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Abstract
The intensive use of synthetic fertilizers and the low use of plants that improve tropical soils have damaged natural resources, because farmers do not see a visible improvement in proportion to the work that their crop entails. With the purpose of considering as an economic investment the contribution of nitrogen and phosphorus (N and P) to the soil by three nitrogen fixing species (Phaseolus vulgaris, Clitoria ternatea, Canavalia ensiformis: Fabaceae), and having an estimation that allows making decisions, the economic investment associated to the rhizosphere contribution of N and P was analyzed in a livestock production soil (Up) and an agricultural one (Ua). A completely random design with three repetitions was used for each species. The soil properties evaluated were total nitrogen (N) and useful phosphorus (P). The initial and final difference of these values were estimated as contribution and converted into American dollars. The results suggested that P. vulgaris was the species that showed the least contribution of N in Up soils (14,723 kg ha-1); and the P content was higher with C. ternatea (26,851 kg ha-1). C. ensiformis was the one that showed the highest investment for N (US$ 70.06) and C. ternatea was the one of highest investment for P (US$ 42.80) with Up, without showing investments for N and P in Ua soil.
