Jane, potash is essentially potassium. It is normally a constituent in balanced fertilizers and therefore there should be no need to add it preferentially in the spring over any balanced fertilizer (10-10-10 or 15-15-15). In the fall, you often will see fall fertilizer for your lawn advertised. It contains higher amounts of potassium because potassium helps with winter hardiness of your grass, but that doesn't explain why your clematis reference is recommending applying it in the spring. People growing clematis use a myriad of products including rose fertilizer, compost, composted manure, alfalfa pellets or tea made with alfalfa, grass clippings, shredded leaves, leaf mold, etc.

Continue to add organic matter to your soil to help improve its tilth. Since the clematis is already planted, this can be accomplished by using a mulch that you will spread around your plant that will decompose over time such as shredded leaves, compost, grass clippings, etc. Sandy soil eats organic matter and needs a lot of organic matter in order to help to hold onto moisture, which clematis love. Also since organic matter decomposes, you need to continuously add it over the growing season.

Glad you are having a great weekend in CT. Here in NC it is hot, dry, and the yards already look like it is the middle of August! :O