I do know that you can use upto at least 25% of composted coffee grinds. I must put 40-60kg on my plot every week to 2 weeks with No problems. I must have put 100kg in my fresh horse manure bins just a few weeks ago and boy does it speed up the process.
If you do put lots on the ground watch out for crusting as it will prevent water getting to the roots so I lightly rake it in after spreading. Plus SLUGS hate it