University of Bonn, Plant Nutrition in the Tropics and Subtropics, Germany
Intensification of the land use in lowland-rice-based cropping systems of Southeast Asia has been associated with the widely observed decline in yields and factor productivity. When faced with declining crop yields, farmers have traditionally opted for the insertion of green manures into the farming system. In the 1970s soil-improving plant species has been cultivated on an estimated 30 million hectares. Despite the growing concern about the sustainability of the emerging intensive cultivation systems, the use of green manures is limited today to less than 1 million hectares. Farmers generally attempt to compensate for declining yields with a higher and increasingly inefficient use of external inputs. Have Asian farmers forgotten an important part of their (agri)cultural heritage or are green manures a thing of the past? Based on a critical analysis of more than 400 published studies, the potential and constraints of green manures use are analysed, possible cropping niches are defined, and future perspectives are proposed.
Green manures can accumulate on average 80kg N ha-1, which is derived to a major share from biological N2 fixation and/or from the sequestration of native soil or added fertiliser N. Yield increases and N use efficiencies achieved with green manures are similar or more than those with mineral fertilisers in rainfed lowlands with sandy soils. However, a low availability of P strongly reduces the performance of nitrogen-fixing species in these environments. Furthermore, only in the rainfed lowlands with a single transplanted crop do land and labour availability allow farmers to cultivate crops for the sole purpose of soil improvement. Once green manures systems provide multiple use solutions, they may become acceptable, since farmers are looking for direct returns to their investments. It becomes apparent that the niches for green manure are limited to marginal environments in infrastructurally little developed areas. Unless there is a significant increase in mineral fertiliser prices and/or progress in the identification of green manures with direct economic functions, the use of soil-improving plant species will continue to play only a limited role in the lowland agriculture of Southeast Asia.
Keywords: Azolla, N use efficiency, Oryza sativa, Sesbania