Larsen, A., et al. March 1, 2001. Re-engineering the toilet for sustainable wastewater management. Environmental Science and Technology Vol 35(9):192-197 

http://www.internal.eawag.ch/~maurer/abstracts/09larsen.html

Current fertilizer production and use consume limited resources and harm the environment. At current extraction rates, reserves of phosphate rock that are economically recoverable with today’s technology will last less than 100 years. (http://minerals.usgs.gov/minerals/pubs/commodity).

In addition to resource limits, phosphate rock has a high heavy metal content, giving rise to hazardous wastes when processed. The cadmium content of phosphate rock, for example, ranges from 0.1 to 850 mg cadmium per kilogram phosphorus. Because these impurities are not entirely removed from the final product, phosphate fertilizer application introduces heavy metals, such as cadmium, which is very toxic, into the soil. This problem will worsen if rock of lesser quality is used in the future as the resource is expended.

There are also impacts associated with hauling raw materials long distances to where they are needed, as well as after their consumption, when nutrients are discharged into lakes, rivers, and oceans, where they cause pollution and are largely unavailable for use in agriculture.