As the major part of soil P is tightly sorbed to mineral particles, bound within organic matter or precipitated as poorly soluble salts, it is mainly exported from soils to water bodies via erosion by water.

The most crucial potential P loss (P Loss) from ecosystems is loss from the soil due to soil erosion by water 2, and this often happens in sudden events 15. ton in 1961 to $700 per ton in 2015 (with large year-to-year fluctuations) 13, 14. The growing demand for P fertilizer globally has caused an increase in the cost of rock phosphate from about $80 per U.S. The following consequences have been instances of farmers riots and death due to severe national shortage of fertilizers in countries such as India, which are totally dependent on phosphate imports 3, 12. In parallel to the 2007-2008 global food crisis, phosphate rock and fertilizer demand exceeded supply, and prices increased by 400% within a 14-month period 11 demonstrating the sensitivity of this market. Ulrich and Frossard 10 argue that the main problem is not the geological P availability, but rather socio-economic (e.g., fertilizer access) or environmental (e.g., water pollution) vulnerabilities, resulting from current and future P production and consumption patterns. Recent literature is controversial as to whether or not P supply from rock reserves in the next decades will be a real physical scarcity 5 or will be limited by economic and technical constraints 6, 7, 9.

However, the socio economic as well as political consequences are still dramatic with the newly discovered P reserves being restricted to a small region of the Western Sahara and Morocco. The imminent threat of such a P limitation has been restrained somewhat as obviously some P deposits had been overlooked or misclassified in the past which will theoretically last for the next 600 years of global P supply 8. The potential threats of global P limitation due to peak phosphorus have been discussed intensively in the recent past 3, 4, 5, 6, 7. The one-way flow of P from mineral reserves to farms (e.g., soils), to freshwaters and finally into oceans, are already considered to be beyond the safe operating space for sustainable human development 2. However, if fertilization with animal waste or human excreta is not available or not organized, P fertilizers stem from non-renewable geological P deposits, which are an increasingly limited resource (this is again in contrast to N, as N fertilizer can be produced as an endless resource via the Haber Bosch process as along as energy and natural gas is available). In natural ecosystems the P that is lost from the soil-plant cycling system has to be replaced by the slow process of rock weathering 1 or added via fertilizer in human managed systems (there is no equivalent to the biological N 2 fixation which is only kinetically limited but potentially not resource limited). Phosphorus (P) being a key element in DNA, RNA as well as ATP and phospholipids is essential for the growth, functioning and reproduction of all life on earth.

Endless space 2 depletion points