Azolla : A potential biofertiliser for rice

Priyanka Irungbam, L Nabachandra Singh, KS Shashidhar, Yumnam Sanatombi, Y Bebila *

The genus Azolla belongs to the single genus family Azollaceae. There are six species of Azolla –A. caroliniana, A. nilotica, A. filiculoides, A Mexicana, A. microphylla and A. pinnata. Out of these, A. filiculoides, A. Mexicana, A. microphylla and A. caroliniana, belong to the sub-genus Euazolla.

The two species under subgenus Rhizosperma are A. nilotica and A. pinnata. A. pinnata is the most common species found in India. Azolla has been of interest to botanists and agriculturists for years because of its symbiotic relationship with a nitrogen-fixing, blue-green alga, Anabaena.

The delicate fern provides nutrients and a protective leaf cavity for the Anabaena, which in turn provides nitrogen for the fern. Under suitable field conditions, the fern-alga combination can double in weight every 3 to 5 days and fix atmospheric nitrogen at a rate exceeding that of the legume- Rhizobium symbiotic relationship.

Azolla can accumulate 2 to 4 kg N ha-1 d-1 (equivalent to 10 to 20 kg of ammonium sulphate), and since the Azolla-Anabaena combination grows in aquatic conditions, it can be a potential nitrogen source for flooded crops such as rice.

Benefits of Azolla

1. Azolla (Kang) used for fertilizer management, is one of the most promising biofertilizers for a variety of crops, including rice, wheat, taro and soybean. Its application improves soil fertility by increasing total nitrogen, organic carbon and available phosphorus in the soil.

2. It is used as a food supplement, fresh, dried or ensiled, for a variety of animals, including pigs, rabbits, chickens, ducks, and fish. Azolla contains essential amino acids, vitamins (Vitamin A, Vitamin B12 and betacarotene), growth-promoting intermediaries and minerals like calcium, phosphorous, potassium, ferrous, copper and magnesium.

3. It suppresses the growth of aquatic weeds by blocking sunlight and providing a physical resistance to weed seedling emergence by creating a heavy, interlocking, azolla mat.

4. It can also be used in the control of mosquitoes. A thick Azolla mat on the water surface can prevent breeding and adult emergence.

5. It plays an important role in ecosystem management including bioremediation of toxic trace metals and organic pollutants. It can act as a bio-filter to remove pollutants. Azolla live biomass acts as potential bio-accumulator for toxic pollutants, while the dead biomass regulates pollutant concentration through bio-sorption.

6. Its application can minimize greenhouse gas (GHG) emission from agriculture as well as mitigate atmospheric GHGs.

The characteristics that make Azolla suitable as a biofertilizer on rice are as follows:

1. A shallow, freshwater habitat such as that found in a flooded rice field is the ideal environment for Azolla.
2. Azolla fixes nitrogen at a substantial rate.
3. Azolla has rapid growth.
4. Since Azolla floats on the water surface, it cannot compete with rice for light and space.
5. In most climates, Azolla grows best under a partial shade of vegetation, which a rice canopy, in its early and intermediate stages of growth, can easily provide.
6. When rice approaches maturity, due to low light intensity under the canopy and depletion of nutrients, Azolla begins to die and decompose, thus releasing nutrients into the medium.

7. Azolla decomposes rapidly, and therefore, the nitrogen it has fixed and the phosphorus and other nutrients it may have absorbed from the water, perhaps in competition with the rice, are rapidly released back into the medium and made available for uptake by rice during grain development.

8. Azolla has a greater ability than rice to accumulate potassium in its tissues in low potassium in its tissues in low potassium environments; thus, after decomposition, it makes this nutrient available to rice.

9. In contrast with chemical nitrogenous fertilizers, Azolla has long-term residual effects, in particular, the improvement of soil fertility by increasing total nitrogen, organic carbon, available phosphorus, potassium, and other nutrients.

10. If chemical nitrogenous fertilizers are applied, an Azolla mat reduces the volatilization of ammonia that normally occurs.
11. A thick Azolla mat in a rice field has the side-benefit of suppressing weeds.

Azolla production technology

Azolla production technology is simple and not very expensive, and at the same time, it is very efficient in terms of biomass accumulation and nitrogen fixation. The rice-growing season is also conducive for the growth of Azolla plants. The most common mode of application of Azolla in the field is as green manure or as a dual crop along with rice.

In case of application as green manure, Azolla collected directly from nursery plots, ponds, ditches, canals etc. is applied in the field. A thickmat of Azolla will be formed after application in about 2-3 weeks and can be incorporated in the soil. Rice can also be transplanted in the field subsequently. Single super phosphate (25-50 kg ha-1 is applied in split doses).

Azolla application by this mode contributes around 20-40 kg N ha-1. In dual cropping, Azolla is grown along with rice and each crop of Azolla contributes on an average 30 kg N ha1. After 7-10 days of transplantation, fresh inoculums of Azolla are applied in the field at the rate of 0.50-1.0 ton ha1.

Single super phosphate is applied at the rate of 20 kg ha-1 in split doses. In about 15-20 days, a thick mat of Azolla is formed. Azolla, thus incorporated, decomposes in about 8-10 days and releases the fixed nitrogen. Another crop of Azolla can be raised in a similar way during the crop cycle of rice.

The dual application does not have any negative influence on the rice crop. The most effective application for increasing soil fertility is to first culture Azolla as a monocrop in the fallow period without diminishing the rice-growing period, incorporating it before transplanting and subsequently culturing it as an intercrop with two incorporations.

Azolla production can be carried out in nursery plots, ponds, ditches, canals, concrete tanks, polythene lined ditches and even paddy fields. The field selected for Azolla cultivation needs to be thoroughly prepared and leveled uniformly.

Generally, 20m x 2m size plots are made in the field with suitable bunds and irrigation channels with a water depth of at least 10 cm and each plot is filled with water (20 liters) and inoculated with Azolla (8-10 kg).

Single super phosphate (100 g) in 2-3 split doses is applied to each plot at an interval of 4 days. Furadon or carbofuran (3% active granules) can be applied in the plots (100 g plot-1) with or after a week of inoculation. 100-150 kg fresh Azolla can be harvested from each plot after 15 days. The same method can be used to produce Azolla in bigger plots.

Cultures of Azolla must be maintained in nurseries throughout the year. The water should be changed or cold water added if the water temperature rises above 35oC. For adequate growth during field cultivation, the daytime water temperature should stay within the range of 15-35oC. Very high humidity and high temperature or very low humidity and low temperature are both detrimental to the growth of Azolla.

Azolla grows best within a pH range of 5 to 7 and can survive a range of 3.5 to 10.

Conclusion

The application of Azolla as a biofertilizer on agricultural crops, reducing or replacing chemical fertilizers, can play a significant role in maintaining or improving ecological balance. Utilization of Azolla in agriculture also improves socio-economic status by reducing the agricultural input cost, generating employment opportunities for small-scale industry involved in its propagation.

Azolla can also reduce the energy demand for fertilizer production, thus, reducing the impact on environment due to energy production. Considering its various uses, it can, therefore, be concluded that Azolla is a multi-faceted biofertilizer which will help in attaining sustainability in crop production.

Department of Agronomy,
College of Agriculture,
CAU, Imphal

For further details contact:
Public Relation & Media Management Cell,
CAU, Imphal