Soil development along mountainous slopes is often affected by pronounced altitudinal variations of climatic parameters. This study was conducted to assess the quality of agricultural soils in an agro-ecosystem that stretches from 2400 to 3300 m above sea level in the Andes of northern Ecuador. We specifically studied the effects of the soils' climate-dependent pedological development on their inherent fertility and their runoff-erosion behavior, and speculate about possible impacts of future climate change in this context. Example profiles of the major soil types were pedologically characterized, a number of soil fertility parameters were analyzed for 100 agricultural fields, and runoff-erosion behavior was tested in rainfall simulation experiments on 15 agricultural topsoils. Altitudinal differences in temperature and leaching regime have led to differential pedological development on the volcanic slopes of the study area. At higher elevations, cool and humid conditions have favored the accumulation of organic matter and the precipitation of active amorphous materials, leading to the formation of Andisols. At lower elevations, organic matter contents are low, the colloidal fraction is dominated by halloysite, and the soils classify as Entisols and Inceptisols. These differences profoundly affect the soils' inherent fertility and runoff-erosion behavior. As a consequence, a number of soil fertility characteristics, such as organic matter content, pH, cation exchange capacity, phosphate sorption and phosphorus availability, show significant altitudinal variations, and the soils' resistance to erosion increases dramatically with elevation. Some of the inherently unfavorable soil conditions can be offset with adequate management practices, while others are more difficult to mitigate. The low water and nutrient storage capacity of the sandy low-elevation soils represents a particular challenge for rainfed agriculture; and projected future climate warming could yet aggravate this situation by further decreasing the already low organic matter contents of these soils. In the long term, changing moisture and leaching conditions could change the soils' colloidal composition and thus considerably alter their inherent quality.