It seems to me you would not want to apply any more than a thin dusting if you don't want to kill what is already growing. You could probably get away with more for prepping an empty field or garden bed.
I wonder if you could "dilute" by mixing a small amount in water -- but keeping in mind that water percolated through ash is how they make LYE <-- VERY CAUSTIC.
In one of Emilia Hazelip videos they showed sprinkling ash using tea strainer or powdered sugar shaker in the bottom of the trench for planting peas. If I remember correctly, trench, manure, sprinkling of ash, then cover with inch of soil, THEN sow peas, then cover the seeds with soil and tamp down.
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