Salt Build Up in Soil

Asked August 12, 2017, 11:13 PM EDT

I'm not sure I agree with the tenet that salts build up in the soil media of potted plants--at least those potted plants in containers with drain holes. I understand the theory that salts build up when salt-laden water evaporates leaving behind the salts that were dissolved in solution and thus increasing the salinity of the soil. However, during the subsequent watering events with the same (and lower salinity level water), any salt present in the soil that is in higher concentration than that in the water, will dissolve back into the water. My contention is that then is that soil salinity will only be as high as that of the water used to irrigate the plant. What am I missing?

Orange County California soil

1 Response

Thank you for your question. I think the missing 'link' is the definition of "salt." In soil science,

"A salt, in chemistry, is defined as the product formed from the neutralisation reaction of acids and bases. Salts are ionic compounds composed of cations (positively charged ions) and anions (negative ions) so that the product is electrically neutral (without a net charge). These component ions can be inorganic such as chloride (Cl−), as well as organic such as acetate (CH3COO−) and monoatomic ions such as fluoride (F−), as well as polyatomic ions such as sulfate (SO42−).

There are several varieties of salts. Salts that produce hydroxide ions when dissolved in water are basic salts and salts that produce hydronium ions in water are acid salts. Neutral salts are those that are neither acid nor basic salts. Zwitterions contain an anionic center and a cationic centre in the same molecule but are not considered to be salts. Examples include amino acids, many metabolites, peptides and proteins.

When salts are dissolved in water, they are called electrolytes, and are able to conduct electricity, a property that is shared with molten salts. Mixtures of many different ions in solution—like in the cytoplasm of cells, in blood, urine, plant saps and mineral waters— usually do not form defined salts after evaporation of the water. Therefore, their salt content is given for the respective ions.

Salts can appear to be clear and transparent (sodium chloride), opaque, and even metallic and lustrous (iron disulfide). In many cases the apparent opacity or transparency are only related to the difference in size of the individual monocrystals. Since light reflects from the grain boundaries (boundaries between crystallites), larger crystals tend to be transparent, while polycrystalline aggregates look like white powders. Of course, some salts are inherently opaque.

Salts exist in all different colors, e.g. yellow (sodium chromate), orange ( potassium dichromate), red ( mercury sulfide), mauve ( cobalt chloride hexahydrate), blue (copper sulfate pentahydrate,ferric hexacyanoferrate), green ( nickel oxide), colorless (magnesium sulfate), white, and black (manganese dioxide). Most minerals and inorganic pigments as well as many synthetic organicdyes are salts."

So, "salt" doesn't just mean NaCl. It extends to the 16 macro and micro nutrients, as well as ions of the three 'building block' elements: oxygen, hydrogen and carbon.

Here is a link to a brief Extension article about 'curing' salt buildup. And this one explains the differences between rainwater and tapwater. Or, if you want to eliminate these ions completely, use distilled water, which is 'pure' H20, since the mineral ions have not 'survived' evaporation.

Hope this is helpful.