I was looking around this site and discovered a story about how locally available (and free!) lave rock was used to remineralize completely farmed out land. Notice that the first step mentioned was a complete soil analysis. John J. Ruegg maintains his land with several different types of powdered lava rock. All of this is free to him and he uses a hand grinder to powder it himself.
Here is similar information from the government about remineralization. They give several examples of sources for remineralization. There are many other pages that talk about soil remineralization. Just click on the agenda link at the bottom of any of the above pages. The general idea is that remineralization is needed, but the sources can be easily attained. One such source could be the "local" rock quarry.
Offered by Roger.