Firstly ‘RAW’ Biochar is unlikely to produce positive results as it is like an inert sponge and it will soak up nutrients. So it is advisable to precharge the Biochar before adding to the soil.
There are many ways of carrying this out and in general this will depend on the individuals experience and preferences. It is possible to buy precharged Biochar inoculated with fungi, bacteria and organic nutrients. For the more experienced person soaking it in compost tea maybe one possibility or it can simply be mixed through good garden compost and leaving to stand for a few days.
Rates of Application
In general adding 5% to 10% of Biochar to soil, by volume, is widely accepted as the desired amount; but if adding to open soil, 1kg to 5kg per M2 is acceptable. It is more desirable to add a little over a period of time.
Any type of biomass can be pyrolysised, but for soil improvement, using unprocessed biomass is preferable. In this context unprocessed refers to virgin wood (not chip board or old pallets that may have been chemically treated). Also, as the science is not yet fully complete, pyrolysising FYM might be best avoided. After all FYM is already good in its current form for soil improvement.