Potential and limitations of green manure technology in lowland rice

The growing popular concern about the sustainability of agricultural systems and an increasing awareness about environmentally-friendly agricultural production and healthy nutrition stands in striking contrast to a worldwide decline in the use of green manures. It is timely to assess the future role that soil-improving legumes may play in agricultural systems. This paper reviews potential and limitations of green manure technology, using lowland rice cropping systems as the example. A review of more than 250 published references indicates that the average amounts of N accumulated by green manures can entirely substitute for mineral fertilizer N at current average application rates. With similar N use efficiencies, green manure N is less prone to loss mechanisms than mineral N fertilizers and may therefore contribute to long-term residual effects on soil productivity. The considerable genetic variability in the available legume germplasm allows the selection of appropriate legumes to most conditions with positive effects on soil physical and chemical parameters. However, the use of green manure legumes for lowland rice production has declined dramatically world-wide over the last 30 years. Socio-economic factors like the cost of land, labor, and mineral N fertilizer are seen to determine the cost-effectiveness and thereby farmers' adoption of green manure technology. Hydrology and soil texture determine the agronomic competitiveness of a green manure with N fertilizers and with alternative cash crops. With a rapidly-growing demand for rice and growing land and labor scarcity, green manure use is not seen to become a relevant feature of favorable irrigated rice-growing environments in the foreseeable future. On the other hand, in environments where soil properties and hydrology are marginal for food crop production (flood-prone rainfed lowlands with coarse-textured soils), but which farmers may be compelled to cultivate in order to meet their subsistence food requirements, green manures may have a realistic and applicable potential.