Charcoal is a hot topic, simmering nicely, intimate with global warming issues! So last autumn I made three beds, two with charcoal (one on top and one with charcoal incorporated) and one bed with manure instead of charcoal.
At first this spring there was little difference in growth but now the bed with no charcoal is looking more abundant. The charcoal beds are fine and similar in growth, but with smaller plants. It looks like the charcoal has no especial store of nutrients itself but I think it is more a long term idea, that it conserves nutrinents which may otherwise leach, though there is no particular evidence for this, more a surmise from the Terra Preta soils of the Amazon.
It looks to me like adding charcoal is not like adding manure and compost, it is more about soil structure, air and root development. Worth using for long term soil quality.