A casing layer provide a moist porus structure for the primordia of the particular species of fungi to develop. It is a way of imitating an ideal soil structure. Some mushrooms, such as the Stropharia Ruguso-annulata require a microbial casing layer. Grown outdoors this will develop naturally but poses challenges for artificial cultivation. Other species can be grown on a substrate such as rye grain then when fully colonised a casing layer is added whether the substrate is in a tray or jar. The mycelium will then colonise the casing layer in preparation for fruiting from the the surface.


  • 10 parts peat
  • 10 parts vermiculite
  • 1 part CACo3 (Limestone)
  • 1 part CASO4 2H2O (Gypsum)


  • mix peat, CASO4 and CACo3
  • add water slowly while mixing
  • add vermiculite and continue to add water
  • test for field capacity


  1. Mix the peat, limestone and gypsum and then slowly mix in the water until moist
  2. add the vermiculite and continue to add water until you reach the require “field capacity”
  3. Field capacity is a level of moisture before saturation, the maximum water content before it would be considered water logged. Too much water whether in your substrate or in a casing layer will create an anaerobic environment and increase the risk of contaminants. It’s hard to describe but basically, squeeze a handful of the casing layer it a “rivulet” or quick stream of water should come out for 1 to 2 seconds before reduced to just a few drops.
  4. Now put the casing mix into jars, lids on but not tight, cover with tinfoil and sterilise in a pressure cooker at 15psi for at least 30mins.
  5. Leave to cool for 6-8 hours, now the casing can be added to the layer of substrate in whatever cropping container you are using.
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