Ashes. Here is a
bulletin from the University of Maine that gives a pretty good rundown of all the nutrients in
wood ash. It will help you with your pH, potassium, and phosphorous problems. The only thing is, you want to apply it sparingly and often as (1) you don't want to burn plant
roots and (2) these things can leach out pretty easily. If there are little bits of unburned charcoal still in the ash, that's even better, as that
biochar will also help build the soil. I like to mix the ashes with a lot of
water and apply it that way. Unfortunately, with all the solid bits in it you can't really use a sprayer, so you have to go lo-tech with a large
bucket and a small bailing can.
If there is a park near you with cookout grills, take your trowel and bucket and help keep them tidy. Clean them out, even the half burned charcoal briquettes that might be in there. Briquettes fresh from the bag are not good biochar to be adding to your garden because of the binders and "easy-light" chemicals that are added, but after they have been set on fire once, the "easy-light" substances are the first to go and what is left is perfectly fine to add to the garden.