Spatial variability of soil fertility parameters in Kaski District, Nepal: A geostatistical approach

Mapping of soils and depicting spatial variability across agro-ecological regions remains underexplored, especially in Nepal’s hilly areas. The current study aimed to assess spatial variability and map key soil properties such as pH, soil organic matter, total nitrogen, available phosphorus, and available potassium across the Kaski district, Nepal. A total of 263 composite, geo-referenced samples from 0–20 cm depth were collected using a proportional random sampling design across different land- use types. Fertility maps were generated using Inverse Distance Weighting (IDW) interpolation in ArcGIS 10.8. Descriptive statistics revealed significant spatial variability among soil parameters, with a coefficient of variation from 15.2% (low) for pH to 137.93% (high) for available phosphorus. Most soils were moderately to strongly acidic with a mean of 5.48 ± 0.004. Soil organic matter, total nitrogen, and available phosphorus showed medium fertility levels (SNI = 1.91, 2.19 & 1.95), whereas available potassium reflected a low soil nutrient index (SNI = 1.61), indicating potassium deficiency. Spatial autocorrelation analysis confirmed significant positive clustering for SOM and potassium, while nitrogen exhibited a random distribution pattern (I = 0.25, z = 1.46, p = 0.14). Trend analysis revealed directional gradients with saddle-shaped curves in pH, SOM, and total nitrogen, influenced by topography, elevation, and fertilizer management practices. This study provides a comprehensive characterization of soil fertility, highlighting fertility constraints and offering insights for precision agriculture and land-use planning aimed at sustainable soil management in the Kaski district of Nepal.