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

Winter 2001, No. 7


Ever wonder where the term potash originated? Of course, potash is now commonly used in reference to potassium fertilizers. But, in colonial days it referred to a crude potassium carbonate salt that was produced by leaching wood ashes and drying or evaporating the leachate in iron pots…hence pot-ash. Early settlers used this salt as a source of potassium fertilizer and for soap and glass making, wool scouring, and cloth dyeing. In fact, the first patent issued in the U.S. was for a process to extract potash from wood ashes.

In fertilizer terminology, potash refers to potassium oxide, or K2O. For example, a fertilizer with 0-0-60 analysis will contain 60 percent K2O equivalent by weight. This is somewhat confusing since the fertilizer material doesn’t actually contain K2O, and plants do not take up K2O. It’s simply the standard that has been adopted and used for some time now. Occasionally, in scientific literature, percent potassium is used instead of percent K2O. To convert potassium to K2O, multiply by 1.2; multiply K2O by 0.83 to convert to potassium.

Potash fertilizers range from 20 to 62 percent K2O. They are all water-soluble and therefore agronomically effective. They consist of potassium in combination with chloride, sulfate, nitrate, and other elements. Common potash fertilizer sources include,

In addition to potassium, these fertilizers provide other needed nutrients. For example, MOP contains 60 to 62 percent K2O and about 45 percent chloride. Sulfate of potash contains 50 to 53 percent K2O and about 18 percent sulfur. Sulfate of potash magnesia contains 20 to 22 percent K2O and sulfur and 10 to 11 percent magnesium. Potassium nitrate contains 44 percent K2O and 13 percent nitrogen.

Muriate of potash is by far the most commonly used of the potash fertilizer sources. It comes in red, white, and colors in between. The question is sometimes asked, “Does the color of MOP make a difference in its agronomic effectiveness?” The answer is an emphatic No. Some crops may be sensitive to the chloride in MOP. Therefore, SOP or potassium nitrate may be the best source for crops such as potatoes, tobacco, fruit trees and others with low tolerance to chloride. Sulfate of potash magnesia is routinely used wherever there is a need for at least two of the three nutrients in that material.

Potassium is a major essential nutrient in crop production. Where it is deficient in the soil or where crop demands during specific growth stages exceed the soil’s ability to supply adequate potassium, it must be supplemented through fertilization. All potassium fertilizers are agronomically effective and in most cases will perform similarly. Crop sensitivities, the need for accompanying nutrients, and market availability are factors that should be considered when selecting the best source for a specific situation.


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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