"Take your fingers and go, boom boom boom boom," he says, demonstrating how to pack a mixture of peat moss, dirt, lime, perlite and other components into galvanized metal boxes equipped with a lever.

It’s often mixed into peat-free composts, making up about 5-20% of the total, but you can also buy it on its own as a compressed block or as loose fibres. The blocks expand into a fluffy compost when ...