326 Organic waste recycling: technology and management
Green manure
Green manure, in a strict sense, is plant matter cultivated for its fertilizer value
to other crops. However, certain species of aquatic weeds that grow wild in rice
fields and are ploughed into paddy, e.g. Limnocharis flava and Sesbania
bispinnosa, are sometimes referred to as green manure. Thus, the distinction
between aquatic weeds, which grow wild and are used as manure and fertilizer,
and green manure which is cultivated, is not always maintained. Certain types of
aquatic weeds are cultivated as green manure or biofertilizers to add nitrogen to
the soil, and this practice is useful since it lessens dependence on commercial,
inorganic fertilizer.
The cultivation of the fern Azolla pinnata, with its symbiotic nitrogen fixing
blue-green algae was developed in Vietnam, where it is treated not as a weed
but as a valuable crop (Ngo 1984). Edwards (1980) cited a literature that
reported that the cultivation of the fern has recently spread to southern China
also. In both countries, Azolla is produced to fertilize areas of rice and other
crops. In northern Vietnam, just before or after rice transplanting, Azolla is
scattered in the fields at the rate of 4-5 tons/ha, and in January and February it
grows along with the rice. During this time, when the mean daily air
temperature is 16-17 °C, the paddy is young and available solar radiation and
nutrients allow Azolla to grow normally, with a result that it completely covers
the surface of the water. Towards the end of March, when the temperature rises
to 22-24°C, the paddy has grown up and shades most of the surface, resulting in
the death of Azolla and release of P and K and newly acquired nitrogen to the
paddy. The Azolla produces about 18 tons of fresh material/ha, and assimilates
more than 100 kg N/ha in the 3-4 months growing period. A negligible portion
of the fixed nitrogen is released when Azolla is growing and it becomes
available only on the death of the plant.
Before being applied to agricultural lands and fields, Azolla is produced in
small, special field where it receives appropriate fertilization and care. In these
fields the Azolla biomass can double every 3-5 days in good weather. A one-
hectare field fully covered with Azolla can produce 1-4 tons of fresh biomass
per day, which contains 2.6-2.8 kg of nitrogen (Ngo 1984). Within 3 months
green manure enough to fertilize 4,000 ha of rice fields can be produced from 1
ha of Azolla fields. Thus, if year round production of Azolla is maintained, one
ha of Azolla can save 5-7 tons of costly imported chemical fertilizer (ammonium
sulfate) per year. Ngo (1984) reported that increase in yield of rice by 30-50%,
of sweet potato by 50-100%, and of corn by 30%, have been achieved in fields
green manured with Azolla.
BecauseAzolla is cultivated as green manure in only limited areas of Asia,
there can be management problems in other areas. It grows in abundance in