Farmers can and do “mine the soil”, you can look it up>>>> http://www.google.com/search?num=100&hl=en&client=firefox-a&hs=W5q&rls=org.mozilla%3Aen-US%3Aofficial&q=%22mining+the+soil%22&aq=f&aqi=&aql=&oq=
Your soil is a bank. Not all farming is renewable. It can be done so you eat into your capital. The historical record is full of civilizations that have done this and ......
Yes, I know...the current farming practices are destroying the potential of our future, especially in the Mississippi basin. But there are ways of replenishing agricultural conditions and output, without even considering mineral mining for fertilizer.