Soil salinization, the accumulation of soluble salts in the soil to a level that negatively impacts soil quality and plant growth, presents a significant environmental challenge. While this process can occur naturally, certain agricultural practices accelerate the problem. This article explores how farming contributes to this issue.
Irrigation Water as a Salt Source
Irrigation, a common agricultural practice, introduces dissolved salts into the soil, directly contributing to salinization. Even water sources considered fresh, such as rivers, groundwater, or treated wastewater, contain varying concentrations of these dissolved mineral salts. Most irrigation water contains between 200 to 800 milligrams per liter (mg/L) of dissolved salts.
When irrigation water is applied to fields, only a portion is utilized by plants. A significant amount evaporates from the soil surface. As the water turns into vapor and leaves the soil, the dissolved salts are left behind. This continuous process leads to a gradual concentration and accumulation of salts within the topsoil.
Repeated irrigation over time, especially with large volumes of water, continuously adds more salts to the soil. Each subsequent cycle of evaporation further concentrates these accumulated salts, steadily increasing their levels in the root zone. This input and concentration of salts from irrigation water drives agricultural salinization.
Waterlogging and Capillary Rise
Agricultural practices, particularly excessive irrigation combined with inadequate drainage, can lead to waterlogging. This facilitates salinization through capillary rise. Waterlogging occurs when the soil becomes saturated, and the water table rises close to the surface. This often results from applying more water than the soil can absorb or drain.
When the water table is high and close to the surface, water from this saturated zone is drawn upwards through the tiny pores and channels within the soil. This phenomenon, known as capillary action, operates similarly to how water rises in a narrow tube. The forces of adhesion and cohesion pull the water upward through these microscopic pathways.
As this upward-moving water reaches the soil surface, it evaporates, leaving behind any dissolved salts it carried. These salts originate not from the irrigation water itself, but from deeper soil layers or underlying groundwater, which can contain high concentrations of salts. Capillary rise thus mobilizes existing salts from beneath the surface, depositing them in the upper soil profile as the water dissipates.
Changes in Land Use and Water Dynamics
Significant alterations to natural landscapes for agricultural purposes can indirectly lead to soil salinization. This occurs by disrupting regional water balances. Clearing deep-rooted native vegetation for shallow-rooted annual crops significantly impacts the water cycle. Native plants typically draw substantial amounts of water from deep within the soil and release it into the atmosphere.
With the removal of this deep-rooted vegetation, less water is extracted from the soil. This reduction in water uptake means more rainfall infiltrates the ground, increasing groundwater recharge. Over time, this can cause a rise in the regional water table. For instance, in parts of Australia, clearing forests has led to replenishment of shallow aquifers.
As the water table rises, it brings naturally occurring salts from deeper geological layers closer to the surface. These deep-seated salts are then drawn upward by capillary action. Once near the surface, this saline groundwater evaporates, depositing these ancient salts and contributing to surface salinization, even in areas not directly irrigated.