Spatial variation of acidic sodic soils in high-andean environments and theoretical estimation of gypsum requirement

Abstract

High-Andean forage systems are characterized by extensive management of cultivated forages, primarily through rotation of forage oats (Avena sativa) and alfalfa (Medicago sativa), under marginal edaphoclimatic conditions. Within this context, soil sodicity severely constrains the productivity of these systems in the districts of Ayaviri and Macarí, Melgar Province, Peru. However, spatially explicit information on gypsum requirements (GR) for sodic-soil reclamation remains unavailable. To address this gap, this study aimed to: (i) develop a weighted Soil Quality Index (SQIw) for sodic soils, (ii) delineate sodicity-affected areas across districts, soil classes and environmental conditions and (iii) generate high-resolution maps of gypsum requirements using regression kriging (RK). A total of 1273 soil samples were analysed in high-altitude areas of Ayaviri and Macarí. Key sodicity-related variables included exchangeable sodium percentage (ESP), clay flocculating charge and Modified Soil-Adjusted Vegetation Index (MSAVI) selected through Pearson correlation and principal component analysis to construct the SQIw, while regression kriging was used to generate spatially explicit gypsum requirement maps. Results showed wide variability in pH and ESP, reflecting heterogeneous sodicity conditions across the study area, with Macarí presenting higher soil quality (SQIw = 0.61) than Ayaviri (SQIw = 0.44). Sandy loam soils and acid saline–sodic soils exhibited the greatest vulnerability. Regression kriging achieved high predictive accuracy for GR estimation (R² = 0.97 in Ayaviri; R² = 0.85 in Macarí), enabling precise delineation of priority intervention zones. The resulting GR maps provide a robust basis for site-specific gypsum application, improving amendment efficiency, reducing unnecessary costs and supporting sustainable sodicity management in vulnerable High-Andean agroecosystems.