In this article we will discuss about:- 1. Meaning of Biofertilisers 2. Advantages of Biofertilisers 3. Microbial Mass 4. Problems.
Meaning of Biofertilisers:
Biofertilisers are low-cost, effective, and renewable sources of plant nutrients used to supplement chemical fertilisers. The role of biofertilisers in agriculture production assumes special significance, particularly in the present context of the escalating cost of agriculture inputs.
Biologically active products containing selective strains of micro-organisms which can contribute to plant nutrients through microbial activity are known as biofertilisers. Biofertilisers are mixture containing specific strains of microorganisms like bacteria, fungi, algae or their combinations.
When these preparations are incorporated with seeds, setts, seedlings or soil, they enhance crop productivity and soil health, by way of biological nitrogen fixation, solubilisation and uptake of other nutrients and synthesis of growth-promoting substances.
Biofertilisers have a number of advantages over chemical fertilisers. Biofertilisers are very cheap. They supply other nutrients, may control plant pathogens, also supply vitamins and plant growth hormones and prevent soil erosion by producing capsular polysaccharides. They also convert immobilised chemical into soluble forms and make them available to the plants.
Advantages of Biofertilisers:
The biofertilisers are found to make their contribution as agriculture inputs due to the following advantages:
1. Biofertilisers are supplements to chemical fertilisers.
2. Biofertilisers are low cost and can help to reduce consumption of such fertilisers.
3. They contain micro-organisms which provide biological nitrogen directly to plants.
4. They help in solubilisation and mineralisation of other plant nutrients like phosphates.
5. They improve plant growth due to better synthesis and availability of hormones, vitamins, auxins and other growth-promoting substances.
6. On an average crop yield increases by 10-20 per cent with their use.
7. They control and suppress soil-borne diseases.
8. They help in the proliferation and survival of beneficial micro-organisms in the soil.
9. They improve soil texture by increasing amount of humans and maintain soil fertility.
10. They are eco-friendly and pollution free.
Broadly, biofertilisers are categorised into two main groups, viz., biological nitrogen fixing biofertilisers and phosphate solubilising or mobilising biofertilisers. Biological nitrogen fixing biofertilisers consist of micro-organisms which have the ability to fix biological molecular nitrogen (N2) either symbiotically or a symbiotically in the plants; whereas, phosphate solubilising biofertilisers are capable of solubilising or mobilising the fixed insoluble phosphates of the soil.
Types of Biofertilisers:
Broadly biofertilisers are divided into five main categories.
These five types are again divided in sub-types as follows:
1. Symbiotic- Rhizobium, Frankia, Anabaena azollae.
2. Free living- Azotobacter, Clostridium, Blue green algae, Azolla, Acetobacter, Nostoc, and Anabaena.
3. Associative symbiotic- Azospirillum.
i. Phosphate Solubiliser:
Bacteria – Bacillus megaterium, Phosphaticum, Bacillus circulans, Pseudomonas striata, Pseudomonas sp., Fungi – Penicillium sp, Aspergillus awamori.
ii. Phosphate Absorber Biofertilisers:
Arbuscular mycorrhiza- Glomus sp., Gigaspora sp., Acaulospora sp., Scutellospora sp. and Sclerocystis sp.
Ectomycorrhiza- Laccaria sp., Pisolithus sp., Boletus sp., Amanita sp. Orchid mycorrhiza: Rhizoctonia solani.
Thiobacillus novellas, Aspergillus.
Micronutrients supplier- Silicate and Zinc solubilisers- Bacillus sp.
Plant growth promoters- Pseudomonas- Pseudomonas fluorescens.
4. Organic Matter Decomposer and Microbial Cell Mass:
Cellulose decomposer, Lignin decomposer.
Production of Biofertilisers:
Rhizobium:
Rhizobium is a gram negative rod shaped bacteria, it fixes atmospheric nitrogen symbiotically by forming the nodules on the roots of leguminous plants. The root nodule formation process is highly specific. Rhizobium isolated from root nodules of bean group plant, do not induce root nodule in the pea group plant. It is essential to use specific Rhizobium species as a fertiliser for specific plant.
