VEGETABLE PRODUCTION SYSTEMS in the tropics and elsewhere are mostly intensive, because vegetables are high-value cash crops. High fertilizer rates are commonly applied to maximize yields. There is an urgent need for the implementation of alternative methods to reduce excessive use of mineral fertilizers and to improve soil fertility and vegetable quality.
The age-old practices of green manuring, application of compost, crop rotation, and inter- and relay-cropping, which were used in various soil fertility programs for developing countries until the early 1960s, have declined with the increased use of mineral fertilizer (Singh, 1975). A major benefit of legume green manures (GM) is the contribution of N to the soil via N2 fixation. Because the role of N from organic sources such as GM is tied to complex microbial cycling of C and N, the availability and effects of legume N are more difficult to predict than those of chemical fertilizer N (Groffman et al., 1987). Most recent research on GM has focused on staple crops, especially on rice (Oryza spp.) (Ladha et al., 1989). Few published results exist for tropical vegetable production systems, although organic manuring is still a common practice in some vegetable farms in India and Nepal (Babha Tripathi, Nadia, India, personal communication). Stivers and Shennan (1991) and Abdul-Baki and Teasdale (1993) reported tomato yield following legume GM and mulch comparable to those obtained with synthetic fertilizers in the USA, but Lennartsson (1990) showed that vegetable yields following GM did not outfield those grown after fallow in the UK. Investigations are needed to evaluate the potential role of legume GM in tropical horticulture and to estimate the risks to production before promoting it as a widespread practice for farmers.
The objective of this study was to assess the feasibility of meeting N needs of tomato with legume GM at one location in southern Taiwan and two locations in the Philippines through integration of the legumes into established vegetable cropping patterns of these areas. This cropping strategy was tested for its location specificity by quantifying legume biomass, N2 fixation, N accumulation, and tomato yield and N uptake. Tomato N nutrition was monitored by NO3–sap samplings in the southern Philippines. To quantify the contribution of various N sources to tomato plants, legumes were labeled with 15N in an additional experiment in the northern Philippines and 15N was traced in tomato plants.