Here’s a great story from Rodale’s New Farm about soil fertility. I think this is a really useful way to think about soil. When he says that soil is not a hydroponic medium, what he means is that it isn’t some inert substance designed to hold and deliver the nutrients you inject into it. It’s a place where nutrients are constantly digested, recycled, reused, lost, gained. In other words, it’s dynamic and constantly changing.
“Soil is not a hydroponic medium. You can’t spoon feed fertility to roots, even if you apply all the fertilizer a corn crop needs. In a corn-soybean rotation on most soils, if you put on 15C pounds of N, less than 50 percent is likely to end up in the corn plant. It can be as little as 10 percent. The N that does reach the plant may come from last year’s fertilizer or from the last decade’s. Adding new fertilizer may cause more N uptake, but even that is mostly ‘old’ nitrogen that enters the plant because the nitrogen cycling system kicked into a higher gear.
It’s like a biological banking system. The dollars you get out are not the same dollars that you deposited. “