[csense]

> Crops aren't solely viable because of climate -- a large portion of crop-viability is due to soil quality, which is built up over many many years.

I live in a rather less urban area than most HN readers, but I actually don't know all that much about farming.

I'm fascinated about some of the implications of your comments.

Having seen potting soil and fertilizer for sale in greenhouses and garden centers, I've always assumed soil fertility is a portable, manufacturable commodity. That is, if you want to start a farm in a place with the correct climate but unsuitable soil, you can simply buy healthy soil from somewhere else to get started, and then after a while your crops will establish themselves and create a somewhat self-sustaining ecosystem on your fields that only requires input of sunlight, water and nutrients that are commodity chemicals (nitrates and phosphates that can be mined or chemically manufactured).

Where does the bought soil come from? Established farms. Presumably crops have some capability to create soil from barren earth over time, so you can safely harvest and export 5% of your soil a year, or something. Or you can use dedicated compost piles which produce soil from crop / livestock waste. Perhaps occasionally importing rocky / sandy / clay soil to replenish the base mass.

You seem to disagree with this view; your comment instead implies that soil fertility is more fixed, not fungible. I find this interesting, and want to learn more.