Sterilise the growth medium and add the pure culture of Rhizobium to prepare inoculum. Incubate on a shaker at 30 to 32°C for 3-4 days. Transfer this inoculum under sterile precautions to a large fermenter; incubate under aeration for 4-9 days. Rhizobium biomass is cultivated and mixed with lignite, charcoal powder, peat or farmyard manure. These materials will act as carrier. Lignite is most commonly used. Lignite is obtained from mine. It is sieved through a sieve of 85 mesh sterilised and cooled.
The pH is adjusted to 6.5 to 7.5 by using sterile CaCO3 or lime. Lignite has 30-40 per cent moisture which prevents death of bacteria during storage. Rhizobium mass is thoroughly mixed with carrier by using rotatory drum. This mixture is allowed to stabilise for 24 hours. Then it is packaged in polyethylene bags and properly sealed, otherwise, carrier get dried leading to the death of micro-organisms during storage.
On the seed surface- Mix the fertiliser in clean water to prepare a thick slurry. Use 250 grams of fertiliser for 10 kg seeds. It is recommended to mix 10 per cent sugar or jaggery or 4 per cent gum in the slurry. Spray uniformly this slurry on the surface of seeds. Intermittent mixing of seeds helps for uniform distribution. Dry seeds in the shade and sow immediately.
Frankia belongs to actinomycetes. Frankia is also a symbiotic nitrogen fixer. It is a filamentous bacterium and bears spores in chain. It forms root nodules in the forest crops. 24 genera and 8 Angiosperm families of forest crops are known to possess actinorrhizal root nodules. Casurina sitophilia and Alnus, forest crops common in India bear actinorrhizal root nodule. Root nodule formed by Frankia are hard like wooden and large (5 to 6 cm in diameter) resembling a tennis ball.
Cultivation is done by using specific medium under microaerophilic condition incubating at 28-30°C at least for 4 weeks. Frankia is a slow grower.
Azotobacter is non-symbiotic nitrogen fixing bacterium commonly present in soil. Azotobacter is gram negative, small rod and forms microcyst as a resistant structure in old culture. Azotobacter species: A. chrococcum, A. vinelandi, A. beijerinckii, A. macrocytogenes. B. A nitrocaptans.
Azotobacter is isolated from soil by using suitable nitrogen free medium. Procedure for mass cultivation and mixing is similar to that of Rhizobium.
1. On the Surface of Roots:
250 grams Azotobacter is mixed in 10 litres of water. Roots of transplanting crops or potatoes or sugar cane pieces are dipped in this mixture for 2-3 minutes, just before the transplantation.
2. In the Soil:
This method is rarely used. Mixture of fertiliser in fine soil is sprayed on the soil having grown up crop. Then biofertiliser is mixed in the soil by using spade.
It was found that some tropical forage crops like maize, wheat, sorghum and rye possess nitrogen fixing potential due to activity of Spirillum in their roots, Spirillum was then re-examined and named as Azospirillum. Azospirillum reside inside the roots and aerial parts of plants. They are absent in dicotyledons.
Azospirillum Species:
A. brasilense, A. lipoferum, A. halopraeferans and A. amazonens. A. amazonense are observed in acidic soil, A. halopraeferans are found in salaine soils.
It is similar to that of Rhizobium except the chemical composition of media for mass cultivation is different.
Application:
Application of Azospirillum in the field is similar to Azotobacter.
Blue Green Algae (cyanobacteria):
Blue green algae like Aulosira, Anabaena, Tolypothrix, Cylindrospermum, Nostoc, Plectonema fix the atmospheric nitrogen. They usually represent 30 per cent of the total algae occurring in the soil.
Cyanobacteria population may be 70 per cent of total algae occurring in moist soil. Blue green algae, bear heterocyst, a site of nitrogen fixation. Cyanobacterial growth is favoured by excess of water which is a need of rice crop.
Mass Cultivation:
A small pit (m2) is prepared in the soil and lined with polythene sheets, 10 kg of sieved soil, 250 grams super phosphate, 19 grams sodium molybdate are added in the pit. Water is added in the soil to maintain the level 7-10 cm depth. Furadon or carbifuron may be added as pest control.
