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I'd just add a thin layer no more than once a week. Myself, probably once every two weeks. And cover.

That's just I how I do things. I'm not building a bin all at once. I'm building as I go on a weekly or every other weekly basis.

Wood ash isn't going to go bad on you so add a little and if that doesn't seem to zap things add some more. If you see something like the process slowing down, back off a bit. Your ash will still be there a week later or two months later.

I'm guessing but I'd figure a 5 gallon pail at once would be too much and a quarter pail won't cause a major disaster.

The worst case scenario is the pile slows down a bit. Not a major disaster.

I'm not the paranoid type so if you're worried, wait for the others to chime in.

to sense
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From what I've learned recently, adding ashes to soil in San Francisco are can be disastrous. The soil pH is high.. Adding ashes makes it even higher.

You'd be be wise to phone your county's University of CA Extension Service office and learn the facts for where you live.

Here's a clickable map from USDA where you can locate the address & phone number of your county office.
http://www.csrees.usda.gov/Extension/

Here is a link that might be useful: USDA map to Extension Service offices.