Hi Gordon,
I doubt it's potash, or any other mineral. By the sounds of things the problem is more with the soil structure, I'll bet it is very low in humus.Humus is what compost becomes when compost/organic matter has broken right down. This, in effect becomes an invisible jelly which coats every soil particle, at least it does, when you've got enough of it.
This is the 'magic' ingredient of soil, which holds the moisture and in that moisture are the dissolved nutrients, for the plants.
The solution is long term, it takes two or three years to start to get it right, it all depends on how much compost you can get dug in. One thing, try not to grow carrots or parsnips on freshly composted soils, fine if it was for the previous crop otherwise they bifucate [lots of little roots instead of one big one]
Watering will help in dry conditions, but without the humus it will drain out, taking nutrient with it, so a little weak feed, every ten days or so will help.
Get digging! good luck.
Paul Rix [oldgeezer]