Hello Hootie and welcome to the forum.
By potash I guess you mean a mix of minerals that contain water soluble K (like KCl, K2CO3, K2SO4 and KNO3). Once the capsules are hydrated the K will become water soluble and not stick in the substrate, basically the same thing as adding K2SO4 to your water column. K is only part of plant nutrition as you rightly say. Once enough K is provided, plants will need sufficient N, P, and C and other macro and micro nutrients in plant available form. If one is not enough you will run in the next deficiency.
I myself have not used osmocote but they are a source of NH3 so they can trigger an algae bloom. Other users in the forum had problems with overdosing their substrate and had consistent high levels of NO3. The problem is that once they are in the substrate you cannot easily take them out nor can you control how fast they release their nutrients. This all is not to say that other users were not successful with it and if used sparingly it can provide enough nutrition for a low input/output tank.
Tell us more about your aquarium. Do you have any pics of the deficiency in your tank ?