Answer: Well aged horse manure (and stable bedding) can be worked into the soil and will provide some nitrogen as well as some organic matter. The actual analysis would depend on what the horse had eaten and how it had been stored and for how long it had been stored, but it does add some nitrogen. Fresh manure however is very hot with a high nitrogen content and should be used only very sparingly as a side dressing kept well away from tender young plants. In my experience, if you have fresh manure it is better used as a source of nitrogen for a home composting program.
Wood ashes can provide some phosphorous as well as a good amount of potash (a typical analysis would be 0 - 1.5 - 7) but this depends on the type of wood. Ashes are difficult to use in quantity because they can also affect the soil pH. In addition, wood ashes tend to leach very quickly through the soil. Usually, a mix would include both organic and mineral sources of potash, either granite dust or greensand.
In terms of creating a mix, you would need to test your soil first and see what it is lacking (and possibly what it already has in abundance) and work from there. In practical terms I would expect these to be applied individually rather than together as a mix because the application techniques are so different. Your County Extension (270-2781) would be able to help you with the soil tests and interpreting the results based on what you want to grow.
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