Aim
Salinity and sodicity are critical soil degradation processes in arid and semi-arid regions, severely constraining agricultural productivity and water resource sustainability. The Sistan Plain in northeastern Iran, with annual precipitation of only 55 mm and evaporation exceeding 4,800 mm, exemplifies such fragile environments. The region depends on the Hirmand River and Chah-Nimeh reservoirs for irrigation. However, extensive salinity and sodicity, especially in northern and eastern parts, threaten agricultural sustainability, particularly following recent large-scale irrigation projects covering 46,000 hectares. Given that previous reclamation studies are outdated due to changes in water quality, land use, and climate, this study had four primary objectives: (1) evaluate the reclamation potential of representative saline-sodic soils through field-scale experiments, (2) determine precise leaching water requirements for achieving target salinity reduction levels appropriate for crop production, (3) assess the necessity and actual effectiveness of gypsum application for sodicity amelioration across different soil types and salinity-sodicity classes, and (4) develop site-specific leaching and desodification curves as practical, science-based management tools for farmers, agricultural extension services, and land-use planners. The central hypothesis was that reclamation success is predominantly governed by soil physical properties, particularly texture, permeability, and the presence of restrictive layers, and that gypsum application is not universally required across all salt-affected soils in the region.
Materials and Methods
Three representative soil series were carefully selected to represent the range of salinity-sodicity conditions and physical characteristics in the Sistan Plain: Akbarabad (Hi1-11: initial ECe 11.5 dS m⁻¹, silty clay, S3A2), Charak (Nim2-13: initial ECe 65.3 dS m⁻¹, ESP ~73%, silty loam with heavy clay layers at depth, S4A4), and Arbab (NP-20: initial ECe ~60 dS m⁻¹, loam, S3A2). At each site, four GRP lysimeters (1 m diameter, 60 cm height) were installed, inserted 15–20 cm into soil to prevent lateral flow and ensure predominantly vertical water movement. Leaching used intermittent ponding with 100 cm water applied in four 25-cm increments. After each increment, soil samples were collected from one lysimeter at 0–25, 25–50, 50–75, 75–100, and 100–150 cm depths, with each lysimeter representing a distinct leaching stage to avoid disturbance from repeated sampling. Gypsum was applied at 34 t ha⁻¹ (Charak) and 13.5 t ha⁻¹ (Arbab), calculated from initial ESP, CEC, bulk density, target ESP of 15%, and 92.6% purity. Leaching-only was applied to Akbarabad, while both leaching-only and leaching-plus-gypsum were applied to Arbab for direct comparison. Soils were analyzed for ECe, soluble cations and anions, SAR, ESP, and CEC using standard laboratory methods. Normalized leaching curves used X = (leaching water depth)/(soil depth) and Y = (ECf − ECeq)/(ECi − ECeq) for salinity, with analogous equations for ESP, to eliminate external factors such as evaporation and drainage conditions. Best-fit mathematical models (power, exponential, inverse, and logarithmic) were tested and selected using SPSS statistical software.
Results
Results revealed strongly contrasting reclamation responses, confirming the overriding influence of soil physical properties. For Akbarabad, leaching alone was highly effective: after 100 cm water, ECe of the upper 50 cm decreased from 11.5 to <2 dS m⁻¹, with SAR and ESP dropping to safe levels (<13 and <15%). The desalination curve indicated ~45 cm net water was sufficient to reach ECe of 2 dS m⁻¹. This success was attributed to adequate permeability and native calcium sources (calcium carbonate and possibly natural gypsum) facilitating sodium displacement without external amendment. In stark contrast, Charak showed extremely limited reclamation. Despite 100 cm water and 34 t ha⁻¹ gypsum, ECe of the top 25 cm decreased only from 65.3 to 14 dS m⁻¹, and ESP from 73% to 49.5%. SAR remained extremely high (>100). The heavy clay layer (60% clay at 50–75 cm) severely restricted water flow and solute transport, acting as a permeability barrier. Desalination curves predicted that reducing ECe to 6 dS m⁻¹ would require >110 cm water, and desodification to ESP of 10% would demand nearly 200 cm—unrealistic volumes given severe water scarcity. Thus, the primary limitation was not calcium deficiency but poor physical conditions preventing effective water movement and solute exchange. For Arbab, both treatments were effective. In the leaching-only treatment, ECe of the upper 50 cm decreased from ~60 to <6 dS m⁻¹ after 100 cm water, with SAR and ESP reaching safe levels. The desalination curve indicated ~58 cm net water was required to reach ECe of 6 dS m⁻¹. Gypsum addition (13.5 t ha⁻¹) provided only marginal benefits: after the first 25 cm water, ECe in the top 25 cm decreased from 19.2 to 9.9 dS m⁻¹ with gypsum, compared to 14.5 dS m⁻¹ without. However, this minor advantage disappeared in later stages, and final salinity and ESP values showed no significant differences between treatments. The desalination curves were nearly identical, indicating that gypsum did not meaningfully enhance permeability or solute removal. This was attributed to adequate soil permeability, sufficient native calcium, and calcium/magnesium in irrigation water, which together allowed effective sodium displacement by leaching alone, making gypsum application unnecessary and economically unjustifiable in this profile.
Conclusion
This study conclusively demonstrates that reclamation potential of saline-sodic soils in the Sistan Plain is strongly controlled by site-specific physical and chemical properties, not simply by initial salinity levels. In soils with favorable permeability and adequate native calcium, leaching alone can effectively reduce salinity and sodicity to acceptable levels without gypsum. In contrast, in soils with heavy clay textures, very high initial salinity, and extremely low hydraulic conductivity, even large water volumes and high gypsum rates yield only marginal improvements, making complete reclamation economically and practically unfeasible under water-scarce conditions. Gypsum should not be prescribed indiscriminately; its necessity depends on exchangeable sodium, native calcium availability, permeability, and irrigation water quality. Given acute water scarcity, leaching objectives should be aligned with crop salt tolerance and rooting depth rather than targeting excessively low salinity. The leaching and desodification curves provide a robust scientific basis for estimating site-specific water requirements and prioritizing reclamation investments. Adoption of salt-tolerant crops with limited, targeted leaching is more pragmatic than complete reclamation in severely affected areas. Future research should address long-term sustainability of reclamation, secondary salinization risks from capillary rise and evaporation, and economic feasibility of alternative physical and chemical amelioration methods under local climatic and economic conditions. The findings of this study can inform policy decisions regarding land-use planning, irrigation water allocation, and agricultural development strategies in arid and semi-arid regions facing similar salinity challenges.