Nitrogen manufacturing plants
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CTA. 1987. Nitrogen manufacturing plants. Spore 12. CTA, Wageningen, The Netherlands.
Permanent link to cite or share this item: http://hdl.handle.net/10568/44737
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Two virtually unknown, nitrogenfixing plant species - Sesbania rostrata, an African shrub and Azolla, a fern from Asia - show potential for replacing chemical fertilizers and increasing crop production. Current work is designed to pave the way for...
Two virtually unknown, nitrogenfixing plant species - Sesbania rostrata, an African shrub and Azolla, a fern from Asia - show potential for replacing chemical fertilizers and increasing crop production. Current work is designed to pave the way for their use by small farmers but researchers are also looking at long-term ways to incorporate their characteristics into food plants themselves so that in the future farmers maysowself-fertilizing crops. Nitrogen is indispensable for plant growth but it is not always present in sufficient quantities in the soil, especially when continuous, intensive cultivation does not allow the soil to regenerate its stocks. Nitrogen inputs that can be directly assimilated by plants (e.g., nitrates and urea) are also important for maintaining healthy growth. In many African countries, however, these imported chemical fertilizers are rarely applied because they are too expensive for the farmers. Plants capable of fixing atmospheric nitrogen are therefore of great interest to these countries. Two wild plants that have recently been studied by researchers in Africa show considerable potential in this regard. Sesbania rostrata, which occurs naturally in the wet lowlands of the Senegal River valley, is a leguminous plant which means that it can assimilate free nitrogen thanks to the symbiotic relationship between its roots and the rhizobial bacteria found in the soil. These micro-organisms penetrate the roots and inoculate the cells of the root skin which results in the formation of the nodules in which nitrogen fixation takes place. The special feature of Sesbania is that it also has such nodules on its stems, which enables it to assimilate all available nitrogen even when the soil is waterlogged. Sesbania can produce about 40 kg of nodules per hectare compared to an average of 5-6 kg for other leguminous plants. If cut after 50 days of growth, this shrub, which can reach 3 metres in height, can be used as a green manure. In this way, it can provide about 200 kg of nitrogen per hectare per year, compared to 100-150 kg for other leguminous plants. Doubling yields Research by scientists in the Casamance, Senegal, has helped to determine the optimum growing conditions for this plant. Optimum development is obtained by long days and high temperatures (30C). To ensure good nodule development on both the stems and roots, the soil must be inoculated with rhizobium. Site-specific tests have shown that by ploughing in such green manure one can double the yields of sorghum and irrigated rice and increase the yield of rainfed rice by 70 %. The International Institute of Tropical Agriculture (IITA) has also experimented with interplanting rows of Sesbania and lowland rice. The results show that the prunings of this plant spread between rice rows represent the equivalent of up to 120 kg of urea per year. Further research is currently underway in Cote d'lvoire and Martinique on the use of S. rostrata to fertilize banana fields. This plant has also shown potential for use as a mulch in the rainy season when lowland areas are flooded. Research in northern Cameroon is concentrating on the growth of Sesbania itself. It appears that this plant grows better on moist than well-drained soils. It is thus particularly suited to lowland or irrigated rice fields. Although such research has only just begun, it already shows the considerable promise of this plant, whether it is used as green manure or mulch. It remains to be seen, however, if such success on experimental farms can be easily repeated on the fields of small farmers. The first problem to overcome is the need to produce at low cost sufficient quantities of the inoculum required to guarantee a high level of nitrogen fixation. Considerable effort will also be required to train farmers who have no experience in the use of green manures and to whom its benefits may not be immediately evident. In addition to growing the plant, it must be cut and then ploughed in, a process that demands both time and tools that are not always available. Finally, this technique also requires careful planning because it takes about six months to prepare a crop of S. rostrata before planting the rice. At the scientific level, this plant is also the subject of considerable interest because of the potential of harnessing its symbiotic relationship with rhizobium to fix nitrogen. It is hoped to be able to transfer this plant's ability to form caulinary nodules to other leguminous plants. Research in this direction is too recent to expect great results, but it has already improved our knowledge of the nitrogen fixation process Floating fertilizer Azolla is the second of these nitrogen factories' that is holding the attention of scientists. This aquatic fern, which floats on the surface of calm waters and has lona been used as a green manure by farmers in Asia, is also found in Africa. In the Koular Valley of Senegal, for example, rice fields which have never been treated with urea have concentrations of nitrogen and carbon that are four times higher than those of adjacent fields. This is due to the presence of Azolla which grows naturally in these lowland plots and is spread by the farmers when they transplant their rice The use of Azolla in Africa was first mentioned in expedition-reports dating from the beginning of the century. After being harvested from the surface of waterways and dried, it was burned so that its potassiumrich ashes could be used to make soap. This small fern is thus a plant of many uses. Each of its leaves contains a central cavity that harbours the cynobacterium Anabaena azollae, more commonly known as the blue-green algae, which are responsible for nitrogen fixation This is a very fast-growing plant capable of doubling its biomass in two or three days. Its reproduction is also prolific: when it reaches 1-2 cm in diameter, its older parts break off and form new plants. The greatest problem with Azolla is the risk of it becoming an aggressive weed. Under unfavourable conditions sexual reproduction takes over and helps to ensure its survival. Furthermore, Azolla thrives under varying ecological conditions, including different pH, light, temperature and salinity levels, as long as it has a good water environment, preferably a shallow basin that enables its roots to touch the soil, and a source of phosphorus. This little plant is virtually a 'nitrogen factory' given its ability to produce 23-170 kg of nitrogen in 60 days. Most experience with Azolla has been in rice fields where it is either dried and spread like a fertilizer, or used like a green manure. Trials in Senegal have shown that when grown with rice, Azolla reduces evaporation, minimizes the need for weeding, and increases production by about 50 % when used with double-cropping systems instead of traditional cropping methods. To avoid any problems, a good inoculant is essential. It can be produced in shaded plastic or clay bowls that are covered to avoid contamination. Research scope As with Sesbania, there has been little use of Azolla as a green manure outside experimental farms. Its potential seems greater, however, because it can be grown at the same time as rice and does not require much more labour. There are two major obstacles that need to be overcome. First, researchers have not yet discovered a way to grow the cyanobacterium without its host plant, which considerably reduces the scope for genetic manipulation. Second, it is difficult to induce sexual reproduction in Azolla, which makes it difficult to conserve its strains. Efforts are being redoubled, however, to understand the interactions between this cynobacterium and its host plant in order to exploit this natural process. The development of more resistant strains, notably those that tolerate changes in pH levels, varying photoperiodicity and high light intensity, is another promising research direction. To combat decreasing soil fertility in Africa and elsewhere, green manure appears to be a promising solution for the future. These two plants warrant considerable attention but it must be remembered that implementation techniques need to be developed to make such progress accessible to small farmers. If research in this direction eventually results in breeding food crops that can fertilize themselves, it will open the door to another, more accessible green revolution: one that will reduce - rather than increase - the need for chemical fertilizers.
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