The aquatic term, 'azolla' together with the blue-green algae (anabaena azollae) provides a symbiotic association that can fix agronomically significant amounts of nitrogen (N).

The usefulness of azolla for increasing rice yield has been reported in various countries. Azolla has been used in rice production for centuries in China, Vietnam and recently in India and the Philippines. Azolla can be grown.

(a) as a green manure and incorporated into the soil during land preparation,

(b) as a manure grown in another area and transported to the rice field, or

(c) as an intercrop with rice.

Azolla can be most potential source of green manure for tropical rice production for their high N fixing capacity and the availability of this nitrogen to the standing rice crops.

The positive effects of azolla incorporation, either before or after transplanting on rice field was confirmed at many international networks on soil fertility and fertilizer evaluation for rice (INSFFER) sites.

The productivity of azolla is much higher in open paddies than in dual culture even with a light rice cover. By widening the distance between rice rows, azolla was continuously grown under the rice canopy with higher productivity.

In 1975, a conference on azolla for southern China, which summarized findings from 1500 experiments in 7 provinces reported that azolla increased rice yield by 600-700 kg/hactare.

Literatures reveal that the incorporation of one crop of azolla before transplanting followed by split application of 30 kg N/ha urea gave more grain yield than that of 60 kg N/ha urea, which shows that combination of organic and inorganic fertilizers is better than the application of chemical N fertilizer alone.

Biologically N fixing (BNF) systems offer an economically attractive and environmentally safe means of reducing external inputs of chemical fertilizers and improving internal resources.

Yield tests show that the application of azolla increased rice yield by 0.4-0.5 t/ha over control plots. Azolla is the genus of a small free floating aquatic fern that grows in ponds, canals and rice paddies, which floats freely on the surface of the water.

It has a worldwide distribution in temperate and tropical regions. Presently, six azolla species are known, of which only one, A. pinnata has so far been cultivated for agricultural purposes.

Its capacity to fix molecular dinitrogen from the atmosphere is due to symbiosis with a heterocystous N-fixing blue-green algae, Anabaena azollae, found within the very small cavities of the upper part of azolla fronds.

With the development of agriculture and the extension of multiple cropping systems to paddy fields, the selection of azolla varieties for adaptability to new cropping patterns and for multiplication capability has become the important objective in the present day agricultural productivity in the world.

The cavities are the main fixing points for nitrogen from the atmosphere. In the open paddy field, the N contents of azolla range from 30 to 146 kg N/ha and the average N-fixing rate ranges from 1.0 to 3.6 kg N/ha per day.

Several studies show that under favourable experimental conditions azolla can fix as much as 40-60 kg N/ha within a period of about one month. The daily N-fixing activity of azolla usually varies with light intensity, temperature and the population density of azolla. The amount of N fixed at night is about 25-30 per cent of that fixed during the day.

Nitrogen-fixing activity was 8-10 per cent higher on sunny days than on cloudy or rainy days. In open field condition, a layer of azolla covering a 1-ha field contains abut 10 ton green matter and ensures about 25-30 kg N/ha.

This amount of azolla biomass can be doubled and tripled by growing a second or third layer after the first crop (layer) has been incorporated. The annual N production of azolla thus cultivated throughout the year reaches 450 kg N/ha. Indeed, azolla rapidly decomposes in soil after incorporation.

As a result, the yield response to azolla in rice production is as good as that to inorganic N. The biological potential of azolla as a green manure is clear.

Because of its high capacity to fix N from the air, azolla can be a complete or partial substitute for chemical N fertilizers in rice cultivation, at least biologically.

By applying and incorporating azolla, farmers can significantly reduce the quantity of inorganic N and still maintain almost the same level of rice fields should be drained so that it is easily and effectively turned under by rake or weeder nutrients mainly from the water.

Good water control is critical for azolla multiplication. Water depth should be kept at about 3-5 cm., when azolla is incorporated in the field. For a successful growth, the fern azolla needs optimum temperature between 16-30xC, shading lights and relative humidity of about 60 per cent is beneficial for its growth. Soil pH of 5.5-7 is good enough.

The potential use of azolla as a green manure is great, but the constraining factors to its use are also significant. The economic return of using azolla varies much from one nice-growing factors. The potential of using azolla as a green manure is useful in rice cultivation.

The economic potential of azolla is greatest in countries where the opportunity cost of labour is low. Labour cost increases of azolla application become critical in countries, where agricultural wage rates approach 2 dollars/day.

The total nitrogen organic carbon amount available P contents of soil increase due to azolla application indicating that N and P in azolla was released in the soil after its decomposition.

The addition of azolla increased organic carbon content of soil and therefore, might have also enhanced the nitrogen fixation by heterotrophic nitrogen fixers macronutrient contents of azolla expressed in per cent of dry weight are approximately:

N 4-5; P 0.5; K 1.0 - 2.0; Ca 0.5; Mg 0.5 and Fe 0.1. Azolla increases rice production. The frequency of azolla incorporation into the soil may have a positive correlation with rice yield. Thus, the application of azolla has far-reaching effects on the growth and yield of rice plants.