Electromagnetic induction methodology for mapping sodium adsorption ratio (SAR) and ionic composition in salt-affected soils for developing soil database and reclamation planning

Spatial assessment of the sodium adsorption ratio (SAR) and soil salinity (ECe) at the field scale is essential for effective soil and crop management in salt-affected lands. In this study, a cost-effective, rapid, and non-invasive geophysical technique, electromagnetic induction technique (EMI), was employed to measure soil SAR, while the Electrical Conductivity Sampling Assessment and Prediction (ESAP) software was used for its prediction and quantification. The investigation was conducted over a 12-ha experimental field at Nain, Panipat, Haryana, India. EMI survey was carried through EM-38 instrument in horizontal and vertical modes on a 20 × 20 m grid. To calibrate EMI readings, 21 optimal sampling locations were identified, and soil samples were collected up to 90 cm depth at 15 cm intervals (0–15, 15–30, 30–60, and 60–90 cm). Laboratory analyses of these samples included electrical conductivity of the saturated extract (ECe), saturation percentage (SP), moisture content (MC), cations (Ca²⁺, Mg²⁺, Na⁺), anions (CO₃²⁻, HCO₃⁻, Cl⁻), and SAR using standard wet chemistry procedures. Spatial variability of ECe, dominant cations and anions, and SAR was mapped using inverse distance squared (IDS) interpolation. Multiple linear regression (MLR) revealed that apparent conductivity (ECa) strongly correlated with measured SAR (R2- 0.81) and ECe (R2-0.86) in upper 0–15 cm soil layer. High correlations were also observed between ECa and Na⁺ (r = 0.79) and Cl⁻ (r = 0.72), indicating sodium chloride (NaCl) as the principal contributor to both SAR and ECe in the study area. A quantitative evaluation of the mean soil profile (0–90 cm) demonstrated that 28.07% of the field area exhibited SAR values below 50, 30.36% ranged between 50 and 100, 38% between 100 and 200, 3.51% between 200 and 300, and only 0.06% exceeded 300. Similarly, spatial analysis of soil salinity (ECe) indicated that 7.5% of the area had ECe < 4 dS m⁻¹, 8% ranged from 4 to 8 dS m⁻¹, 27.8% from 8 to 16 dS m⁻¹, 47.3% from 16 to 30 dS m⁻¹, while 9.4% exhibited highly saline conditions with ECe > 32 dS m⁻¹. Areas of high SAR coincided with high salinity (ECe), primarily due to elevated Na⁺ concentration. These findings demonstrate that the combined use of EMI and ESAP software provides an efficient approach for quantifying and mapping SAR and salinity, offering a reliable basis for rehabilitation and management of salt-affected soils according to their severity.