Biofertilizers are defined as " biologically active products or microbial inoculants which help plant's growth."
When plant nutrients are available in abundance the soil is said to be fertile. Microorganisms have a great role in increasing soil fertility.
Biofertilizers either fix atmospheic N₂ or solublilize plant nutrients like insoluble phosphate or stimulate plant growth through synthesis of growth promoting substances.
Depending upon their activities they are classified as follows :
- Symbiotic nitrogen fixers
- Asymbiotic nitrogen fixers
- Bluegreen algae and Azolla fertilizers
- Phosphate solubilizing bacteria ( PSB )
- PGPR (Plant growth promoting rhizosphere)
- Mycorrhizae
- Organic fertilizers.
Disadvantages of Chemical Fertilizers :
- Increased use of chemical fertilizers causes damage to soil physio - chemical properties.
- It causes environmental problems like leaching carcinogenic compounds and damage to ozone layer (green house effect) by releasing nitrous oxide (N₂O), carbon monoxide, etc..
- Chemical fertilizers are expensive.
Advantages of Biofertilizers :
- It is economical.
- Biologically fixed N₂.
- Consumes about 25-30 % less energy.
- It is ecofriendly.
- Biologically originated therefore biodegradable.
- Non - toxic and non carcinogenic.
- Easy application with less labour.
- No sophisticated technology involved.
- Natural, renewable and economical.
- Crop yields increases by 15-20%.
- No adverse effects on crops.
- They improve the physicochemical properties of soil such as pH, structure, texture and water holding capacity of soil.
- Simple to use without side effects.
- Thus, the government promotes the use of biofertilizers.
Limitations of Biofertilizers
- Short shelf - life (less than 6 months), and care in storage is essential.- Not applicable to all crops, due to specificity between a biofertilizer and plants.
1). Symbiotic nitrogen fixing bacteria
- Rhizobia are gram-negative, aerobic, non-spore forming soil bacteria.
- They are able to enter into symbiotic relationship with leguminous plant roots, forming root nodules.
- In the nodules they fix large quantities of N₂, into NH4+ ions which is available to plants for synthesis of amino acids, proteins, etc.
- Efficient rhizobia for different crops and location which are tolerant to various stresses like drought, temperature, high or low pH, salinity, etc. are therefore isolated and cultivated on large scale.
- They are added to carriers like peat or lignite to prepare biofertilizer.
- Generally they are mixed with 10% sugar or jaggery solution and sprinkled on the seeds, before sowing.
- When the seeds are sown, bacteria grow along with seed growth. germination and form root nodules, enhancing plant growth.
- Actinomycetes Frankia species can form symbiotic association with many shrubs and fix nitrogen.
2). Asymbiotic nitrogen fixers
- Azotobacter and Azospirillum ( also Bacillus polymyxa ) when applied to rhizopshere, fix atmospheric N₂ and make it available to crop plants.
- Azospirillum colonizes not only roots, but also above ground parts of the plant through symbiosis.
3). Algal fertilizer ( BGA + Azolla)
- Blue green algae (BGA) and Azolla (an aquatic fern) are useful biofertilizers in lowland paddy farms.
- Composite cultures of Blue Green algae including the genera Anabaena, Nostoc, Plectonema, etc. have been more effective than single cultures. Azolla harbours the BGA in leaf cavities providing symbiotic association.
- BGA fixes atmospheric N₂ which is made available to the crop. They also produce growth promoting substances and provide partial tolerance to pesticides. They help reclaim saline and alkaline soils.
4). Phosphate solubilizing bacteria (PSB)
- Phoshorus is a limiting nutrient in soils.
- Bacillus megaterium, var. phosphaticum, B. polymyxa Pseudomonas striata, P. rathoenis and Aspregillus awamori convert nonavailable inorganic phosphate, into soluble organic phosphate which can be utilised by crop plants.
5). Plant Growth Promoting Rhizobacteria (PGPR)
- PGPR includes Pseudomonas fluorescens and P. putida. They are important new biofertilizers.
- Pseudomonas species produce pseudobactin a siderophore which chelates Iron and makes it unavailable to harmful fungi in rhizosphere.
- PGPR has resulted in Increase in yields of potato, radish and other crops upto 30-144% .
- They also produce plant growth promoting compounds, such as indole acetic acid (IAA).
6). Mycorrhiza
- Mycorrhiza is a symbiotic association with roots of plants and fungi.
- The fungi enhances absorption of nutrients from soil by plant roots resulting in health plants.
- Fungi in turn gets nutrients mainly carbohydrates from plant host.
- There are two types of mycorrhizae : Ectomycorrhiza and Endomycorrhiza.
- Ectomycorrhizae are found on the roots of plants and the fungal growth surrounds the root surface. They belong to class Basidiomycetes, Zygomycetes and Ascomycetes.
- The fungi absorbs nitrogen, phosphorus, potassium and calcium from soil and make it available to plants.
- They also convert complex organic molecules into simpler available forms.
- They also protect the roots from pathogens and produce growth promoting substances.
- Endomycorrhizae are found in the roots of most fruits and horticultural crops.
- They help the plants in obtaining phosphorus.
- They also produce growth promoting substances and offer resistance against pathogens.
- Mycorrhizal biofertilizers have been used for fruit trees such as mango, papaya, tamarind, vegetables and rubber.
- AM (Arbuscular mycorrhiza) biofertilizers are obtained by growing the fungi using a suitable host plant growing in soil, or sand or its mixture. AM fungi are obligate symbionts and they cannot be cultivated in laboratory media, which is a major limitation for its use and inoculum development.
7). Organic fertilizers :
- Organic fertilizers include animal dung, animal urine, bone meals, slaughter house wastes, crop residues, oil cakes, urban garbage, sewage/sludge, sea - weed extracts, etc.
- All these compounds can be biodegraded and mineralised by soil microbes and the soil fertility can be increased.
- Humic acid, fulvic acid, etc. are also used in organic fertilizers to improve soil texture and water - holding capacity.
