Aquatic weeds and their utilization 3257.5.3 Soil Additives
Mineral fertilizers are expensive for many farmers in developing countries, yet
there is a greater need now than ever to increase food production. As an
alternative, the 1974 World Food Conference and, more recently, the Food and
Agricultural Organization (FAO) of the United Nations have stressed the
urgency of reassessing organic fertilization. This includes the possible ways in
which aquatic weeds can be used as organic fertilizers, namely, as mulch and
organic fertilizer, ash, green manure, compost, or biogas slurry.
Mulch and organic fertilizer
Mulching involves the laying of plant material on the surface of the soil to
reduce evaporation and erosion, to smother weeds and for temperature control.
Both sand and clay soils need conditioning to make them productive. Working
plant material into the soil improves its texture, and also, by acting as manure,
improves the nutrient content. Several species of aquatic weeds that can be used
as manure are Pistia stratiotes,Hydrilla verticillata,Salvinia spp., Eichhornia
crassipesandAzolla (Frank 1976; Gopal and Sharma 1981; Gupta and Lamba
1976; Little 1968).
The high moisture content of aquatic weeds makes them suitable as mulch.
But the time and labour involved in harvesting and distributing even sun-dried
material would preclude such a use, except on a small scale adjacent to the
watercourse in which the aquatic weeds occur.
Ash
Although it has been suggested that the ash of water hyacinth can be used as a
plant fertilizer, some reasons showing that it is not feasible are:
- Burning of the plant to ash results in the loss of nitrogen and organic
matter, which reduces the fertilization potential of the plant. - Burning of the plant requires previous drying which involves
additional labour and expenditure and is restricted to dry days of the
year, while growth and distribution of the weeds is most profuse in
wet months. - Ash needs immediate bagging and storing in order to prevent it from
being washed away by rain or carried away by wind.
Thus, the cost of labour and energy required to obtain ash from aquatic
weeds seems to far exceed the value of the ash obtained as a fertilizer (Little
1968).