A6 Salinization and Chemical Soil Degradation

Topic

Salinization consists of the accumulation of soluble salts within the soil profile, reaching concentrations that eventually inhibit plant growth; for many crops, this inhibition threshold is reached when the soil's electrical conductivity exceeds four decisiemens per meter.

Within the soil, these salts move via the same transport mechanisms as any solute dissolved in water: they are carried along by the water flow itself—a process known as advection—while simultaneously redistributing through molecular diffusion and hydrodynamic dispersion; this causes the salt to spread across a much larger volume of soil than that originally occupied by the water that transported it.

Primary salinization occurs when the water table is shallow (less than two meters deep) and saline groundwater rises via capillary action to the plant root zone; secondary salinization, conversely, is induced by agricultural irrigation itself, as irrigation water introduces salts into the soil that become concentrated as the water evaporates, leaving the salt behind.

When the soil also accumulates a high proportion of sodium relative to its calcium and magnesium content, sodification occurs; this excess sodium disperses clay particles, completely destroying the granular structure that normally holds the soil together.

Acidification via leaching, meanwhile, reduces the soil's capacity to retain and exchange nutrient cations; it is precisely this loss of cation exchange capacity that causes the soil's aluminosilicate clay to dissolve, releasing aluminum in a form that is soluble and toxic to plants.

The quantitative model integrating these processes combines Richards' equation—which describes water dynamics within the soil—with the reactive transport of the solutes dissolved in that water; Thus, from the salt accumulation with which this document begins to the toxic aluminum released by the acidification with which it concludes—encompassing salt advection and dispersion, primary and secondary salinization, and the sodification that destroys soil structure—salinization and chemical soil degradation can cause a single soil profile to lose its capacity to support crops through several distinct chemical pathways simultaneously.

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