Applicability of visual and analytical soil quality indicators in environmentally diverse catchments of the Ethiopian Rift

Abstract

This research focuses on two science-based soil quality assessments, i.e., visual soil assessment (VSA) and analytical (lab-based) soil quality (SQ) assessment. The VSA, especially when conducted by farmers, offers a cost-effective approach to studying soil in extended study areas. However, little is known about the applicability of VSA in areas subjected to different degradation processes. Our study takes place in the lowland, midland, and highland agro-ecological zones (AEZs) of the Shafe and Sile-Elgo catchments, in the Southern Main Ethiopian Rift Valley. It aims to contribute to the state-of-art knowledge on VSA by evaluating its reproducibility when conducted by farmers and extensionists and its correlation to analytical SQ indicator scores, and to generate insight into SQ of the cropland in the various AEZs of two selected catchments. To that aim, 36 cropland fields were selected, half of these fields receiving soil and water conservation (SWC) practices, while no SWC was applied to the others. In each field, the VSA was conducted by farmers and experts, followed by laboratory analyses and calculation of an SQ index. Our finding reveals that overall VSA scores assigned by both groups were very strongly correlated (r=0.909–0.975, p<0.001) and reproducible at any level of stratification in the highly diverse, tropical study area covered in this research. However, the VSA indicator soil texture turned out to be non-reproducible. Both VSA and analytical SQ indicator scores are significantly (p<0.05) higher in fields receiving SWC practices and integrating both methods provided a clear contrast between fields that have good and poor SQ conditions in different AEZs of the catchments. The VSA method is less capable of capturing problems related to toxicities or low nutrient and water availability as a consequence of acidity, salinity and sodicity. Therefore, when the VSA method is applied to monitor SQ status of fields subjected to diverse degradation processes, it would be better to include specific indicators of pH and EC.