Backyard Blackberries! A gift from nature.

Yes, I know this is the Grassroots Motorsports forum, but some few of you know the answer to every question.  Berkeley Alexa.

Back when we first moved to this property 20 years ago, we fenced an area, built a doghouse and a lean-to for shade, and kept my wife's two crazy outdoor dogs there.  Somewhere down the road, a small stand of blackberry began to thrive, and produced large, tasty blackberries.  Year after year they thrived and gave us more and more berries.  I would refer to them as "Jackson's Blackberries" after the stalwart Chow mix that tended them.  Years passed, as did the dogs, and still the berries grew.  Neighbors would come over and pick a few quarts, leaving us more than we could eat.  The berry patch had grown to where it was not possible to reach the middle, and in places, it measured as much as fifteen or twenty feet across.

A few years back, we began to notice a loss of vigor in the plants.  At first we chalked it up to "a bad year for berries," but a year or two more of decline convinced us that something wasn't right.  Pokeweed plants, honeysuckle, and a few rogue trees would pop up in the middle of the patch, and I would prune them out, but still, the berries were in decline.

So, what to do?  Did the former supply of dog pee give the plants the nutrients they needed?  Or have they simply depleted whatever naturally-occurring goodness was present in the soil?  What sort of nutrients might I add to the soil?

Following to see what I can learn to help mine do better. 

Are they pruned?  Seems that they know that they have taken over so that they produce less fruit. 
I prune mine every year once harvest is done- cutting all of the fruit bearing cane to allow the new growth to do their thing. And topping the new growth at ~3ft to allow multiple branches for fruit, too. 
And I hear it's important to make sure the new growth doesn't compete with itself too much, so even thin out the new stuff. 

alfadriver said:

Are they pruned? 

In short, no.  As we've had room for them to do their thing, I had chosen not to mess with success.  Their continued spread always produced fruit on new growth, and the old growth seemed to fruit nearly as well.  My wife has taken clippers to them from time to time, but I've seen no positive result.