Geochemical Assessment and Spatial Drivers of Soil pH Distribution in the Sumani Watershed, West Sumatra, Indonesia

Abstract

Soil acidification strongly influences agricultural productivity and watershed resilience in tropical volcanic landscapes. This study examined the spatial distribution and geochemical controls of soil pH in the Sumani Watershed, West Sumatra, Indonesia. The mean soil pH was 5.51 (SD = 0.42), with significant correlations to erosion drivers—positively with the USLE R factor (r = 0.17*) and negatively with the LS factor (r = – 0.21**). Exchangeable Ca, Mg, K, Na, and available Si were positively associated with higher pH values. USLE modeling indicated moderate to severe erosion in upland areas, contributing to persistent acidification, while depositional zones showed slightly acidic soils. The lowest pH (4.64) was found in intensively cultivated vegetable fields, where frequent nitrogen fertilization and pesticide use increased exchangeable acidity. In contrast, irrigated rice paddies maintained higher pH (mean = 5.61), supported by irrigation water and volcanic ash inputs. These results emphasize long-term acidification trends and highlight the need for integrated management combining liming, silicon supplementation, balanced NPK fertilization, and erosion control to sustain rice yields above 10 t ha-1 and ensure watershed sustainability.