Ichi's new litter

For the past 2 days, when I go out with my 5 youngest cats to the garden, Ichi urinates on a pile of ash. The gardener sometimes burns old leaves/dried grass in a corner and now there's a permanent "ashbox" The other 4 just watch Ichi but have not gone near the ashes. Is this okay?

i did a quick search but couldn't find any health hazards of ashes used as litter. In fact all I did find was that before "kitty litter" many households used ashes and sand in cat boxes for their litter. The only problem was that it was just so messy

Thank you very much!!! Why didn't I think of researching before asking my question?