I would imagine one of those asplundh machines would work. Provided you could feed it in. And it would be very messy. I would just hack at it with a mattock or something similar.

A half inch (or finer) soil screen, used like a cheese grater, will quickly break up clumps in compressed bales of peat and screen out any large stacks/twigs and what few stubborn clumps resist breaking up. You could break up a 3 cu ft bale in less than 10 min. I always screen the peat to break it up when making the 5:1:1 mix.

Al

@charles kidder By asplundh machine you mean a wood chipper? It's a bit large for this application?

@tapla (mid-Michigan, USDA z5b-6a) The sideway scraping motion against a 1/2 inch screen does break up the larger chunks quickly. But I was finding myself wanting to then run the particles that pass 1/2 inch through a 1/4 inch screen, and that got a bit time-consuming. I have smaller screens than what you photographed.

It would be great to have a large grain mill dedicated to something like this, but finding the right form factor might be hard, and it's expensive.