The use of biofertilizers is important while practicing integrated plant nutrient management as well as organic farming. Organic fertilizers are preferable alternatives as they feed the microorganisms, are less likely to burn plants and are too mild to affect the water cycle except when present on a massive scale.

Some organic fertilizers include:
  • Compost
  • Composted horse, poultry, sheep, goat and pig manures or waste. These can be source from farmers, or commercially under brand names such as Dynamic Lifter, Dino-Fert, and Blood-And-Bone.
  • Cow manure (un-composted is fine)
  • Worm castings and worm juice
  • Algae, sea-weed or fish waste in the form of diluted water from your fish tank, or commercial products such as SeaSol and Charlie Carp liquid fertilizers
  • Home-made liquid fertilizers brewed from weeds, green-waste or manures.

In case of Rhizobium, Azotobacter, Azospirillium and Phosphate Solubilizing Bacteria, the total viable count shall not be less than 1 x 107 CFU per g of carrier material in the form of powder or granules or 5 x 107 CFU per ml in case of liquid formulations during the entire period of shelf life.

In case of Mycorrhizal Biofertilizers, the viable propagules shall not be less than 80 per g.
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Some of the commonly used Biofertilizers are as follows:
  1. Rhizobium and Bradyrhizobium

It induces better root nodulation and stem modulation in inoculated plants and thereby brings down the requirement of nitrogen fertilizer for the cultivation of pulses, oil seeds and legume green manures. Commercially it is available as carrier-based inoculum. Method of application is seed treatment.

  1. Azotobacter

Suitable for upland crops like vegetables, tapioca, plantation and orchard crops. It is available as carrier-based inoculum. It fixes about 15-20 kg N/ha under ideal upland conditions and thereby reduces the requirement of nitrogen fertilizers by 10-20 per cent. Methods of application are seed treatment, seedling dip and direct soil application.

  1. Azopirillum

It is suitable for both upland and wetland conditions and is available as carrier-based inoculum. It fixes about 20-25 kg N/ha under ideal conditions, thereby effecting a reduction of 25 per cent in the quantity of N fertilizers required. Treatment with Azospirillum also induces better root formation in inoculated plants. Hence this biofertilizer is also recommended for root induction in polybag raised seedlings of plantation and orchard crops as well as vegetables. The isolates of Azospirillum Brasiliense strains AZR 15 and AZR 37 from Kuttanad soils are highly effective for rice, vegetables and nursery plants. 

Method of application

Seed treatment; For treating 5-10 kg seeds, 500 g culture is required. Moisten the seeds by sprinkling water or rice-gruel water. Take 500 g culture in a plastic tray/basin, add moistened seeds, mix well and dry in shade for 30 mins. This may be sown immediately.

Seedling root dip (for transplanted crops): Slurry of the culture is prepared by mixing 500 g culture with 50 ml of water and the roots are dipped in the slurry for 15-20 mins before transplanting.

Soil application: Mix the culture with FYM or compost in the ratio 1:25 and apply directly in the soil.

Inoculation for paddy: Mix in 2 kg of culture in 60 liters of water and soak the seeds required for 1 ha (60 kg) for 24 hours before sowing. At the time of transplanting, dip the roots of seedlings for 15-20 mins in the culture slurry prepared by mixing 2 kg inoculum with 50 liters of water. This slurry can be used for treating seedlings required for 1 ha. Another 2 kg culture may be applied in the field along with FYM or compost.

  1. Blue Green Algae (BGA)

Mainly recommended for wetland rice cultivation. It is available as carrier-based inoculum and it fixes about 25-30 kg N/ha under ideal conditions. However, the use of this biofertilizer is not feasible in acidic soils with pH below 6.0.

Method of application

Broadcasting in the rice fields @ 10 kg/ha one week after transplanting.

  1. Azolla cultivation

Azolla, aa floating fern which harbors the nitrogen-fixing blue-green algae can be cultivated in shallow cement tanks or pits lined with polythene sheets. Pits or tanks of convenient length and breadth and 15 cm depth are made. Soil has to be spread with uniform thickness at the bottom of the tank at the rate of 7 kg per m square. Fresh cow dung at the rate of 2.5 kg per m square has to be made into a slurry and poured uniformly in the soil in the tank. Rajphos or Mussoriphus at the rate of 15 gm per m square  has to be given along with cow dung slurry. Water has to be added to the tank to a depth of 9 cm. Healthy azolla at the rate of 250 to 500 g per m square has to be spread uniformly in the tank. Azolla starts multiplying after a period of one week.

 Azolla cultivation can also be done in partially shaded place on terrace.

  1. Approximately 50 per cent shade is required for the healthy growth of azolla in tanks on terrace.
  2. Polythene sheets of thickness of minimum 150 gauge is conductive for making azolla tanks
  3. Once started multiplying, 250 to 450 gm of azolla can be harvested daily from one sq. meter area.
  4. The harvested azolla can be used as organic manure for vegetable crops ands as feed for poultry.
  5. Removal of a little quantity of water from the azolla tank and the addition of fresh water, addition of cow dung slurry and phosphorus at the rate of 0.5 kg and 10 gm respectively per sq. meter area of tank are to be done at weekly interval.
  6. Removal of one fifth of soil and addition of same quantity of fresh soil to the tank has to be done at monthly interval.
  7. The whole tank has to be replaced with new tank once in six months.
  8. The production cost of azolla comes to about 50 paisa per kg.
  9. Mosquito will not grow in tanks where azolla is grown.
  10. Phosphate solubilizing bacteria and fungi

Recommended mainly for upland crops raised in neutral and slightly alkaline soils. Available as carrier-based inoculum. Enables the efficient utilization of cheaper sources of phosphatic fertilizers such as rock phosphate by the crop plants.

