A matrix of smaller and larger particles can be combined to create an ideal habitat for mycelium. The smaller particles stimulate quick growth ("leap-off'). The larger particles encourage the mycelium to form thick, cordlike strands, called rhizomorphs, which forcibly penetrate through and between the cells.The larger chips become nutritional bases, fruiting platforms, giving rise to super-large mushrooms. A simple 50:50 mixture (by volume) of sawdust and chips, of varying particle sizes, provides the best structure for the mushroom habitat [1].

Straw and sawdust are nitrogen poor and semi-selective for mushrooms. Usually, these substrates are supplemented with nitrogen to increase mushrooms yields. When
nitrogen is added, however, selectivity is somewhat lost (other fungi will grow in the substrate). If high levels of nitrogen are used, temperatures in the substrate increase by respiration as the mycelium of the mushroom or other organisms grow. As temperatures rise due to the biological growth (called thermogenesis), other molds loose dormancy as temperatures approach 100 F. Below this, the mushroom consumes these organisms. Temperatures of 75-85 F are ideal. Adding nitrogen to a substrate always increases the risk of contamination.

Mushroom growers exploit the ability of fungi to digest substances that many organisms cannot, by pairing mushroom crops with semi-selective substrates that are nutritionally inaccessible to potential competitors (Stamets 2000).


  1. Paul Stamets: gourmet and medicinal mushrooms, P301.
  2. Sawdust as Substrate