Assessing the Spatial Variability of Soil Acidity in Assosa District, Northwestern Ethiopia

This research aimed to quantify and map the spatial variability of soil acidity and related properties in Assosa District, Northwestern Ethiopia, to support targeted acid-soil management. The hypothesis was that soil acidity exhibits significant spatial heterogeneity due to both natural soil formation and human activities, requiring site-specific interventions.
A total of 100 geo-referenced composite surface soil samples were randomly collected across cultivated and potential agricultural lands during the 2023-2024 cropping season. Samples from 0–20 cm depth were analyzed in the laboratory for pH in water and KCl solutions, exchangeable acidity and aluminum, organic matter, cation exchange capacity (CEC), and soil texture. Statistical and geostatistical methods, including variogram modeling and ordinary kriging, were applied to assess spatial patterns and dependencies.
The findings show that soils are predominantly acidic, with water pH ranging from 5.06 to 6.60 (mean 5.56) and KCl pH from 3.95 to 5.17 (mean 4.43). Nearly 76% of the area has suboptimal pH (<5.5) for many crops. Exchangeable acidity and aluminum levels indicate significant toxicity risks in some areas. Soil organic matter and CEC showed moderate to high variability, influenced by texture and management history. Clay dominated the soil texture, contributing to buffering capacity and slowing amendment movement.
Spatial analysis identified a moderate spatial dependence of acidity parameters, with Gaussian models best describing the spatial structure and ranges of about 19 km. This spatial information allows the identification of acidity hotspots for precise lime and organic amendment applications to improve soil fertility and crop productivity.
Overall, the data provide essential baseline information for sustainable, site-specific soil acidity management in the district, optimizing resource use and supporting agricultural productivity in acid-affected tropical soils.