Azolla is a free floating freshwater fern. The plant body consists of crowded mass-like leaves on a branched fragile stem and submerged roots.

The leaves are arranged in two alternate rows. The leaf is triangular or polygonal in shape. It has an upper lobe called aerial lobe and a lower lobe called submerged lobe. The ventral surface of the aerial lobe bears mucilage cavities harbouring the blue green algae, Anabaena azollae.

Anabaena azollae is a filamentous, heterocystous alga lacking mucilage sheath. The cells are barrel shaped. Azolla synthesizes sugars and provides them to Anabaena. On the other hand, Anabaena fixes the atmospheric nitrogen in the heterocysts and provides the reduced NH3 to the Azolla. Thus Azolla and Anabaena are living together as symbionts.

Azolla biomass gets doubled within 5-7 days by vegetative methods. It can fix 40-80 kg nitrogen/ha/year. Besides this, its forms a good green manure for flooded rice. An increase of crop yield upto 15-20% has been observed while fertilizing the rice with Azolla. There are seven species of Azolla- Azolla carolinina, A.filiculoides, A.mexicana, A.nilptoca, A.microphylla, A.pinnata and A.rubra. All these species harbor the unique blue green alga Anabaena azollae. Azolla pinnata is the common species in India.

Azolla microphylla is a heat tolerant species. It was crosses with Azolla filiculoides- A cold tolerant European species- to raise hybrids. The hybrids are growing faster, tolerant to heat and cold, and have 4-5% more nitrogen.

Azolla can be mass cultured in nurseries or in the main fields. A simple method of Azolla production is given here under:

The land is ploughed well and leveled properly. It is divided into small plots of 20*2m size with bunds and irrigation channels. Each plot is a nursery plot.
The plots are irrigated through the channels to a height of 10 cms.
The soil pH is adjusted to 8 by adding lime or sulphuric acid.
A 10 kg of fresh cattle dung is mixed with 20 liter of water to make a slurry and the slurry is sprinkled uniformly all over the plot.
The 8 kg of Azolla inoculums is spread in each plot.
100gm of super phosphate of added to each plot by top dressing.
100 gm of furadon-G is top dressed over the plot 7 days after spreading the inoculums to control pests.
Azolla forms a dense mat with in 15 days after inoculation. After 15 days, Azolla is harvested and used as a source of primary inoculums for main fields. About 40-50 kg fresh Azolla can be harvested from a plot.
Azolla is added to the main fields either as a green manure or as a dual crop.

The main field is ploughed and leveled about 15 days before transplanting the rice seedlings. Then Azolla is raised throughout the main field. After forming a dense growth , Azolla is mixed with the mud by gentle ploughing and leveling just before transplantation.

Azolla is grown as intercrop with rice 1-2 weeks after transplanting the rice. Usually it forms a dense growth with in 3-4 weeks. Thereafter, the Azolla biomass is buried in the mud by using weeder. Azolla may once again be cultivated in the same field. By this method , 0.5-1 ton of Azolla biomass can be added to one hectare of rice field during a season.

Azolla saves 20-30% nitrogen fertilizers required for rice.

Azolla reduces the multiplication of weeds and their dispersal in paddy fields. Azolla reduces about 75% of Echinocola and 98% of Marsilia and eliminated Cyperus in paddy fields.

In China, fishes have been cultured in paddy fields. Azolla is grown in such paddy fields. The fishes feed on Azolla and excrete there wastes in the water. Azolla takes up the excreta for its growth.

Azolla has also been used as a nitrogenous feed for carps growing in pond.

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