Efficiency of Nano-Gypsum and Elemental Sulfur Application in Reclaiming Saline-Alkali Soils in Siwa Oasis, Egypt

The calcareous, salt-affected soils of Egypt’s Siwa Oasis, characterized by shallow groundwater and marginal irrigation, create combined risks of sodicity and salinity, severely limiting crop recovery. This study aimed to identify the optimal, cost-effective application rates of nano-gypsum (NG) and elemental sulfur (S⁰) for achieving six-month reclamation targets. We evaluated cost per unit ESP reduction and per 0.01 increase in SF*, accounting for dose–response and timing differences. Field trials compared NG (120, 240, 480 kg ha-¹) and S⁰ (0.5, 1.0, 2.0 t ha-¹) on non-saline sodic (NSS) and saline–sodic (SS) soils across sandy, loam, and clay-loam textures. The amendments were incorporated to 0–20 cm, followed by immediate leaching. Soil plots were sampled at 1, 3, and 6 months to measure chemical (pH, ECe, ESP, SAR) and physical (hydraulic conductivity, water retention, available water) properties. The structural factor (SF) and its normalized form (SF*) were subsequently derived. Statistical analysis was performed using a three-way factorial RCBD with repeated measures (Texture × Treatment × Time) and ANOVA with Fisher’s LSD (α = 0.05). In NSS soils, all textures successfully reached an ESP < 15% within six months, accompanied by coherent declines in SAR/ECe and gains in hydraulic conductivity (ks) and structural factor, confirming functional desodification. In SS soils, both amendments reduced ESP, SAR, and ECe; however, progress was texture-limited. While sandy and loam soils achieved ESP ≈ 14–15%, the clay-loam soils remained above the sodicity threshold (>15%) at six months, with ECe levels persistently above 4 dS m-¹, indicating a need for continued leaching. Economic analysis revealed that NG rates of 240–480 kg ha-¹ were consistently on the cost-effectiveness frontier for both ΔESP and ΔSF*. Conversely, S⁰ at 0.5–1.0 t ha-¹ was dominated, and the 2.0 t ha-¹ rate was only occasionally non-dominated, with steep incremental cost-effectiveness ratios (ICERs, i.e., higher costs per unit improvement). We recommend prioritizing NG at 240–480 kg ha-¹ to secure ESP reduction and structural recovery within six months. S⁰ ≤ 2 t ha-¹ should be reserved for carbonate-rich, fine-textured soils to sustain acid dissolution. These applications must be paired with immediate leaching, maintained drainage, and extended leaching cycles in SS soils until the target ECe < 4 dS m-¹ is met.