Green Manure and Legume Covers in the Tropics

Abstract

Growing leguminous plants for multiple uses has lately received renewed interest. Costs of fossil-based nitrogen fertilizer, after being unrealistically low for decades, have increased. Supplying nitrogen through biological fixation is attractive, but systems that accomplish this are needed in both temperate and tropical farming systems. Biological fixation as a source of nitrogen is natural for leguminous crops, yet the consumers of most nitrogen fertilizer are the major food cereals. One way these can obtain biological nitrogen is through the judicious use of green manures and legume covers.

This report provides an overview of literature describing various legumes and the cropping systems in which they are used in the humid tropics. Systems described include rice and green manures, plantation crops and legume covers, root crops and green manures, and some upland crops and green manures. A survey of current use of green manures and legume covers was conducted in 1981 as a preliminary part of the study.

Alternative sources of nitrogen are both more attractive for use in complex tropical farming systems and more in demand because of the scarcity of other nitrogen sources and of nitrogen ferHlizer in many tropical countries. It seems apparent that biological nitrogen should be viewed as a complement to fertilizer nitrogen. The appropriate mix of the two is hard to assess because of inadequate or expensive techniques for measuring the amounts of nitrogen contributed by the green manure or legume cover. We emphasize the need to assess the advisability of legume use in particular situations from a farming systems research and development perspective because of the biologic, economic, and social complexity of tropical farming systems.

As a result of the study, we emphasize that knowledge gaps or research needs exist as follows: 1. Techniques to measure nitrogen contributed by the green manure or legume cover are inadequate. 2. Viable seeds for legume germplasm evaluation are hard to get. 3. There is not enough information on the characteristics of various candidate green manures and legume covers and on their climate and soil requirements. Such data could be used in screening candidate legumes.