From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335

Fall 1999, No. 6


There are several potassium fertilizer sources. Examples include muriate of potash or MOP, sulfate of potash, sulfate of potash magnesia, potassium nitrate, and potassium thiosulfate. Of all the potassium fertilizer materials, MOP is by far the most commonly used. Over 90 percent of all potassium applied to cropland in the U.S. is applied as MOP. Chemically, MOP is potassium chloride (KCl) and is a naturally occurring mineral. Most fertilizer MOP is derived from evaporites that were precipitated when ancient shallow seas evaporated over geologic time.

Granular MOP can be either red or white in color. The color difference sometimes causes confusion. Let’s look at a couple of commonly asked questions about red and white MOP.

Why is some MOP red and some white? Both red and white MOP come from the same evaporite ore deposits. The ore usually contains sodium chloride (common table salt) and other impurities. Naturally occurring MOP ore has a reddish color due to minor amounts of iron impurities. The color difference in the final fertilizer product is due to different methods of recovery and processing.

In the production of red potash the ore is crushed fine enough to provide relatively pure, single mineral grains. The ground ore is then put into fluid suspension. A flotation process is employed where the MOP is floated to the surface of the suspension and skimmed off. The MOP is then further processed and screened. The resulting fertilizer material is nearly pure (about 95 percent) potassium chloride. It retains the reddish color since the iron impurities are not removed in this process. The color may vary from dark red to pink depending on the iron content.

White potash is produced by a process of dissolution and recrystallization. Potash ore is dissolved under pressure in hot brine, and MOP is precipitated as the brine cools and pressure reduces. The iron is removed in this process, and the resulting MOP fertilizer is white. White MOP is generally at least 98 percent potassium chloride.

Is one agronomically better than the other? One of the most important factors affecting the agronomic effectiveness of any fertilizer is solubility. Simply put, plant roots take-up mineral nutrients via soil solution. Thus, a material that is not in soil solution (soluble) is not of much use to the plant at that specific point in time. One text (Fertilizer Technology and Use, 3rd Edition) states that “Fertilizer potassium as KCl added to the soil is 100 percent water soluble.” Since both red and white granular MOP are highly water soluble, they are of equivalent agronomic effectiveness. Both also have near equal amounts of chloride, approximately 47 percent.

In summary, there is no difference in the agronomic value of red and white granular MOP. The difference in color between the two is due to minor amounts of iron in the red material. Whether red or white, MOP is an excellent source of not just one, but two nutrients essential for crop growth and health…potassium and chloride. 


For more information, contact Dr. W.M. (Mike) Stewart, Great Plains Director, PPI, P.O. Box 6827, Lubbock, TX 79493. Phone (806) 795-3252. E-mail:
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