Improving soil fertility the 'green' way


A hedgerow of Leucaena leucocephala, a nitrogen-fixing legume, is intercropped with maize to improve soil fertility in a field in Wenchi, Ghana. Ghana/18309/P. Cenini

Crops need more nitrogen than any other soil nutrient. The rapidly expanding use of industrially manufactured nitrogenous fertilizers has been one of the main factors behind the fast growth in agricultural productivity over the past 50 years. But rising concerns over the possible harmful environmental effect of these fertilizers, as well as their cost for small-scale farmers in developing countries, have highlighted the importance of expanding the use of agricultural production methods that are both agronomically and economically sustainable.

To address these issues, FAO recently brought together leading technical specialists to review current knowledge on biological nitrogen fixation (BNF) in agriculture. The meeting was also aimed at identifying those BNF technologies that offer the greatest environmental and economic benefits for specific agro-ecosystems in developing countries. Based on the consultation's recommendations, FAO hopes to secure donor funding for a programme to encourage wider use of BNF technologies.

Environmental benefits

"For many poor farmers, BNF is a viable, cost-effective alternative or complementary solution to industrially manufactured nitrogen fertilizers," says Eric Kueneman, Senior Technical Adviser in FAO's Agriculture Department. "Most BNF technologies have the potential to generate global environmental benefits by reducing greenhouse gas emissions and water pollution, protecting biodiversity and promoting more sustainable use of agricultural land." By contributing to better soil cover and the build-up of soil organic matter, he adds, the BNF systems envisaged also promote rainfall infiltration, protect soil from erosion and enhance carbon sequestration.

Within BNF systems, nitrogen is fixed by a number of algal and bacterial species. Some of them live free in the soil, but the most important ones are bacteria that form close symbiotic relationships with leguminous plants, such as soybean, alfalfa, beans and groundnuts, as well as a number of leguminous trees and shrubs. These bacteria in association with specific plant species are able to convert gaseous nitrogen, which makes up 78 percent of the atmosphere, directly into plant-usable forms of the nutrient.

"Biological nitrogen fixation already plays an important role in agriculture," says Rabindra Nath Roy, Senior Officer in FAO's Land and Water Development Division. "Worldwide, it is estimated that crops fix about 40 million tonnes of nitrogen annually through BNF -- almost half of the 90 million tonnes from fertilizer production."

More research needed

The participants at the FAO consultation concluded that there are many potentially viable but under-used opportunities for the expansion of BNF-related technologies in the farming systems of developing countries, especially those in which maize, rice and wheat are the dominant crops. The consultation stressed that while there has been growth in understanding of how nitrogen is fixed in sugar cane and how nitrogen-fixing properties might be assumed by non-legumes, many years of research and development will probably be required before cereal crops can be adapted and enabled to fix a large part of their nitrogen needs from the air. In the meantime, the intercropping of legume species and their inclusion in rotations within cereal-based farming systems provides the best opportunity for increasing nitrogen availability to cereal crops.

The inclusion of nitrogen-fixing legumes within farming systems could generate important benefits for small-scale farmers, including increased farm output and improved livelihoods and food security. Intercropping and crop rotations with legumes also reduce the risk of pests and diseases, enhancing ecosystem sustainability.

Because most BNF technologies require few externally purchased inputs and are easily adapted for wide-scale use, they are particularly well suited for farmers with limited resources. According to the consultation participants, however, use of BNF technologies is often discouraged by national policies. Among these are subsidies on inorganic fertilizers, which reduce the competitiveness of BNF technologies and may contribute to wasteful or inappropriate use of synthetic fertilizers.

The consultation concluded that better efforts must be made to promote BNF-based farming systems within the framework of FAO's Integrated Plant Nutrition Systems (IPNS) approach. They cited participatory agricultural extension methods, particularly those that encourage farmer-led experimentation, as especially effective means of fostering use of BNF technologies appropriate to the various needs and constraints of farmers in developing countries.

17 April 2001

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