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

Spring 1997, No. 3


Potash, as the name sounds, comes from pot ashes—so called because it was first obtained by evaporating the lye of wood ashes in iron pots. Early settlers used pot ashes as their primary source of potassium for fertilizers. Today, we use potassium-containing minerals for fertilizers. And, whether they contain potassium sulfate, potassium chloride, or some other form of potassium, they're all potash fertilizers.

Potash is sold on the basis of its oxide, or K2O equivalent. That's confusing because potash does not contain any K2O and plants do not take up K2O. A fertilizer with an analysis of 0-0-60 would contain 60 percent K2O equivalent by weight. There has been interest in expressing the potassium in potash as percent K rather than percent K2O, but for now, the oxide form is the standard. Fortunately, converting K2O to K and vice versa is a simple calculation:

% K = % K2O x 0.83
% K
2O = % K x 1.2

Potash fertilizers contain from 22 to 62 percent K2O equivalent, and all are water soluble. They consist of potassium in combination with chloride, sulfate, nitrate and other nutrients. Common potash fertilizers include:

Potash fertilizers are excellent nutrient sources because they supply other essential plant nutrients in addition to potassium. MOP contains 60 to 62 percent K2O equivalent, varies in color from white to pink or red, and supplies about 45 percent chloride. SOP is a white material containing 50 to 53 percent K2O equivalent and 18 percent sulfur. Potassium magnesium sulfate has 22 percent K2O equivalent, 11 percent magnesium and 22 percent sulfur. Saltpeter contains 44 percent K2O equivalent, and 13 percent nitrogen.

Muriate of potash is the most widely used potassium fertilizer. It is applied directly to the soil or used in blends with nitrogen, phosphate and other nutrients. Potassium sulfate is used most often on potatoes, tobacco, turf grass and other crops which may be sensitive to large amounts of chloride. Potassium nitrate is also good for chloride-sensitive crops and is most widely used for fruit trees and high cash value crops like cotton and vegetables. Potassium-magnesium sulfate is routinely used wherever soils are deficient in at least two of the three nutrients in the products.

In most situations one potash fertilizer is as effective as another if the material is being used for its potassium content alone. Accompanying elements can be important and should be considered in choosing among the various sources.

Regardless of source, potash supplies potassium, an essential plant nutrient. Potassium is often deficient in the soil, making potash fertilizer necessary to grow high yielding, high quality crops.


For more information, contact Dr. Terry L. Roberts, Western Canada Director, PPI, Suite 704, CN Tower, Midtown Plaza, Saskatoon, Saskatchewan, Canada S7K 1J5. Phone (306) 652-3535. E-mail:
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