Dirt Diggers Digest – Issue 13, August 2022

Today’s installment has to do with soil sodicity, or the imbalance of dissolved sodium in soils compared to other ions such as magnesium and calcium specifically, and others, indirectly. We have already learned about soil salinity and its effects on plant water use (see Dirt Diggers Digest – Nov 2017 and July 2018) but a brief review is needed.

A Short Review

All soils and natural waters contain dissolved ions due to the dissolution of minerals from rocks and sediments, or released from decomposing organic matter. These ions are critically important to life as they provide the mineral nutrition needed by all plants and animals. That said, an over-abundance of dissolved mineral ion content in soils and waters can make it difficult for plants and animals to extract life-sustaining water from the soil, or draw it in across the root membrane against an osmotic potential gradient. The soil water becomes too concentrated and actually holds water back from the plants growing in it. Salty irrigation water on the foliage of plants can actually draw water out of the leaf and desiccate, or “burn” the tissue.

Soil Surface Chemistry in a Nutshell

With that as a reminder, let’s dive a little deeper into the dirt! Soil particles (especially clay minerals) and decomposed organic matter surfaces, have electrically charged sites on them to which charged mineral ions in soil water can adhere (think of magnets on a refrigerator door). These sites are largely negatively charged, and attract positively charged cations to their surfaces, hence this property of soil being called cation exchange capacity (or CEC). In balanced soil systems, there is a wide-ranging variety of cations on the exchange sites, which serves as a retention mechanism for important nutrients plants and animals can obtain from the soil. Most of the critical nutrient elements such as Nitrogen, Potassium, Calcium, Magnesium, Sodium, Iron, Zinc and others, have positively charged ionic forms that can be held on this exchange complex.

Interestingly, each cation imparts some of its own properties to the surface chemistry of the exchange complex (yes, you can take, and yes I have taken, a course on the surface chemistry of soils—it is NOT for the mildly interested). In particular, ions such as calcium and magnesium (particularly calcium) being divalent (meaning they have 2 units of positive charge) tend to bridge neighboring particles closer together in a magnetic handshake of sorts—sharing one unit of charge with each of the two neighboring particles. Such a condition in soils is termed, flocculation.

Sodium ions on the other hand only have one unit of charge, which is used in interaction with the surface of only one soil particle. As it turns out, sodium ions also have a large diameter, so if sodium is the dominant, or even abundant cation in solution, the exchange complex becomes saturated with them, the available surface charge is masked, and soil particles are repelled apart and stay isolated from one another. Such a condition in soil is termed, dispersion. In the case of a soil with moderate clay content, small, dispersed clay particles can be easily moved about and begin to plug pores in the soil matrix, or settle all in the same orientation to gravity’s pull and form crusts at the surface or compacted layers below the surface. All of these are highly undesirable soil conditions which restrict air and water movement into and through the soil.

Determining Sodium Hazard

Much research has been done to quantify the tipping point at which an abundance of sodium begins to have these negative effects on the physical condition of the soil. Studies indicate that once 15 percent of the exchange complex is saturated with sodium ion, soil structure begins to break down, clay particles become dispersed, and soils begin to exhibit the ill effects of sodium excess. Measuring the actual exchangeable sodium percentage (or ESP) of a soil is quite complicated, so a correlated measure where the dominance of sodium in an extract of soil water is determined—the Sodium Adsorption Ratio (or SAR)—has become a surrogate measure by which we determine sodium hazard.

Soils that have a generally high content of dissolved ions in solution are deemed to be “saline” in character (the electrical conductivity of the soil is greater than 4 dS/m). Soils that have an SAR of greater than 13, regardless of the total salinity of the soil, are deemed “sodic.” Soils that have both high salinity and an SAR greater than 13, are deemed “saline-sodic.”

Sodic Soil Reclamation

Sodic soils present myriad problematic conditions we previously mentioned (pore plugging, crusting, etc.) and must be reclaimed if they are to be productive. Whereas we previously discussed salinity in the past, you may recall that reclamation of a saline condition is accomplished by leaching soluble salts out of the soil matrix. Reclaiming a sodic soil is much more involved. First, sodium must be replaced on the exchange complex, and secondly, leached from the matrix in order to establish a desirable balance between sodium and divalent cations like calcium and magnesium.

Sodic soil reclamation, then, is a stepwise process. The first step being the replacement of sodium on the exchange complex with a divalent cation, preferably calcium, that will release sodium into the soil water, begin to flocculate clay particles together, thus allowing for improved water movement through the soil matrix. Only then can one leach the matrix of the sodium and excess salts in the soil in order to establish an acceptable sodium balance.

Sources of divalent cations vary, but the main source is generally gypsum (calcium sulfate salt), or, in the case of a soil containing lime (solid calcium carbonate) adding an acidic material that solubilizes calcium from the lime and allows it to react with the exchange complex. There are other sources as well that may be locally available. For instance, here in Utah, we have used calcium nitrate sludge—a byproduct of chemical manufacture—which contains about 12% soluble calcium and has been used effectively. Chloride salts of calcium and magnesium could also be used (these are common ice melt salts used in winter highway maintenance or used for dust suppression on dirt roads). Chloride, however, has some specific toxicity on plants, particularly in direct contact with the foliage, whereas calcium sulfate and calcium nitrate have the advantage of adding additional nutrients like sulfur and nitrogen.

To effectively reclaim a sodic soil, one needs to determine the “gypsum requirement” which involves detailed soil analysis of the soil SAR, CEC and total salinity. Just adding a soil conditioning amendment is not likely to be effective without the information necessary to refine the amount needed, and is often a waste of time and money.

Additional Considerations

Here are a few additional things to be aware of:

1. The pH of your soil can help you determine whether your soil is likely to experience a sodium issue. Most of our soils in the west contain excess lime (calcium carbonate) and have pH’s of between 7.3 and 8.3 in equilibrium with that mineral. If your soil pH exceeds 8.3, this is often due to a shift from calcium carbonate to sodium carbonate as the dominant mineral buffering pH. In sodic soils, pH’s can range from 8.5 to nearly 10. The pH of a soil is a routine measurement in a standard soil test, and can give us a clue to the potential need to more deeply test the sodium balance of the soil.
   
   
2. Soils very low in total dissolved mineral content (i.e. salinity) can experience dispersion and sodic soil-like infiltration and aeration problems if they become further diluted by the addition of a low electrolyte water, like rain water. This is not likely to be an issue in Utah, but there are soils in the Intermountain West where this may occur. In such cases, the treatment would be to add a soluble calcium source to the soil to promote flocculation, but there would be no need to then leach the soil of excess salt.
   
   
3. Conversely, soils very high in total dissolved mineral content (i.e., high salinity) are somewhat protective against dispersion simply because there are so many ions in solution and random exchange reactions on the surface of soil particles keep dispersion in abeyance. However, in such cases, if one simply begins leaching to remove excess salt, but is not aware of the sodium balance, soil structure and permeability may rapidly deteriorate as the ion content is reduced, making further reclamation very difficult. Make sure you are well informed on your soil’s sodium balance in such cases so you don’t inadvertently make soil conditions worse.

Author

Grant Cardon