Genesis, fertility, and erodibility of volcanic ash soils in the Andes of northern Ecuador
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The volcanic ash soils of northern Ecuador have supported agricultural activities for thousands of years; however, at the dawn of a new millennium, the sustainability of agricultural production seems to be threatened on the slopes of volcano Cotacachi, where crop yields have been declining over the past decades. This study was undertaken as part of an interdisciplinary effort to devise a natural resource management plan and had the objective to characterize the soil resource of the area. Nature and culture in the studied Andean mountain environment are dominated by its verticality, and altitude is the master variable that governs soil development, determines the soils’ susceptibility to runoff and erosion, and affects many aspects of soil fertility. At higher elevations, organic matter accumulates, amorphous constituents form, and the soils are Andisols. They have stable aggregate structure and show high infiltration capacity and resistance to erosion. However, their N- and P-supplying capacity may be impaired by slow mineralization of the former and strong sorption of the latter. At low elevations, organic matter contents are low, halloysite predominates clay mineralogy, and the soils are Inceptisols and Entisols. They exhibit lower infiltration capacity and are more susceptible to erosion; however, compared to soils from other parts of the world, their interrill erodibilities are classified as low. The cultivation of arable crops in these lower zones may be limited by the soils’ low water-storage capacity and susceptibility to nutrient leaching. Irrespective of elevation, the young soils in the southern part of the study area have inherently low potassium contents and would greatly benefit from K additions. While erosional degradation does not pose a major threat, water and nutrient management appear to be key elements of agricultural sustainability in the Cotacachi area.