http://www.soilassociation.org/farmersgrowers/technicalinformation/compostsplantraisingmediaandmulches/mushroomsubstrates
Mushroom substrates : Soil Association

Mushroom substrates

Product Name Classification Company Name Telephone Number (Sales) Product Description Product Use Standards Category

Mushroom Casing

Mushroom Compost Mc Ardle Peat (028) 38 891506 Please contact company for more information Please contact company for more information Permitted

Mushroom Casing

Mushroom Compost Mc Don Mushroom Casing Ltd (028) 87 784096 Mixes of peat and limes for mushroom growing For mushroom growing purposes Permitted

An explanation of the "standard categories" for use of organic products are as follows:

Permitted:   You may use this product.

Permitted with justification provided at inspection:
You may use this product but you must be able to justify its use at your inspection with evidence such as test results, records, forms a plan etc. For example you must record why you needed to use that particular product

Prior approval required:    
Before you use the product you must have been given approval by your certification body. This approval may cover more than one use. For example you may submit an annual plan to your certification officer that will then need to be approved before you can use the product.