Phosphorus

The role of diet in phosphorus demand
Geneviève S Metson et al 2012 Environ. Res. Lett. 7 044043 doi:10.1088/1748-9326/7/4/044043

Phosphorus (P) is an element essential to modern industrial agriculture. As Science Daily reported:

Phosphorus is just as important to agriculture as water. But a lack of availability and accessibility of phosphorus is an emerging problem that threatens our capacity to feed the global population. Like nitrogen and potassium, it is a nutrient that plants take up from the soil and it is crucial to soil fertility and crop growth.
"Unless something is done, the scarcity of phosphorus will cause problems of a global dimension. As early as 2035 it is calculated that the demand for phosphorus map outpace the supply," says Dana Cordell, who presented her thesis at the Department of Thematic Studies -- Water and Environmental Studies, Linköping University, Sweden on the implications of phosphorus scarcity on global food security.
Phosphorus is extracted from phosphate rock, a non-renewable resource that is used almost exclusively in agriculture. Two thirds of the world's resources are in China, Morocco, and Western Sahara.
"The demand for phosphorus has increased and prices soared by 800 percent between 2006 and 2008," says Dana Cordell.

But, as Metson et al. point out, most of the drivers for phosphorus use are changes in diet.That the source-to-fork process is so inefficient that only one part in five of mined phosphorus makes it to our food doesn't help either.
But poultry litter is a good source of P. But the litter from industrial poultry operations, while high in P, is often to high in P. And the excess tends to wash off the fields and into local waterways, causing algal blooms and killing aquatic life  through oxygen depletion.We saw a lot of this in the '70s, and it lead to legislation to protect waterways--legislation that is currently under attack or fully repealed.
The current thinking, as reported on Science Daily again, is to wash litter and rapidly remove and recover phosphorus in solid form. The litter can then be used as a nitrogen fertilizer (washing doesn't remove the N apparently), or re-used as litter.
But all this presupposes industrial agriculture the way it is currently practised.  And that presupposes continued access to cheap hydrocarbon energy with no consequences from burning it. Sustainable farming has long used manure to fertilize and rebuild soil, and chickens walking about on a patch of ground don't tend to over-fertilize. The use of chicken tractors means that chickens are moved to fresh pasture before they denude and over-fertilize. But the focus remains on maintaining the status quo, and keeping financial power where it currently resides.