K₂O to Elemental Potassium

Potash is the other half of the oxide convention. The K figure on a fertiliser label is potassium expressed as K₂O, an oxide that is not present in muriate of potash, sulphate of potash, or anything else in the bag. The factor to elemental potassium is $78.196/94.195 = 0.830$, and back again is 1.205.

The word "potash" is itself a relic and worth a moment. It comes from "pot ash": wood ash leached in a pot, evaporated, and sold as the crude potassium carbonate that was the world's only potassium fertiliser before mineral deposits were mined. It was North America's first chemical export.

Because the factor is 0.83 rather than 0.44, the potassium error is less dramatic than the phosphorus one — about 20% rather than 130% — which paradoxically makes it more dangerous. A 20% discrepancy looks like ordinary variation between recommendations and rarely triggers suspicion, whereas a factor of two makes somebody check.

The place it bites hardest is comparing a soil test to a fertiliser plan. Soil tests report exchangeable K in ppm or cmol(+)/kg, always elemental. Getting from one to the other needs both this conversion and the depth-and-bulk-density step, and the two are frequently confused with each other.