Potash fertilizer — is there a problem?
Is K fertilizer harmful?
This is the area in which the authors take a rather odd turn, claiming that adding fertilizer K reduces crop nutritional value, damages soil structure, and (as KCl) can be toxic to plants. Evidence given on all of these counts is rather flimsy, and there’s little to say other than that any such effects are, based simply on quantities, minor or so small as to be non-detectable, or at worst, detectable but unimportant.
Claims that adding fertilizer K will lower “quality” of corn or soybean grain are without merit, and can be ignored. Corn and soybean grain K content can increase with increasing K rate in studies, but there is some recent evidence that grain K levels in both crops might be even lower than the “book values” used for years. Levels, and any problems claimed to be associated with such levels, are certainly not increasing.
The next claim has to do with the idea that K “makes the soil hard” and damages structure. As a monovalent cation, K can, if added in large quantities and given time, displace some of the other cations on soil exchange sites. If most of these exchange sites carried monovalent cations such as K or sodium (Na), soils would tend to “puddle” and be difficult to manage. The number of exchange sites is measured as the CEC, which range from single digits to the 40s or higher, depending on soil texture and organic matter. Each CEC unit occupied by K translates to 780 lb of K in the top 7 inches of soil. The great majority of exchange sites carry divalent cations such as calcium and magnesium, and this will continue to be the case even if we add hundreds (and in many soils thousands) of pounds of fertilizer K per acre.
There have been a number of efforts over years to show that the chloride in KCl can lower crop yield. How this would happen in Corn Belt soils is not clear. Like its cousin NaCl (table salt), KCl is a salt, and roots that encounter high salt concentrations in the soil can be damaged. But KCl can usually be banded with no negative effect, though there have been some reports of lower yields with banded K. Reasons for this are often not clear, but are more likely to be due to salt effects than to the chloride itself.
As a negatively-charged anion, chloride moves through the soil quickly as water moves. Chlorine gas (Cl2) is harmful to all life, but its formation in the soil is chemically unlikely, and only tiny quantities would ever form. Chlorine is actually an essential plant nutrient, though some crops (like wheat) respond more to it than do corn and soybeans. It’s also known to reduce damage from some plant diseases. Plants, like people can deal with unneeded Cl by simply excluding it or getting rid of it once it’s been taken up.