Potassium hydroxide (the main active ingredient in wood ash) is used
to raise the ph of hydroponic solutions as plants need potassium.
Sodium hydroxide creates sodium compounds which are generally
considered bad for plants. Both hydroxides will rapidly convert your
skin and flesh into new chemical compounds if you put your hands into
very concentrated solutions of them. It is a fertlizer if it is
neutralized down to a low ph by the soil it is added to.