Part of soil depletion;
"Soil helps regulate the carbon and water cycles — it’s a reservoir for both cycles, buffering them from shocks and feeding us, all at the same time. But, Amundson et al. warn:
Profound changes are on the horizon for these interconnected functions — particularly sparked by changes to climate and food production — that will likely reverberate through society this century. Ultimately, the way in which we directly and indirectly manage our planet’s soil will be interwoven within our future success as a species.
We are already running into a hard limit when it comes to soil nutrients. Plants need nutrients like phosphorus, nitrogen, and potassium to grow. Microbes, certain plants, and human factories can pull nitrogen out of the air (there’s plenty of it in the atmosphere), but the other nutrients have to come either from mining or recycling.
For a lot of reasons (contamination with prescription drugs, heavy metals, and pathogens in the sewage system) people dislike the idea of turning municipal sewage into fertilizer.
Those flushed nutrients never leave the system in the largest sense, of course: They end up in lakes and oceans and landfills. Phosphorus in the ocean can turn into an algal bloom, which turns into fish, which birds eat and poop out, which we mine for fertilizer. But that cycle takes place far too slowly to meet the needs of hungry humanity.
The only other option is to mine those nutrients, and we are running out:
The growing demand for P [phosphorus] has recently caused an increase in the cost of rock phosphate from about $80 per U.S. ton in 1961 to up to $450 per ton in 2008. Prices since then have fluctuated but are now at about $700 per ton … K [potassium] prices were ~$875 per metric ton in 2009 yet are expected to reach $1500 by 2020.
And the authors point out that these elements are unevenly distributed. The biggest phosphorus mine in the U.S. will be depleted in 20 years, and geopolitical balance of power may get shaken up as nations and corporations begin competing for the remaining reserves in places like Morocco. Oil wars are one thing; at least you can replace oil with other forms of energy. But it’s physically impossible to replace a basic element like P or K."
http://grist.org/food/the-next-big-war- ... gn=climate