Soil salinization is defined as the accumulation of soluble salts in the soil profile to a level that impairs plant growth and productivity. Approximately one-third of the world’s irrigated land is currently affected by salt accumulation, which reduces crop yields and can lead to the abandonment of agricultural land entirely. This issue is exacerbated by poor irrigation practices and the use of saline groundwater. Addressing salinization requires a combination of physical flushing, advanced water management, and chemical or biological soil treatment strategies.
Leaching and Subsurface Drainage Systems
The primary physical method for reclaiming salinized soil is leaching, the controlled application of low-salinity water to dissolve soluble salts. This process moves the dissolved salts out of the plant’s root zone and deep into the subsoil. The effectiveness of leaching is measured by the leaching fraction, which is the amount of water that must pass through the soil in excess of the crop’s water needs to maintain a desired root zone salinity.
Leaching alone is insufficient for long-term reclamation if the saline water remains beneath the root zone, as capillary action can draw the salty groundwater back toward the surface where it re-evaporates and re-salinizes the soil. Subsurface drainage systems are therefore a necessity, providing an exit path for the highly saline water that has been flushed downward. These systems often utilize perforated pipes, known as tile drains, or created channels like mole drains, installed below the root zone at depths typically ranging from 1.5 to 2.0 meters.
The design of the drainage infrastructure is critical to its success, with pipe spacing and burial depth optimized for the specific soil type. For instance, studies in silty loam soils often find that a burial depth of 1.5 meters with a pipe spacing of approximately 20 meters offers high efficiency for salt removal. The functioning of the drain lowers the water table, preventing the upward movement of salt, while simultaneously collecting and diverting the leached, saline water away from the field.
Sustainable Irrigation Practices
Long-term management of salinized soil relies heavily on adopting irrigation practices that minimize salt input and manage water movement. A foundational step is the accurate assessment of the irrigation water quality, typically measured using Electrical Conductivity (EC). Monitoring the EC helps managers calculate the precise leaching requirement needed to keep the root zone salinity below the threshold that would harm the cultivated crop.
Localized water application methods, such as drip irrigation, offer a significant advantage over traditional flood or sprinkler systems in saline environments. Drip irrigation applies water directly to the plant’s root zone, creating a localized “wetted bulb” of low-salinity soil where the roots can thrive. This technique pushes salts to the periphery of the wetted area, away from the dense root mass, and reduces the overall volume of water and salt introduced to the field.
The high-frequency nature of drip irrigation is also beneficial, as it maintains a consistently high soil moisture content, which dilutes the salt concentration within the root zone. Managing irrigation schedules to allow for controlled deficit irrigation can also be part of a sustainable strategy. This technique encourages the crop to utilize shallow, slightly saline groundwater, effectively lowering the water table and reducing the risk of capillary rise and re-salinization.
Soil Chemistry and Plant-Based Approaches
Chemical amendments and biological methods are complementary strategies for soil remediation, especially where sodium accumulation is a problem. Sodic soils, characterized by a high proportion of exchangeable sodium ions, suffer from poor structure due to clay dispersion. This prevents water from infiltrating and draining efficiently, requiring chemical amendments before effective leaching can occur.
Gypsum, or calcium sulfate, is the most common and cost-effective amendment used for sodic soil reclamation. Applying gypsum introduces soluble calcium ions (Ca²⁺) into the soil, which readily displace the destructive sodium ions (Na⁺) from the soil exchange complex. Once the sodium is replaced by calcium, the clay particles can flocculate, restoring the soil structure and significantly improving water infiltration. The displaced sodium salts are then flushed out during subsequent leaching events.
In soils that are naturally high in calcium carbonate (lime) but have a high pH, acidifying agents can be used as an alternative to gypsum. Elemental sulfur, when applied, is oxidized by soil bacteria to form sulfuric acid. This acid then reacts with the native calcium carbonate to release soluble calcium, which performs the same sodium-displacement function as applied gypsum.
Phytoremediation, the use of plants to clean up contaminated soil, offers an environmentally sound and low-cost biological mitigation method. This approach utilizes salt-tolerant plants, known as halophytes, which can withstand high salt concentrations. These specialized plants are cultivated to absorb and accumulate the salts from the soil into their above-ground biomass.
Once the halophytes have grown and sequestered a significant amount of salt, the biomass is harvested and removed from the field, thereby physically extracting the salts from the soil system. Examples of effective halophytes include species like Atriplex (saltbush), which can be grown for forage or biofuel production, adding an economic benefit to the reclamation process. Phytoremediation is a slower process than physical leaching but provides a continuous method for reducing soil salinity over time.