble phosphates in the root zone of plants and satisfy the phosphorus nutrition of plants.
VAM is an example of endomycorrhiza meaning, the storage organelles of phosphates like vesicles and arbuscles are seen intracellularly. Vesicle is a globose structure and arbuscle is a tree like branching structure present in the root cortical cells (Figure 11.9). VAM fungi are naturally most prevalent in angiosperms. gymnosperms, pteridophytes and bryophytes.
Root based inoculum is used for preparing VAM biofertilizer (Figure 11.10). The selected spores of VAM fungi are allowed to infect plants like onion, sorghum and other grasses. After 3-4 months, the roots along with the soil are macerated or pelleted with an inert material and packed in polythene pouches which can be used as biofertilizer.
Blue green algae are single celled or filamentous prokaryote capable of nitrogen fixation and photosynthesis.
Most of the filamentous forms have specialized large, thick walled cells called heterocysts which are sites of nitrogen fixation.
Example: Nostoc, Anabaena is examples for filamentous BGA. Gleocapsa is an example of unicellular BGA. Some of the filamentous forms do not possess heterocysts but still fix atmospheric nitrogen. Since they need standing water for their growth, BGA can effectively colonize paddy fields and enrich the soil with nitrogen.
Applying BGA to paddy fields can reduce the amount of chemical nitrogenous fertilizer applied for the growth of paddy crop. Therefore cultivation of BGA in large quantities is necessary. Mass cultivation of BGA has the following steps.
1. Isolation of BGA
2. Mass cultivation of BGA
BGA can be isolated from soil or paddy fields. Appropriate dilutions from serially diluted algal sample are inoculated in the liquid flasks containing algal media like BG-11 or Pringsheim’s media. After several weeks of incubation at 28°C, the individual colonies are picked up, identified and stored. This can be used as starter culture for mass cultivation. Mass culture can be done in 2 ways.
The dried algal flakes around 10kg/ha can be applied in paddy fields after transplantation.
Azolla is a floating freshwater fern. The plant has a branched stem, deeply bilobed leaves which are arranged alternately on the stem and each leaf has a dorsal and ventral lobe (Figure 11.9). The dorsal lobe houses the cyanobacterial symbiont Anabaena azollae (Figure 11.10). The fern and the cyanobacteria exhibit symbiotic relationship in which Anabaena provides the fern with fixed nitrogen and fern provides niche for the cyanobacteria free from competition from other microorganisms.
Azolla can be used as a nitrogenous biofertilizer for paddy crop. When applied into the paddy fields, Azolla provides nitrogen nutrition to standing rice crop and can reduce the need for synthetic fertilizers.
Azolla is grown on the flooded rice fields prior to planting for 2-3weeks. Then water is drained and Azolla is incorporated into the soil followed by rice transplantation within a week’s time.
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