Soil salinity is the accumulation of soluble salts in the root zone. This high salt concentration creates an osmotic effect, making it difficult for plants to absorb water even when the soil appears moist, functionally causing a physiological drought. High levels of specific ions, particularly sodium and chloride, can also become toxic, interfering with nutrient uptake. This leads to visible damage like leaf burn, stunted growth, and a significant decline in plant health and crop yield.
Understanding the Problem
Soil salinity arises from both natural processes and human agricultural practices. Naturally, salts are released through the weathering of parent rock material and can accumulate in arid or semi-arid regions where rainfall is insufficient to flush them below the root zone. High water tables can also contribute, drawing saline groundwater up toward the surface where the water evaporates, leaving the salts behind.
Human-induced causes often relate to water management, especially in irrigated agriculture. Using irrigation water that contains salt, combined with high evaporation rates, leads to salt accumulation over time. Poor drainage allows water to sit and evaporate, concentrating salts rather than moving them away. The excessive or improper use of certain fertilizers can also introduce soluble salts to the soil solution.
Identifying a salinity issue begins with recognizing visual symptoms in plants, such as yellowing, stunted growth, leaf tip burn, and wilting despite adequate moisture. A precise diagnosis requires testing, which involves measuring the soil’s Electrical Conductivity (EC). For soils with excessive sodium, the Sodium Adsorption Ratio (SAR) is also calculated, since high sodium levels destroy soil structure and water infiltration.
Primary Reduction Technique Leaching
Leaching is the primary method for reducing soil salinity, involving the application of excess, low-salinity water to dissolve salts. This process moves the soluble ions down and out of the root zone, physically flushing them away from where plants absorb water. Successful leaching depends entirely on having good subsoil drainage, as applying large volumes of water without an exit point will raise the water table and worsen the problem.
The amount of water required for effective salt removal is quantified by the Leaching Requirement (LR). For severely saline soils, applying approximately 12 inches of quality water will remove about 70 to 80 percent of the salt from each foot of soil. The leaching water should ideally have a low Electrical Conductivity (EC), though slightly saline water can be used on sodic soils to maintain soil permeability during the initial flushing.
Leaching is most efficient when water is applied in several intermittent cycles rather than one continuous ponding event. Intermittent leaching allows the water to drain to field capacity, facilitating a more effective downward movement of the salt. Deep, less frequent watering is more effective than frequent shallow watering, which concentrates salts near the soil surface as the water evaporates. The goal is to move the salt concentration well below the active root zone.
Chemical and Organic Amendments
While simple leaching works for most saline soils, high-sodium or sodic soils require chemical intervention before salts can be effectively washed away. Sodic soils suffer from poor structure because sodium causes clay particles to disperse and clog the pores. This destruction of soil structure prevents water infiltration, making traditional leaching impossible.
The primary amendment used to reclaim sodic soil is gypsum, or calcium sulfate. The calcium in the gypsum replaces the problematic sodium ions attached to the soil particles in a process called cation exchange. Once the sodium is released into the soil solution, it can then be leached out of the root zone with subsequent water application. Gypsum is relatively inexpensive and directly provides the necessary calcium.
In soils that naturally contain calcium carbonate (lime), acid-forming amendments like elemental sulfur or sulfuric acid can be used instead of gypsum. Elemental sulfur is oxidized by soil microbes to form sulfuric acid, which reacts with the native lime to release calcium into the soil solution. This biological process is slow, especially in cool temperatures. Sulfuric acid is a much faster-acting alternative but requires careful handling due to its corrosive nature.
Organic amendments, such as compost and manure, improve the physical properties of the soil. They do not directly reduce salt levels but enhance aggregation and create better pore space. This improved soil structure facilitates water infiltration and drainage, making the leaching process more effective. Organic matter also provides a food source for soil microorganisms, contributing to overall soil health.
Long-Term Management and Prevention
Preventing the recurrence of soil salinity requires a shift in irrigation and drainage practices to manage the water balance effectively. Switching from overhead or sprinkler irrigation to high-efficiency methods like drip or micro-sprinklers minimizes the amount of water lost to evaporation from the soil surface. Drip irrigation also applies water directly to the root zone, reducing the upward capillary movement of salts from deeper layers.
Proper drainage is a foundational requirement for long-term salinity control. Maintaining or installing subsurface drainage systems, such as tile drains or drainage ditches, is often necessary to provide an exit route for the saline water that has been flushed from the root zone. Without adequate drainage, all reclamation efforts are temporary, as the salts will simply build back up.
Where soil reclamation is slow, a practical management strategy is to select crops that are naturally tolerant of higher salt concentrations. Choosing a salt-tolerant crop allows productivity to be maintained while long-term reclamation is underway.
Salt-Tolerant Crops
Crops known to tolerate moderate to high salinity levels include:
- Barley
- Sugar beets
- Cotton
- Olives
- Pomegranates
- Figs