SoilSalt

Soil that has an excess of salts appears dry and cracked.

LOGAN, Utah — 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:

n Calcium sulfate (CaSO4) or gypsum.

n Sodium sulfate (Na2SO4) or Glauber’s salt.

n Magnesium sulfate (MgSO4) or Epsom salt.

n Potassium sulfate (K2SO4) or sulfate of potash.

n Sodium chloride (NaCl) or table salt.

n Potassium chloride (KCl).

n Magnesium chloride (MgCl2).

n 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

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.

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, restricts 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.

Other problems with soil salts include specific plant sensitivity to sodium or chloride, sodium’s dispersion effect on clay particles that 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. Arid/semi-arid zone soils require frequent testing for salinity and choosing plant varieties that can tolerate the level measured.

Soluble salts in soils cannot be neutralized 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. Salt levels are reduced only through physical washing (called “leaching”) from the soil with water of higher quality than the current saline condition. While this may take large quantities of leach water (6 to 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.