Aquatic weeds and their utilization 327Japan, India, China, Philippines and Thailand, and tropical strains that grow at
30-35 °C are also found. Because these tropical, local varieties are more heat
resistant than the one found in northern Vietnam, they could be developed to
become a high-yield and high N-content crop. However, experimentation is
needed to determine the full potential of Azolla in tropical areas.
Attempts have also been made to use free-living, filamentous, nitrogen-fixing
blue green algae to improve fertility of rice fields in Japan and India, but they
are still in an experimental stage.
Composting
One of the most promising methods to utilize aquatic weeds is to use them to
make compost. Composting requires about 60-70% moisture content (see
Chapter 3) and this can be achieved by a few days of wilting in the sun, a great
saving over the other drying methods in terms of cost and equipment. They also
generally contain adequate nutrients (C/N ratio is between 20 to 30, see Table
7.2), which favour the growth of microbes that produce compost. As previously
stated in Chapter 3, the nutrient contents of compost products are usually several
times less than inorganic fertilizers. However, when applied to land, about 25-
30% of inorganic fertilizer nutrients can leach to groundwater or are not
available to the crops. The compost nitrogen is in both organic (or cell biomass)
and inorganic forms and is released into the soil gradually, and is thus available
throughout the growing season.
Compost can be used as soil conditioner, and as organic fertilizer to raise
phytoplankton in fish ponds. The phytoplankton production was found to be
directly related to nutrient content of the composts and field trials of using
composted water hyacinth to fish ponds resulted in a considerable yield of
Tilapia (a herbivorous fish) as shown in Table 3.10 (Polprasert 1984).
Pulp, fiber, and paper
Aquatic weeds have found use in producing fibers and pulp for making paper. In
Romania Phragmites communis (the common reed) has been used to make
printing paper, cellophane, cardboard, and various synthetic fibers. Wood pulp
is mixed with the reed pulp to increase the tear strength and the density of paper.
In addition, the raw weed and pulp mill wastes yield a variety of other products
notably cemented reed blocks, compressed fiber board, furfural, alcohol, fuel,
insulation material and fertilizer (NAS 1976). For centuries common reed is
used to produce peasant crafts, thatching, fences and windbreaks. Its stems are
used in basketwork, as firewood, fishing rods, weaver's spool and as
mouthpieces for musical instruments.