Wood ash is high in potassium...so need to keep that balanced with phosphorus. It will also drive up ph quickly.
Biochar is created through a low-heat, airless burn, so not the same as charcoal.
Char needs to be charged with amendments, compost, etc., before being spread around crops.
http://seachar.org/wp-content/upload...activation.pdf
google biochar activation for more info.