Definitely NOT wood ashes. They are strongly alkaline. Aluminum sulfate will do it, but I don't like putting down aluminum compounds around food plants. Coffee grounds are mildly acidic. Pine needles even more mildy so eventually going neutral when fully composted. Elemental sulfur (wettable ag sulfur) works best, but it's slow. Iron sulfate (also known as 'copperas') can acidify soils even faster but be careful not too apply too much or you can burn your plants. I know some who have used sulfuric acid (new, unused battery acid) mixed into water to accomplish this still faster, but I can't recall what the acid to water ratio was and it's important. .....Alan.