What is the recommended ratio of spawn to substrate for growing mushrooms?

If you’re referring to growing mushrooms in bags or containers, we typically recommend a 10% ratio of spawn to substrate (dry weight). For example 1L/600g of spawn would be suitable for a dry substrate weight of 6kg. 

Alternatively, if you’re referring to growing mushrooms in an outdoor environment such as mushroom bed, we would recommend 1 litre of spawn per square metre.