Why are arid/semi-arid zone soils salty, and why does it matter?
First, let us answer why soils get salty.
Salts are naturally occurring, soluble minerals in areas where soil evaporation and plant water use (or transpiration) are both high, and precipitation is low. In that scenario, dissolved minerals concentrate over time as the extraction of essentially pure water leaves the soluble mineral salts behind.
Salts vary widely in their composition depending on the types of mineral deposits in the region and the composition of irrigation water used. For instance, where geologic marine deposits interact with ground and surface water in a region (such as the Mancos shale formation in many parts of the Western U.S.) high levels of sodium-based salts can occur. In other areas, geologic gypsum deposits result in calcium-based salts accumulating in the soils. Generally, a diverse suite of minerals gives rise to a “cocktail” of soil salts.
Common salts in soils include:
- Calcium Sulfate (CaSO4) or Gypsum
- Sodium Sulfate (Na2SO4) or Glauber’s Salt
- Magnesium Sulfate (MgSO4) or Epsom Salt
- Potassium Sulfate (K2SO4) or Sulfate of Potash
- Sodium Chloride (NaCl) or Table Salt
- Potassium Chloride (KCl)
- Magnesium Chloride (MgCl2), and
- Calcium Chloride (CaCl2)
(NOTE: calcium and magnesium chlorides are the common de-icing salts used on roadways)
Additionally, salts such as calcium carbonate (lime), sodium bicarbonate (baking soda), and many others, are either too insoluble (in the case of lime) or less common (like baking soda) to contribute much to the salinity or “saltiness” of soils.
Secondly, why are salts in soils a concern?
The primary concern common to all salts is that the salinity of soil water is inversely proportional to the free energy of the water; meaning that small magnetic interactions between the salt ions and water molecules prevent the water’s free movement. This in turn, restricts water transfer across the membrane barrier of plant roots forcing the plant to expend energy to overcome these interactions to draw in water.
Soil salinity, therefore, creates a restriction to plant water uptake, resulting in what is often termed a “chemical drought.” Enough salt may dissolve in soil water that even though there may be plenty of water in the soil reservoir, plants cannot overcome the water-salt interactions to draw water into the root.
Hence, salt-affected plants look to be under water stress.
They look wilted and stunted in growth. In the extreme, leaf margins can look desiccated or “burned” compared to leaf centers, eventually becoming fully necrotic. At low salinity levels, even if the plant exhibits no outward symptoms of salinity stress, the plant may have to siphon off precious energy to devote to water uptake at the expense of growth and eventual production.
(NOTE: Some new de-icing salt formulations are termed “safe” salt, but that is not in reference to their effect on plants. Rather, it refers to their propensity to corrode metal car parts. In excess, there is no “safe” salt as far as plant water uptake is concerned).
Other problems with soil salts include specific plant sensitivity to sodium or chloride, sodium’s dispersion effect on clay particles which breaks down soil aggregates and causes plugging and crust formation, and very high pH levels (greater than pH 8.5) in soils as a result of a buildup of sodium carbonate. These special cases may occur under certain circumstances. We will not go now into the specific mechanisms for these additional salt effects, but save them for another post.
Also in a future post, we will discuss the variable tolerance to soil salinity exhibited by different plant species. Suffice it to say right now, that arid/semi-arid zone soils require frequent testing for salinity (accomplished by measuring the electrical conductivity of a soil solution) and choosing plants that can tolerate the level measured.
Just a word in closing about soil salt reclamation.
One cannot neutralize soluble salts in soils with external amendments. There are products available that purport to do so. These are typically organic polymers that adsorb salt ions for a period, but that eventually decompose and release adsorbed salts back into solution. One reduces salt levels only through physical washing (called “leaching”) from the soil with a water of higher quality than the current saline condition. While this may take large quantities of leach water (6-12 inches of water per foot of soil reclaimed) this is a one-time effort, followed by low salinity maintenance through annual fractional leaching to prevent buildup.
Grant Cardon, USU Extension Soils Specialist