[header] How to add potassium to an organic garden [title] Mix in muriate of potash or sulfate of potash. [step] Muriate of potash, or potassium chloride, and sulfate of potash, or potassium sulfate, are natural minerals. Muriate of potash tends to be cheaper, but the chlorine it contains can hurt the helpful microbes living in your garden's soil. 
Sulfate of potash is safer, but it's a bit more expensive. [substeps] Check your product's label for specific instructions on how much to add per square foot or meter.