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Opportunities for harnessing biological nitrogen fixation of non-cultivated herbaceous legumes in order to improve soil productivity on smallholder farms in Zimbabwe were explored in a study initiated in December 2001. Over 30 indigenous legume species were jointly identified with farmers across three agro-ecological regions, ranging from sub-humid (800 mm annually) to semi-arid (< 650 mm). A Gwezu Smell Technique, based on the odour released from freshly harvested legume roots, greatly enhanced the capacity of farmers to participate in the identification process. The legume diversity was highest in Chinyika Resettlement Area where cropping had been going just over 20 years. This was contrary to the loss of diversity in old Communal Areas where dominance of clean weeding practices for over 70 years has led to depletion of the weed seed bank. Legume contribution to the total above ground biomass ranged from 3% in 1-year fallows under semi-arid conditions to 88% in a field abandoned soon after crop establishment under sub-humid conditions. The latter case indicated an opportunity for manipulating legume populations to increase their contribution. Overall, total fallow productivity did not exceed 3.2 t ha-¹ due to extreme conditions of poor soil fertility, where soils had generally< 10% clay with~ 0.4% organic C,< 3 ppm available P and pH (H2O) between 4.8 and 6.0. Evaluation of the fallowed sites suggested that most of the fields fallowed by farmers are often too poor for any meaningful cropping using the currently available low cost soil fertility technologies. Based on results from this exploratory study, this paper introduces the …