The pH is adjusted to 7.0 by lime. 0.15 gram inoculum containing mixture of different cyanobacterial genera is added in the pit. Incubated for one week. Cyanobacteria grow and form scum on water surface. When it dries out, scum is scrapped, dried, powdered and stored in bags.
Application:
10-20 kg/ha dried algae is sprayed in rice field a week after transplantation of rice seeding.
Azolla:
Azolla is an aquatic fern. It has mainly three parts, stem, small leaves and fine rootlets. Leaves contain an endophytic cyanobacterium. Anabaena azollae in its small leaf cavity on upper surface. A. azollae fixes the atmospheric nitrogen and supplies to the plant, whereas algae derives some nutrients from plant. Azolla have 94 per cent water, 5 per cent nitrogen and 1 per cent minerals. Being watery it decomposes rapidly in the soil.
Azolla Species:
A. caroliniana, A. mexicana, A. microphylla, A. nilitica, A. pinnata and A. rubra. A. pinnata is commonly found in India.
Mass Cultivation:
Prepare a microplot (20 m2) with tin or cement. Add water 5-10 cm in depth. Add superphosphate 4-20 kg/ha, fresh Azolla 0.5 to 1.0 kg/m2 to the water. Adjust the pH of water 8.0, optimum temperature is 14-30°C. Furadon may be added as insecticide Azolla grows on the surface forming a mat. Harvest the Azolla after full growth usually after 20 days and dry.
Application:
Cultivate the Azolla either before or after transplantation of rice in the field. Inoculate 0.5 kg/m2 allow to grow and then water is drained to mix the Azolla in soil.
Limitations:
In summer it is very difficult to maintain the level of water for growth of Azolla, growth stops as it touches to soil. Azolla may be washed out due to heavy rain. At temperature above 30°C, growth of Azolla is markedly reduced. Azolla mass is bulky and difficult to transport.
Advantages:
Azolla shows tolerance against heavy metals, so it is the best fertiliser in the soils polluted due to metal wastes.
Acetobacter Diazotrophicus:
Micro-organism resides in the leaves, stem and roots of sugarcane plant, utilise sugar of the plant and fix atmospheric nitrogen. These have 20 times more efficiency to fix atmospheric nitrogen than Azotobacter.
The phosphorus is second important plant nutrient. There are two types of micro-organisms making available this plant nutrient.
Phosphate Solubilisers:
Soil contains large amount of insoluble inorganic phosphates and immobilised organic phosphorus also. Phosphate solubilising micro-organisms (bacteria, algae) solubilise tricalcium, aluminium, iron and rock phosphate and organic phosphorus. These micro-organisms easily convert these minerals in to easily available form to the crops in the soil.
PSM:
Bacteria:
Bacillus megaterium Var. phosphaticum. Pseudomonas striata, P. fluorescens, Achromobacter.
Fungi:
Aspergillus, avamori, A. niger, A. flavus, Penicillium digitatum, cephalosporium.
Algae:
Chlorella sp., Anabaena, novecularis, Tolypotherix tennius. Actinomycetes: Streptomyces sp., Actinomyces sp.
Mass Cultivation:
B. megaterium var phosphaticum. and Pseudomonas striata are generally cultivated as fertilisers. Mass cultivation and mixing with carrier is similar to that Rhizobium by using medium.
Application:
Very similar to Rhizobium application on the seeds or transplantable crops Vesicular arbuscular mycorrhiza (VAM). The symbiotic association between fungus and root systems of higher plants is called mycorrhiza, which literally means fungus roots. VAM fungi- Glomus, Sclerocystis, Gigaspora, Endogone, Acaulospora. VA. mycorrihizae develop special characteristic structures called arbuscles and vesicles.
The finger like projections, arbuscles, help in the transfer of nutrients (especially phosphates) from the soil into the root system. Absorbed phosphate is stored in circular or ellipsoidal or rectangular vesicles. VA. mycorrhizae are type of endomycorrhizae.
