what do you use sodium potash for?

I have not heard of sodium potash before. The only horticultural reference I can find for it on the web is for grass.

I aggree with bulbaholic, but potash is an old word for potassium, one of the 3 main ingredients in garden fertilizer, the other 2 being phosphorus and nitrogen, and every living thing needs it, Derek.

I think you are thinking of potassium sulphate (sulphate of potash) This is used to encourage fruit and flowers and is an ingredient of tomato feed. Use it in spring and summer but not in winter as it is very soluble and would wash away in the rain before the plants were ready to use it. However I do always put it round my gooseberries after their February pruning and they always crop well.