As Used by Monty Don follow the link to his article in the guardian gardening section.
The mushroom compost is low in soluble nitrogen, which means there is little risk of plant burn, the total nitrogen is present in such a form that it will be released over time as further decomposition takes place. Phosphate, potassium. calcium and magnesium levels are quite high. Salinity is well below the levels found in conventional manure, the micro element analysis is good for copper, high for zinc manganese and iron and slightly low for boron.