giant tea bag

It has been suggested that it is a good idea to fill an onion bag with well rotted horse manure and suspend in my water butt like a giant tea bag and use the liquid on my plot. Has anyone heard of this

yes! I've not tried it tho. I expect it has a similar effect as making nettle tea or comphry feed. I would only use it on well established plants tho, in case it scorches tender roots.