Method of application

Seed treatment and direct application

  1. Arbuscular Mycorrhizal Fungus (AMF)

Arbuscular Mycorrhizal Fungus (Glomus spp.) is mostly recommended for upland crops, especially for raising container and tissue culture plantlets and transplanted crops. It mainly improves the uptake of available P by inoculated plants. There is also an enhanced absorption of water and other nutrients such as N and K and certain micronutrients. Mycorrhiza inoculation can improve the survival and establishment of tissue culture plantlets under filed conditions and well as induce better resistance against certain soil borne plant pathogens. It is commercially available as granular inoculum consisting of infected roots and soil with mycorrhizal spores. It is given as soil application.

AMF is suitable for tropical tuber crops. The inoculation can be done by placing inoculum (3-5 per set) beneath the set before planting. The rate of spore load in the inoculum should be to the tune of 50 to 400 spores per 100 g soil medium. For transplanted vegetables, AMF is recommended @ 200 g per m square in nursery.

  1. PGPR mix I

It is a compatible consortium of N, P and K biofertilizers and helps to save 25 %N, P and K fertilizers. Methods of application and dose are same as that of Azospirillum.

Tolerance limit of biofertilizers

  • In case of Rhizobium, Azotobacter, Azospirillium and Phosphate Solubilizing Bacteria, the total viable count shall not be less than 1 x 107 CFU per g of carrier material in the form of powder or granules or 5 x 107 CFU per ml in case of liquid formulations during the entire period of shelf life.
  • In case of Mycorrhizal Biofertilizers, the viable propagules shall not be less than 80 per g.

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Application techniques of Biofertilizers

  1. Seed treatment

Five hundred grams of commercially available inoculum will be required for treatment of seeds for one hector area. For this, thick slurry of the carrier-based inoculum is initially prepared by mixing 500 g of the inoculum in 1.25 liters of water. The stickiness of the biofertilizer on the seed surface can be significantly improved by using 10 per cent jaggery solution or 5 per cent sugar solution supplemented with 40 per cent boiled and cooled gum Arabic solution or rice-gruel water. The required quantity of seed material is then gently mixed with this slurry without damaging the seed coat. The treated seeds are spread evenly over a gunny bag and dried in shade and sown immediately in moist soil. The treated seeds should never be exposed to direct sunlight for a long period of time since the UV rays of solar radiation will reduce the population of inoculated bacteria on seed surface significantly.

  1. Seedling treatment

This method is mainly recommended for transplanted crops. The roots of seedlings to be transplanted are dipped in water slurry of the biofertilizer (500 g in 2.5 liters of water) for 20 mins, prior to transplanting.

  1. Soil application

Soil application is generally recommended for all types of biofertilizers except Rhizobium and Bardyrhizobium. The method is to apply the biofertilizer after mixing with dried FYM, compost or vermicompost at the rate of 1:25. For crops of 6 months duration, the recommended dose is 1-2 kg/ha. This can be increased to 2-4 kg/ha for crops of more than 6 months duration. For perennial crops, 10 to 25 g of the biofertilizer is to be applied in the root zone during the first year and 25 to 50 g during subsequent years. This can be done at the time of sowing, transplanting, or during intercultivation.

Factors influencing the efficient use of Biofertilizers
  1. Use adequate quantity of organic manure (as per the recommendation for each crop) along with biofertilizer application. This is essential to ensure better survival, growth and activity of the introduces microbial inoculum in acidic soils.
  2. Liming is essential if the soil pH is below6.0. In moderately acidic soils, the application of lime @ 250 kg/ha is recommended along with biofertilizer treatment.
  3. Irrigation is essential during summer months after biofertilizer application to ensure the survival of the introduced microbial inoculum in the soil.
  4. Since N biofertilizers can supplement only a part of the nitrogen requirement of the inoculated plant, low dose of nitrogen and full doses of phosphorus and potassium as per the recommendation may be applied. This is essential to ensure better plant growth and yield. Similarly, in the case of P biofertilizers, the full doses of nitrogen and potassium should be applied. However, there should be a gap of at least one week between application of biofertilizer and chemical fertilizer.
  5. Use only biofertilizers, which are manufactured as per the quality parameters prescribed by the Bureau of Indian Standards. In the bacterial biofertilizers, the prescribed standards are that in the final product, the population of the desired bacterium should not be less than 10 million per gram of the carrier material and there should not be any contamination with other micro organisms when examined at 1:10000 dilution. Further, it should have a shelf life of at least six months.
  6. The commercially available biofertilizer should always be used before the expiry date marked on the culture packet.
  7. Top dressing with super phosphate @ 25 kg/ha 10 days after inoculation of BGA will enhance its growth under field conditions.
  8. Since the occurrence of green algae in rice field can affect the normal growth and proliferation of BGA, the population of green algae should be controlled initially by applying copper sulphate @ 4 kg/ha.
  9. In moderately acidic soils of pH around 6.5, root nodulation by Rhizobium and Bardyrhizobium can be improved by pelleting with finely powdered calcium carbonate. (See recommendation under cowpea)
  10. Application of P2O5 @ 1 kg/ha is recommended once in four days in P2O5 deficient soils to ensure good growth of azolla. The development of a reddish-purple color in azolla is a typical symptom of P2O5
  11. Since a floating population of azolla can release its bound nutrients only during decay in the soil, it is essential to incorporate azolla in the soil prior to the transplanting of rice seedlings.