Oh man, we all patiently await these things – new additions of the Dirt Diggers Digest! And today I have a brand new one to post – I know you got sick of re-reading all the previous posts! So sit back, relax and learn something new about dirt…I mean soil…I mean black gold…I mean heaven! Take it away Grant!
Why is the Accumulation of Sodium Problematic in Soils?
In past posts I have touched upon the subjects of soil salinity and soil structure separately (March and July 2018) but in semi-arid and arid zone soils in the Western US, the two soil properties are interrelated. Soil salts can have either a positive or negative effect on soil structure, which I will detail in a minute, but first let’s talk about soil solid surfaces.
Soil particle surfaces, especially the surfaces of clay particles, are charged, and almost exclusively negatively charged. Hence, soils have a measurable property termed “cation exchange capacity.” In other words, the surfaces of clay particles can electrostatically interact with positively charged cations (pronounced CAT-ions) in the soil solution. (NOTE: As mineral salts dissolve into soil water, they release their constituent ions which are charged cations and anions in each compound. For example: Na(+) and Cl(-) in the case of halite, or table salt).
Solution cations, such as dissolved sodium (Na(+)), calcium (Ca(2+)), magnesium (Mg(2+)) and potassium (K(+)) attach weakly magnetically to clay surfaces. This is a significant property of soils, in that many of the plant and animal nutrients (Ca, Mg, K, Fe, Zn, etc.) are cations when dissolved in soil solutions and can therefore be retained on soil surfaces for later exchange back into the soil solution and become available for uptake and assimilation by plant roots and microbes in the vicinity.
Additionally, the electrostatic interaction between negative charges on soil clay mineral surfaces, and positive charges on organic matter particles (which have both positively and negatively charged sites) are the primary forces that are responsible for the organization of solid particles into soil structural units (flocculates, aggregates and peds, not CLODS!). This opens up the soil matrix to increased water infiltration and aeration (see the March 2018 post, It’s All About Soil Structure, for more detail).
Here is where the nexus of soil salinity and soil structure comes into focus. Dissolved ions, particularly cations, play an important role in either stabilizing or destabilizing the interactions between neighboring soil particles called flocculates, and between soil mineral and organic matter surfaces that give rise to soil aggregates.
If the salt cations in the surrounding soil solution are divalent cations (meaning they have two positive charges, such as Ca(2+) and Mg(2+)), soil flocculates and aggregates are stabilized as the divalent cations form somewhat of a magnetic bridge between neighboring surfaces. But what about monovalent cations, especially sodium? Many common soil salts contain sodium, such as sodium chloride (halite), sodium sulfate (Glauber’s salt), sodium carbonate (soda ash) and sodium bicarbonate (aka baking powder—yes Virginia, it’s a salt!). As these salts dissolve, they release sodium cations. Sodium cations are unique in that they form a large hydration sphere (i.e., interact with a large cloud of surrounding water molecules). Due to their low (single, or mono) positive charge, sodium cations don’t compete as aggressively as divalent cations for exchange sites on solid surfaces, but if sodium is the predominant cation, something undesirable begins to happen.
In sodium dominated soil solutions, sodium begins to overwhelm the exchange sites on soil particle surfaces, and because of its large hydrated radius, begins to push neighboring soil surfaces apart. These surfaces begin to lose the weak magnetic interaction they had with nearby surfaces that formed them into aggregates, and, hence, soil structure begins to break down. In highly sodic soils, individual soil particles don’t interact at all with neighboring surfaces and are subject to independent movement with water that can cause the plugging of pores, or settling in non-random orientation in the gravitational field causing layered surface crusts or subsurface compaction zones.
Both conditions (plugged pores and crusts) can greatly reduce, or completely restrict, water infiltration and soil aeration, root penetration, plant seedling emergence, and other undesirable soil conditions. Hence, one needs to protect against the accumulation of sodium in soils, or reclaim a soil already high in sodium content. In an upcoming post, I will discuss the processes and amendments effective for soil conditioning and reclaiming soils from saline, sodic, or combined saline-sodic conditions. Stay tuned!
By Dr. Grant Cardon, USU Extension Soil Specialist – AKA the Dirt King!
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