Dr. Dan Talks Agronomy

Pass the Salt

Dan Davidson
By  Daniel Davidson , DTN Contributing Agronomist
Excessive build-up of salts in soil can be a problem in salt-sensitive crops such as legumes (soybeans and field beans, clovers, and most tree crops) or reduce microbial activity. (DTN file photo by Pam Smith)

A recent reader letter asked me to comment about the merits of spreading potash (potassium chloride) on corn over the spring and summer. He wanted to know the implications of the salts found in muriate of potash (or KCl) on biological activity in the soil.

Chloride is often mistakenly blamed for soil problems. However, chloride is actually a micronutrient and small amounts are actually needed by many crops. Typical applications of potassium chloride should not hurt microbial activity in the soil.

It's true that soil microbes are sensitive to salt and sodium. The problem is fertilizer labeling is confusing since Cl on the periodic table means chlorine. While technically correct, when it comes to fertilizer, it is actually referring to the chloride ion.

Fertilizers, by definition are salts. Fertilizers, comprised of individual mineral salts separate in water into their respective ions. Potash (potassium chloride) splits into potassium and chloride ions. The general rule of thumb to maintain soil quality and microbial activity is that electroconductivity (EC) should be less than 1 dS/m (deciSiemens/meter).

Soil microbes tolerate salt levels of 1 dS/m or less. Most agricultural soils will have EC less than this value. However, natural saline seeps or fields irrigated with salty (sodium-laden) irrigation water will have EC values greater than 1 and this can influence microbial activity and plant growth.

Will the chloride in muriate of potash (KCl) be a risk to microbes in the soil? Potassium chloride is a salt and contains the same chloride ion found in table salt (usually sodium chloride). Plants will take it up and actually have a small need for it.

Salts originate from natural minerals that are weathered in the soil by chemical and physical processes, or application of manures, fertilizer or salty irrigation water. Salts are soluble and mobile. Irrigation and rainfall leach salt below the root zone, but evaporation brings salts back to the surface. Excessive build-up of salts, including potassium and chloride, in the soil can be a problem in salt-sensitive crops such as legumes (soybeans and field beans, clovers, and most tree crops) or reduce microbial activity.

The goal is the keep EC less than 1 dS/M. The best way to correct a salinity problem is through good drainage that allows salts to be leached below the root zone. Practices that increase organic matter and promote soil tilth increase drainage.

Gypsum, no-till and practices that increase organic matter can help improve soil structure and drainage, but gypsum won't otherwise remediate salinity unless sodium is the primary culprit.

Plants are continually removing some chloride along with other nutrients from the soil without any influence on productivity. Salts become a problem when the concentration overwhelms the system. Excess salts in the root zone hinder and reduce the roots' ability to absorb water from the surrounding soil. This lowers the amount of water available to the plant, even if there may be adequate amounts of soil moisture for that particular crop. This causes plant stress and "burning."

It is not the chloride alone that affects the microbes. It is the salt level. As for chlorine, it is not naturally found in the soil or in fertilizer. However, as a disinfectant, it's hard on microbes. To learn more about the chloride vs. chlorine question, click on the link to the following article from A&L Labs in Canada at http://bit.ly/…

Dan Davidson can be reached at AskDrDan@dtn.com

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Dan Davidson