Soil salinization is the process where soluble salts accumulate in the root zone of the soil, impeding plant growth and reducing agricultural productivity. While some salt presence is natural due to mineral weathering, the rapid acceleration of this problem, known as secondary salinization, is overwhelmingly caused by intensive agricultural practices. This degradation affects approximately one-tenth of the world’s irrigated cropland, posing a significant threat to global food security. The salts involved are typically chlorides, sulfates, and carbonates of sodium, calcium, and magnesium.
How Irrigation Introduces Salt Loads
The primary mechanism for agricultural salinization is the water used for irrigation. All natural water sources, including surface water and groundwater, contain dissolved mineral salts. This is particularly true in arid and semi-arid regions that rely heavily on irrigation.
Even when the quality of irrigation water is good, the sheer volume applied over many seasons results in a massive deposition of salt onto the land. For example, a single acre-foot of irrigation water can contain several tons of salt, which remains after the water is used. Plants absorb virtually pure water through their roots, leaving the dissolved salts behind in the soil solution. Over time, as farmers apply water year after year, the cumulative salt load in the topsoil steadily increases, initiating the cycle of salinization.
The Physical Process of Salt Concentration in the Soil
Once salts are introduced through irrigation, several physical and hydrological processes work to concentrate them to toxic levels within the upper soil layers. This accumulation is driven mainly by the movement of water out of the soil profile.
Evapotranspiration and Surface Concentration
Evaporation from the soil surface and transpiration from plants remove water from the soil, but they leave the dissolved salts behind. This constant water removal concentrates the remaining salt in the soil solution, increasing its osmotic potential. The salty soil holds onto water so tightly that plant roots struggle to absorb it, even when the soil appears moist, leading to physiological drought.
In hot, dry climates, intense surface evaporation draws water upward through the soil profile via capillary action. As this salt-laden water reaches the surface and evaporates, it leaves behind a visible white salt crust. This crusting effect results from salts being wicked to the top layer, posing a threat to emerging seedlings.
Poor Drainage and Capillary Rise
The natural solution to salt accumulation is leaching, which involves applying excess water to wash salts down below the root zone. Leaching is only effective if the soil has good drainage, which is often compromised in agricultural areas. Irrigation frequently raises the local groundwater table, especially when water is applied inefficiently or when underlying dense clay restricts downward flow.
When this water table, which is often saline, rises to within two meters of the surface, capillary action pulls the salt-rich groundwater upward. This upward movement, combined with the failure of salts to leach downward, creates a cycle of accumulation in the crop root zone where evaporation concentrates the salts near the surface.
Role of Soil Structure
The physical structure of the soil also plays a role in how quickly salinization develops. Compacted or fine-textured soils, such as those high in clay, naturally have lower permeability, which hinders the necessary downward movement of water for leaching. This poor infiltration traps salts near the surface, preventing them from being flushed away. In addition, high concentrations of sodium salts can cause clay particles to disperse, which further reduces the soil’s porosity and exacerbates the drainage problem.
Farming Practices That Accelerate Salinization
Several common farming practices unintentionally intensify the rate at which salts accumulate in the soil profile, compounding the natural physical and hydrological mechanisms.
Use of Chemical Fertilizers
The use of chemical fertilizers is a direct contributor to the total salt load. Many synthetic fertilizers, particularly those containing nitrogen and potassium, are salts designed to dissolve quickly and deliver nutrients. While plants use the nutrient portion, residual ions—such as nitrates, chlorides, and sulfates—remain in the soil solution, directly increasing overall salinity.
Inefficient Irrigation Methods
Inefficient irrigation methods significantly worsen the problem by contributing to water table rise. Techniques like flood or furrow irrigation apply large volumes of water over a short period, resulting in considerable leakage below the root zone. This excessive deep percolation rapidly raises the water table, accelerating the upward capillary movement of salts.
Land Use Changes
Changes in regional land use also have a profound, indirect effect on salinity, particularly in dryland areas. Clearing native perennial vegetation, such as deep-rooted trees and shrubs, for shallow-rooted annual crops reduces the natural uptake of water from the subsoil. This change causes the regional water table to rise over large areas, bringing naturally occurring geologic salts closer to the surface where they are concentrated by evaporation, even without irrigation.