Just like a Cow?

You put grass in the top and get plant food out of the bottom?
;¬)
Remember this?
Since then, by way of a test, nothing has gone into it except the grass cuttings.  NOT all of them, there wasn't room - BUT - when we took over the back garden was "wall-to-wall" grass, with a small pile in one corner.  The pile turned out to be, almost, pure "soil". 
With that in mind, but a LOT later, I mounted the test.  Checking back I see it has had a mention or two so this may well be the last.
having been reminded online that it's "D-Day" had to give ASG a mensh.