Based on the physical nature and carrier materials used, various types of biofertilizers are manufactured by different producers. These are carrier-based inoculants, agar-based inoculants, broth cultures and dried cultures. New developments in biofertilizer production like (i) freeze-dried inoculants (e.g. BAIF, IARI, India), (ii) Rhizobium-paste (e.g. KALO Inc. USA), (iii) granular inoculant (e.g. Soil implant of Nitragin, USA), (iv) pelleting (e.g. Pelinoc of Nitragin), (v) polyacrylamide-entrapped rhizobia (e.g. Agrosoke) and (vi) pre-coated seeds (e.g. Prillcote of New Zealand), appear to be more promising for inoculation success in tropical legumes.
At present, biofertilizers are supplied as carrier-based microbial inoculants which are added to the soil to enrich the soil fertility. The carrier is a medium that can carry the microorganisms in sufficient quantities and keep them viable under specified conditions, easy to supply to the farmers. The use of ideal carrier material is necessary in the production of good quality biofertilizer.
A good carrier should have the following qualities:
Other essential criteria for carrier selection relating to the survival of the inoculant bacteria should be considered.
Biofertilizers are supplied to the soil either by “seed inoculation”, in which the inoculant (bacteria-carrier mixture) is mixed with water to make slurry-form and then mixed with seeds, or by “soil inoculation”, i.e. by spreading over the field during cultivation. In the case of seed inoculation, the carrier must be a form of fine powder. To achieve a tight coating of inoculant on the seed surface, use of an adhesive, such as gum arabic, methylethylcellulose, sucrose solutions and vegetable oils, is recommended. Seed inoculations may not always be successful due to the low nodule occupancy of the inoculated rhizobia strain as a result of the inoculation or low establishment of the inoculated rhizobacterial strain. This might be due to low population and/or low survival of the inoculated bacterial strain on the seed surface and in the soil. In such instance, “soil inoculation” will be adopted, whereby a large population of a bacterial strain can be introduced into the soil. For soil inoculation in general, granular inoculant is placed into the furrow under or alongside the seed. This enhances the chance for the inoculated strain to be in contact with plant roots. Various types of material are used as carriers for seed or soil inoculation. Peat soil, lignite, vermiculite, charcoal, press mud, farmyard manure and soil mixture can be used as carrier materials. Neutralized peat soil/lignite are found to be better carrier materials for biofertilizer production. For preparation of seed inoculant, the carrier material is milled to fine powder with a particle size of 10–40 μm. For soil inoculation, carrier material with granular form (0.5–1.5 mm) is generally used. Granular forms of peat, perlite, charcoal or soil aggregates are suitable for soil inoculation.
The strength of biofertilizers is determined by two basic parameters: number of cells and efficiency of the microorganisms to fix nitrogen or solubilize phosphates.
Liquid biofertilizers are liquid formulations containing the dormant form of desired microorganisms and their nutrients along with the substances that encourage formation of resting spores or cysts for longer shelf-life and tolerance to adverse conditions. The dormant forms, on reaching the soil, germinate to produce a fresh batch of active cells. These cells grow and multiply by utilizing the carbon source in the soil or from root exudates.
As an alternative to conventional carrier–based biofertilizers, liquid formulation technology, which has more advantages than the carrier-based inoculants, has been developed in the Department of Agricultural Microbiology, TNAU, Coimbatore. The advantages of liquid biofertilizers over conventional carrier-based biofertilizers are listed below:
Among different techniques to produce biofertilizer, the concept of effective microorganisms (EM), which are available in liquid form, was introduced in 1991 by Dr. Teruo Higa of Japan. The major groups of microorganisms contained in the EM include filamentous fungi, yeast, lactic acid bacteria and other soil bacteria. The application of EM aims to function as inoculum of microorganisms to the soil in which it will help to establish or re-establish soil ecosystems. EM is commercially available in concentrated form that needs to be processed before the application. According to the procedure suggested by the EM manufacturer, the concentrated EM (EM Bokashi) can be used directly by mixing with molasses and water. However, the common method is to use EM Bokashi as a starter to ferment the raw materials and produce either liquid or solid biofertilizer. The common raw materials include left-over plant or animal materials in the farms. The fermentation period was suggested to be at least seven days and the product is recommended to be used within three months. Today, the production of ready-to-use liquid biofertilizer from EM is becoming available in the market due to the convenience for small-scale farming or domestic application in which the users do not have space and raw materials available for fermentation.
There are three ways of using liquid biofertilizers.
Seed treatment is the most common method adopted for all types of inoculants. The seed treatment is effective and economic. For a small quantity of seeds (up to 5 kg), the coating can be done in a plastic bag. For this purpose, a plastic bag sized 21” x 10” or larger can be used. The bag should be filled with 2 kg of seeds or more. The bag should be closed in such a way so as to trap the air as much as possible. The bag should be squeezed for 2 minutes or more until all the seeds are uniformly wetted. Then the bag is opened, inflated again and shaken gently. The shaking should stop after each seed gets a uniform layer of culture coating. The bag is opened and the seeds are shade-dried for 20–30 minutes. For large amounts of seeds, coating can be done in a bucket and the inoculant can be mixed directly by hand. Seed treatment with Rhizobium, Azotobacter, Azospirillum, along with PSM can be done. The seed treatment can be done with any of two or more bacteria. There is no side (antagonistic) effect. The important things that have to be kept in mind are that the seeds must be coated first with Rhizobium, Azotobacter or Azospirillum. When each seed gets a layer of these bacteria, then the PSM inoculant has to be coated as an outer layer. This method will provide maximum cell counts of all bacteria required for better results. Treatments of seeds with any two bacteria will not provide a maximum number of bacteria on individual seeds.
This method is used for application of Azospirillum/ /PSM on paddy transplating/ vegetable crops. The required quantity of Azospirillum/ /PSM has to be mixed with 5–10 litres of water at one corner of the field and the roots of seedlings has to be dipped for a minimum of half an hour before transplantation.
Use 200 ml of PSM per acre. Mix PSM with 400 to 600 kg of cow dung farmyard manure along with ½ bag of rock phosphate, if available. The mixture of PSM, cow dung and rock phosphate has to be kept under any tree or in the shade overnight and 50% moisture should be maintained. The mixture is used for soil application in rows or during leveling of soil.
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