|SPORE Bulletin of the CTA No. 40 (CTA Spore, 1992, 16 p.)|
Azolla, a small free-floating fern associated with the nitrogen-fixing alga Anabaena azollae, is used by rice farmers in Asia as a natural, low-cost source of fertilizer nitrogen. However, farmers elsewhere in the tropics have not been so ready to incorporate Azolla culture into their cultivation practices. The benefits of Azolla, and the limitations in its use, should be assessed carefully by farmers in Africa, the Caribbean and Pacific.
Inorganic nitrogen fertiliz ers are expensive, and supplies remain inadequate and uncertain for the majority of farmers in the tropics. Nitrogen-fixing crops and trees, composted crop wastes and livestock manures are least-cost alternative sources of nitrogen which have been adopted by farmers in a wide range of situations. Another option, specifically for those growing rice on flooded or irrigated land, is the use of Azolla.
Azolla plants form dense mats on water surfaces in ponds, ditches and rice fields throughout the warm, temperate and tropical regions of the world. Each leaf lobe has a cavity containing the blue-green nitrogen-fixing alga Anabaena azollae.
The two plants have a symbiotic relationship; Anabaena providing nitrogen compounds in exchange for carbon compounds from photosynthesis.
Azolla has long been used as both a green manure for rice and as a fodder for poultry and livestock in China and Vietnam. Azolla can grow exponentially, doubling its biomass in 5-10 days. Yields of over 30t/ha of fresh weight are possible, containing over 2.5t/ha dry matter. Protein content of 13-23% of dry matter makes the product a useful feedstuff, although fibre and mineral contents are also high.
As a source of fertilizer nitrogen, the Azolla Anabaena symbiosis can fix 100-170kg N/hectare/year. Under field conditions selected species can fix about 1.2kg N/day and in excess of 40kg N in 35 days. Azolla is very responsive to phosphorus and requires a continuous supply for rapid growth. Trials have shown that each kilogramme of phosphorus resulted in more than 5kg of additional nitrogen in the Azolla biomass after 35 days' growth.
Most research on Azolla in the tropics has been carried out in Asia, but trials were conducted simultaneously by the West African Rice Development Association (WARDA) at the Richard Toll Forage Station (Senegal) under semi-arid conditions and at Rokupr (Sierra Leone), which has a wet humid climate. These have indicated its potential but also its limitations in the semi-arid Sahelian zone and in the humid tropic zone of Africa.
In northern Senegal, where farmers apply nitrogen at high rates (120kg N/ha) it was demonstrated that up to 50% of the mineral N can be supplied by Azolla. In the mangrove swamps of Sierra Leone Azolla nitrogen can completely replace mineral N at the recommended rate of 40kg N/ha. Azolla can also control weeds in irrigated rice.
Azolla growth depends on a constant and sufficient depth of water in rice fields. In Asia, where rice is transplanted into flooded paddies, inoculation with Azolla is followed by a very rapid proliferation, a suppression of weeds and a generous production of nitrogen. In sub-Saharan Africa there are few places where water is available in sufficient quantity to maintain constant flooding. As a result, farmers have to balance the cost of pumping water to maintain sufficient depth for Azolla growth against the cost of purchased mineral nitrogen. In many instances the cost of pumping plus the additional labour cost involved in producing Azolla can exceed the cost of mineral nitrogen.
Where post-emergence herbicides are used Azolla can only be inoculated after reflooding the field and the beneficial effects of the Azolla are limited to weed control for the current crop and residual nitrogen for a following crop. In village irrigation systems transplanting is more common, but soils are more permeable, water is more limited and much of the land may be without standing water for several days. In such situations Azolla cannot thrive.
Near most rice areas there are natural depressions, which are flooded by rainwater during the short wet season or by drainage from the larger irrigated areas. These natural water bodies can be inoculated with Azolla, which can subsequently be composted for crop production or dried for animal fodder.
Azolla can play an important role in intensive agriculture systems. One hectare of Azolla can produce over 500kg assimilable protein per month and provide feed for pigs, ducks, chickens and cattle. The manure from such livestock can be returned with benefit to crop land.
Azolla can also be grown as fishfood: Tilapia mossambica readily consumes Azolla as do some of the grass carps. There is no doubt that a flexible approach to Azolla production and utilization would be needed in most of Africa, the Caribbean and Pacific; the optimal growth conditions found in south-east Asia are very much the exception in these regions, but where opportunities do exist the rewards could be substantial.