For more than twenty years supplementing incubated compost has been a widespread practice.
The standard advice was 1 kg of supplement per square metre a safe amount with good results. Tests at the time were based on 100kg/m2 fresh compost.
It's also known that supplementing ‘poorer’ compost is essential and that supplementing ‘richer’ compost gives limited results. To distinguish between poor or rich compost the nitrogen content is used as an indicator as no better figures are available. However, this parameter is not actually adequate. It says nothing about the form in which nitrogen is present, or in which amino acids. A high nitrogen content is therefore no guarantee for a nutrient-rich compost for mushrooms.
With contemporary filling weights more supplements can be added. Taking 90kg/m2 incubated compost as an example with normal compost, 15kg/m2 could be supplemented based on the filling weight alone, in other words around 17kg/ton. With poorer compost you could add more for optimal results 20kg/ton is not too high in this case.
It's very important to mix the supplement thoroughly; this aspect is often neglected when growers add supplements themselves. And good control of the extra activity this generates is essential.
Con Hermans, AdVisie ‘The mushroom cultivation advisors’