In endomycorrhizae, the fungus lives within the cells of the root and establishes direct connections between the cells of the root and the surrounding soil. VA. mycorrhizae occur in roots of most angiosperms, pteridophytes and bryophytes, which includes onion, tomato, brinjal, jowar, bajra, sunflower and groundnut.
Mass Cultivation:
Mass cultivation of VA mycorrhizae is difficult. These fungi are obligate symbionts and have not been cultivated in pure culture. The root biomass heavily infected by a specific VAM fungus serves as the inoculum for mass cultivation. Mass cultivation is carried out by cultivating plant roots, in sterilised soil, in prior sterilised room.
Temperatures 25 to 28°C, relative humidity 70- 80 per cent and light intensity 10 to 15 kilo lux are maintained in the room. Root pieces carrying mycorrhizae and soil around is used as inoculum and mixed in the soil. Seeds are sown. After approximately 65 to 90 days mycorrhizae grow luxuriantly in plant roots. During this period enough water is supplied for plant growth. Then this soil along with root pieces are mixed together and used as biofertiliser.
Advantages:
VAM fungi also supply other nutrients like MO, Ca, Zn, Fe, Cu, Mg, to the plants. VAM fungi absorb water from soil and supply to plant VAM fungi also prevent infection of plant pathogen. VAM fungi is the best fertiliser in acidic soil.
Micro-organisms are known to increase the availability of sulphur in the soil for absorption by plants. Large amount of sulphur in the soil is insoluble in water and hence cannot be absorbed by plant roots.
Sulphur oxidising bacteria convert this insoluble sulphur to sulphate, which is absorbed by plant roots. Sulphur oxidiser- Thiobacillus, Thiothrix, Thioploca, Aspergillus, Penicillium microsporeum. Thiobacillus thioxidans and T. novellus, are commonly used as sulphur biofertilisers.
If these are inoculated in alkaline soils, these produce H2SO4 and drop the pH of soil, making it fit for plant cultivation.
Large amount of complex organic matter is added in the soil in the form of plant leaves or bagasse. Micro-organisms having cellulolytic activity and lignolytic activity if added as a fertiliser, they decompose complex organic matter into simpler form and increase the fertility of soil. Cellulolytic microorganisms like Trichoderma, Cellulomonas and lignolytic micro-organisms like Arthrobacter can be cultivated in the laboratory and added in the soil as a biofertiliser.
Microbial Mass:
Saprophytic microbial mass can be used as bio-organic fertiliser. Microorganisms during growth produce different enzymes for conversion of complex substrates. Microbial mass is derived from the growth of Penicillium by using peanut meal, cotton seed meal, corn-steep liquors, ammonium sulphate, potassium phosphate, sulphate, phenyl, acetic acid as precursor; and zinc, iron, manganese etc., as trace element along with continuous supply of sugar solution.
Microbial mass consists of protein, enzymes, vitamins, organic acids, plant growth hormones. Chitin present in it chelates the toxic metals present in soil. Micro-organisms in microbial mass also consume the waste products liberated by roots of the plants, and thus enhance the growth of plants.
Problems with Biofertilisers:
1. Microbial fertilisers are supplementary to chemical fertilisers but not substitute to it. Microbial fertilisers, usually cause 20 to 30 per cent increase in crop production. They do not cause marked increase in productivity like chemical fertiliser.
2. Specific fertilisers are to be used for specific crops. This is more applicable to symbiotic micro-organisms. If non-specific Rhizobium is used as fertiliser, they do not cause root nodulation and increase in crop production.
3. Strict aseptic precaution is required during production of microbial fertiliser. Contamination is a common problem during microbial mass production.
4. Microbial fertilisers are sensitive to sunlight exposure. They get killed if exposed for long time in sunlight.
5. Microbial fertiliser must be used within six months after production when stored at room temperature. They can be used within two years if stored at chilling temperature.
6. Efficiency of microbial fertiliser is markedly dependent on soil character, e.g., moisture content, pH, temperature, organic matter and types of resident micro-organisms. When these factors are unfavourable microbial fertiliser may not be effective in increasing the soil fertility.