[saalweachter]

Note the step of adding it to your compost, not to your soil directly.

The charcoal acts like a sponge for water soluble nutrients in the soil; if you add it to the soil directly, it will suck them out of the soil first, temporarily depleting it. Long term it acts as a reservoir, but you want it to go into your garden full.

[euroderf]

Okay, so "fully loaded" biochar sounds like a time-release nutrient source.