Organic substitution improves saline–sodic soils: Field evidence linking soil sodicity, structure and water status

The organic substitution of mineral nitrogen fertilizer is increasingly promoted as a sustainable strategy for rehabilitating saline-sodic soils, yet field-based evidence linking reduction in sodicity and improvements in structure to enhanced soil water status remains limited in arid saline regions. We conducted a two-year field experiment in the Hetao Irrigation District of northwestern China to evaluate how organic substitution alters soil salt properties (alkalinity, salinity and sodicity), structural indicators (turbidity of soil suspension and pore-size distribution), water-filled pore space (WFPS), and the associated water balance and crop productivity. Four fertilization regimes were compared: no fertilization (CK), chemical fertilization alone, and organic fertilizer substituting 50% and 100% of urea-N. Relative to CK, organic substitution significantly reduced soil alkalinity (2.9%–3.6%) and salinity (26%–54%) within the 0–40 cm soil layer. These changes were accompanied by pronounced shifts in soil cation composition, with reductions in sodicity by replacing Na+ with Ca2+. Consistent with these chemical improvements, organic substitution markedly decreased the turbidity of soil suspensions, indicating suppressed clay dispersion. Reduced turbidity was closely associated with enhanced macro- and meso-porosity, leading to significant increases (3.4%–4.8%) in WFPS. Water balance analysis further indicated that organic substitution reduced upward capillary flux during the dry year and promoted downward water movement during the wetter year, thereby reducing salinity buildup within the soil profile. Partial least squares path modeling further demonstrated that soil pore structure was the main determinant of WFPS under organic substitution, and turbidity of soil suspension proved to be an effective integrative indicator of sodicity and structural recovery. These improvements in soil physical and hydrological conditions were accompanied by increases in crop yield and water productivity, particularly under the OF1. Overall, our results provide mechanistic, field-based evidence that organic substitution alleviates coupled chemical and physical constraints in saline–sodic soils, regulates soil water balance and salt redistribution, and improves soil conditions critical for sustainable crop production and agricultural water management in arid irrigation regions.