Azolla is a small aquatic fern with a branched stem and bilobed leaves. The roots that emerge from the stem help the plants to float on water. It is generally found floating on stagnant water. There is a small cavity on the upper most part of the leaf which houses as many as 80,000 blue green algae have the capacity to fix atmospheric nitrogen and make it available to azolla. In return the blue green algae gets shelter and food from azolla fern and blue green algae. When the plant dies and decays in the soil nitrogen becomes available to plants.
Production of azolla culture
Azolla can be multiplied easily even by an ordinary farmer. There are mainly two methods of azolla multiplication:
(i) standing water method and
(ii) nursery method.
Under this method, a pond or a field with shallow standing water is chose. The depth of water required for azolla cultivation varies between 5-10 cms. For the rapid growth of azolla, application of super phosphate (4-8 kg P2 O5 /ha) is recommended. Azola inoculum can be introduced in standing water also. In three weeks time azolla multiplies to form a carpet on the water surface, which can be collected and used immediately or dried and preserved for later use. The process is repeated to produce more azolla culture.
Azolla is raised in small nursery plots of 50-100 square metre size with strong bunds all around so that water can be made to stand up to a height of 5-10 cm. However, in a newly constructed nursery plot retaining water is a problem due to high percolation rate. To control this, puddling is done in the paddy field) can be adopted. Compacting the soil can also control percolation. Plastering the bottom and sides with a mixture of cow dung and fine clay is yet another effective method of controlling percolation. Permanent azolla nurseries can be constructed with brick and cement. Spreading polythene sheets at the bottom of the nursery beds can also control percolation. Small nursery beds are advantageous compared to large plots as wind causes drifting of azolla towards one side in large plots.
When the plots are prepared and sufficient amount of water is filled inside, super phosphate (4-8 kg P2 O5 per ha) should be applied before introducing the azolla inoculum. Azolla inoculum consists of azolla plants, full or parts) and spores. Azolla can be multiplied easily through any broken part of the plant. Hence, production and use of spores for azolla multiplication has not been developed. For inoculating nursery beds fresh azolla are spread at the rate of 300-400 gram per square metre. It can produce 8-10 tonnes of green biomass in 20 days that can be collected and applied in the field or dried and stored for later use.
Method of application
Azolla can be applied in two ways: green manure form and dual crop form. Azolla is usually applied in rice fields in both ways.
Green manure of azolla
In this method, azolla biomass is incorporated into the field prior to rice plantation. It can be either grown in the same field before transplanting rice or grown in nursery beds and then transported and incorporated into the field by puddling. Azolla, when incorporated into the soil, decomposes rapidly with 7-10 days. However, nitrogen availability extends from one week to ten weeks. Experiment s have shown that 34% of the total nitrogen is available two weeks after incorporation, 63% after 4 weeks, 76% after 6 weeks and 85% after 8 weeks. Application of azolla in the green from produces better results than dry form.
This method involves growing azolla along with rice crop. One week after the planting of rice seedlings, fresh azolla t the rate of 200-300 gm per square metre should be applied in standing crops. Azolla biomass is formed in three weeks. Water is drained out and azolla is incorporated into the soil using implements.
Preservation of inoculums
Azolla does not thrive under adverse conditions: extreme cold or heat. But it can be preserved even under such adverse conditions in very slow moving water bodies such as streams, canals, sewage channels, small p[ponds and tanks and unused wells. They are known as inoculum banks. The optimum temperature for azolla ranges between 15-35 o C.
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