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The Kano Close-Settled zone (CSZ) has been the site of an intensive farming system for many years, and for the last 30 years at least, all available land has been under annual cultivation. Observers have wondered at the apparent sustainability of this farming system. This paper presents the results of a two year case study into soil fertility management by three farmers within the Kano CSZ. Fertilizer use (inorganic and manure), harmattan dust deposition, and biological nitrogen fixation by leguminous crops were quantified at the field level. Chemical analysis of the main inputs to the farming system and harvested products enabled a nutrient balance for the farming system to be calculated. The balance varied according to farmers' management of individual fields, especially application of traditional and inorganic fertilizers, and the effect of rainfall on the size of the harvest, which is the main determinant of nutrient removal from fields.