I would wait the feeding until the buds for next year's crop are forming--usually late summer, when the new canes are maturing. If your soil is exceptionally poor, some slow release nitrogen, such as alfalfa meal, may also be in order. Also, just how are you pruning it? I have seen some strange, hard to understand instructions on plant labels!

I have a thornless blackberry. Oregon, a really old variety, and it fruits well on a very narrow slot of ground next to the garden shed. I water them but seldom feed them. I have 2 litre coke bottles in soil near the root, for watering. They fruit well, even in dry years. I take out the fruited canes each autumn and tie in any new canes. If no new cane, due to dryness, I keep the best old cane. The berries are not huge, but are adequate for my needs. Hope this helps.

Watering maybe the problem. This year mine are covered with large berries and I would say it is because of the high rainfall and flooding in that area of the allotment.

True. In the wild, they usually grow in swales where water collects naturally. You say that you water it, Hutch, but just how do you do that? I.e., what method, how much, how often, etc. Also, are you gardening in eastern or western Idaho?

To all of you that got back to me I appreciate the advise.
I prune the fruited canes back to the groung in the fall.
the bush is on the end of my veg. garden so it is watered daily it gets a good soaking. I live in The Boise area where in july it is 100 most of the month.
thanks again