Hi Nick, what is the phosphorus level in the report? Since you are encouraging soil microbes, you don't need as high level of phosphorus as suggested. Too much soluble P will discourage arbuscular mycorrhizae association instead. My soil test a few years ago came back at 17 lb/Ac (8.5ppm) for M3 P2O5 and the same amount of recommendation (3 lbs/1000 sq ft). I have several free ranging chickens fed partially on commercial feeds and so far I haven't seen any typical P deficiency symptoms.
As for wood ash to add potassium, maybe you can look around and ask from people using wood stove. I've seen huge piles of wood ash outside car repair shops and the like. Those places are more likely to burn wood to heat up large spaces.
Also, I agree with Doug about micronutrients. Is there any sign some might be lacking?