The use of bio fertilizers is important while practicing integrated plant nutrient management as well as organic farming. Some of the commonly used bio fertilizers in Kerala are as follows.
1. Rhizobium (Bradyrhizobium and Azorhizobium)
It induces better root nodulation and stem nodulation 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.
Suitable for upland crops like vegetables, tapioca, plantation and orchard crops. It is available as carrier-based inoculum. It fixes about 15-20 kg of N per 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.
It is suitable for both upland and wetland conditions and is available as carrier-based inoculum. It fixes about 20-25 kg N per 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 brasilense strains AZR 15 and AZR 37 from Kuttanad soils are highly effective for rice, vegetables and nursery plants.
Methods 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 minutes. 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 minutes 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 2 kg of culture in 60 litres 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 minutes in the culture slurry prepared by mixing 2 kg inoculum with 40 litres 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.
4. 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 per 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 at the rate of 10 kg ha-1 one week after transplanting.
It is suitable for wetland rice cultivation. The required quantity of azolla will have to be raised in the farmers field itself. Fixes about 25 to 30 kg N per ha.
Method of application: Apply fresh
azolla at the rate of 10 t ha-1 at the time of ploughing.
6. Phosphate solubilising 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.
7. Vesicular/Arbuscular mycorrhiza (VAM/AMF)
Vesicular arbuscular mycorrhiza 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 field conditions as 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.
VAM fungus Glomus microcarpum var. microcarpum is suitable for tropical tuber crops. The inoculation can be done by placing inoculum (3-5 g/sett) beneath the sett 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. Method of application is the rooted infected cutting technique.
Application techniques of biofertilizers
1. Seed treatment
Five hundred grams of commercially available inoculum will be required for treatment of seeds for one hectare area. For this, thick slurry of the carrier-based inoculum is initially prepared by mixing 500 g of the inoculum in 1.25 litres of water. The stickiness of the biofertilizer on seed surface can be significantly improved by using 10 per cent jagerry 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.
2. 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 litres of water) for 20 minutes, prior to transplanting.
3. Soil application
Soil application is generally recom-mended for all types of biofertilizers except Rhizobium, Bradyrhizobium and Azorhizobium. The method is to apply the biofertilizer after mixing with dried FYM, compost or vermicompost at the rate of 1:25. For crops of six-months duration, the recommended dose is 1-2 kg ha-1. This can be increased to 2-4 kg ha-1for crops of more than six months duration. For perennial crops, 10 to 25 g of the bio fertilizer 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 inter cultivation.
Factors influencing the efficient use of biofertilizers in Kerala
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 introduced microbial inoculum in acidic soils.
2. Liming is essential if the soil pH is below 6.0. In moderately acidic soils, the application of lime at the rate of 250 kg ha-1 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 case of bacterial biofertilizers, the prescribed standard is that in the final product, the population of the desired bacterium should not be less than ten million per gram of the carrier material and there should not be any contamination with other microorganisms when examined at 1:100000 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 superphosphate at the rate of 25 kg ha-1 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 at the rate of 4 kg ha-1.
9. In moderately acidic soils of pH around 6.5, root nodulation by Rhizobium and Bradyrhizobium can be improved by pelleting with finely powdered calcium carbonate. (See recommendation under cowpea)
10. Application of P2O5 at the rate of 1 kg ha-1 is recommended once in 4 days in P2O5 deficient soils to ensure good growth of azolla. The development of a reddish purple colour in azolla is a typical symptom of P2O5 deficiency.
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.