Ashes, no. You want carbon. Not ash. Here's one way how to make carbon from wood.
Here's another...
I begin a good burn pile, light at center then just cover it in dirt. Low oxygen after a steady burn is what you want. Or buy cheap bags of natural charcoal from Lazzari (Google it) or similar carbon charcoal makers.
Sounds like you are on it as for the rest. Remember that good gardening is not just knowledge, but careful observation as well. Even record keeping can be helpful, tho' I am not inclined that way. But I DO spend PLENTY of time just watching things. The minute interactions of plant and insect or animal. It's so revealing.
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