Soil degradation and infertility are major constraints to sustainable agricultural production in many parts of the world.  Phosphorus deficiencies in particular are widespread with P being second only to N in terms of deficient nutrients in most soils (Syers et al., 2011). Phosphorus rarely occurs in highly concentrated forms in the earth’s crust. Therefore, P is a limiting nutrient to growth and production in marine, freshwater, and terrestrial ecosystems (Elser et al., 2007). Phosphorus  is an essential element in plant nutrition and plays several essential roles.  It is important for photosynthesis, seed and fruit formation, root growth and development (Mengel and Kirkby, 2001; Marschner, 2012). Therefore, P deficiency interferes with photosynthesis, protein synthesis, respiration and biomass production in plants and adversely affects crop productivity in large areas of the world (Plenet et al., 2000).

The  green  revolution and the related increases in crop production largely required a concurrent increase in the use of inorganic fertilizers of which P was a major component (Scholz and Wellmer, 2013). Phosphate rock (PR) is the main raw material for the production of commercial P fertilizers such as diammonium phosphate, triple superphosphate, monoammonium phosphate, and complex fertilizers including their mixes thereof (Khabarov and Obersteiner, 2018; Hellal et al., 2019). Minor sources of P are of organic origins such as manures, guano and human excreta. Modern agriculture also relies heavily on P addition to animal feeds (Cordell et al., 2009). Moreover, the production of biomass for biofuels also relies on P application, and thereby also tends to increase the global demand for P (Vos et al., 2011).

Phosphate rocks are finite, therefore none renewable through biological processes as in the case of nitrogen (Edixhoven et al., 2014). The major challenge facing humankind is therefore utilizing this limited resource in a way that promotes sustainable development to include intra- and intergenerational fairness (Mew et al., 2018). The likely depletion of PR is a serious matter, because the PR deposits took millions of years to form (Edixhove et al., 2014). Although the current geological P resources are still sufficient to meet the growing demand of the near future (Vos et al., 2011), just how much of the PR reserves are available for future use remains contentious (Baveye, 2015). In recent years, many researchers have claimed that world reserves of PR were getting depleted at an alarming rate while others have claimed that such apocalyptic forecasts were frequent in the past but have always been proven unfounded, making it likely that the same will be true in the future (Baveye, 2015). This paper reviews these contrasting viewpoints and the implications on the world’s food security status.

WORLD PHOSPHATE FERTILIZER USAGE

Due to increase in world human population over the years, the demand for more food to feed this population has continued to increase. Between the  years  2017  and 2050, world human population is expected to increase by about 29% (Table 1). In the year 2017, the world population was about 7.5 billion and is expected to rise to about 9.7 billion in the year 2050. However, the overall rate of growth will decline due to decrease in human fertility over the years to 2050 (World Population Forecast, 2017). To feed this population, increased fertilizer use by farmers will be needed to enhance food production. Up to 90% of worldwide phosphate production is utilized in agriculture in the form of feed and food additives, but mainly as phosphate fertilizers (Heckenmüller et al., 2014). The global use of fertilizers that contain N, P and K increased by 600% between the years 1950 and 2000 (International Fertilizer Industry Association, 2006). The world demand for P fertilizer is projected to increase from 41 151 000 to 45 858 000 tons between the years 2015 and 2020 (Table 2) (FAO, 2017). Demand for P fertilizer over the same period is expected to grow annually by 2.44% (FAO, 2017). Currently, a total of 200 to 265 million metric tons of marketable PR concentrate is mined (Geissle et al., 2018) and further processed to produce phosphorus fertilizer (83%) or industrial P (17%) (Prud’homme, 2016).

The major source of phosphorus for world food production is phosphate rocks. The demand for more food to feed the ever increasing human population has necessitated the use of more P fertilizers mainly the inorganic sources over the years. This has exerted increased pressure on the mining of PR deposits which are unfortunately finite. To facilitate proper planning of the utilization of PR sources, many attempts have been made to predict the quantities of existing PR deposits. However, these estimates are fraught with uncertainty as more deposits continue to be discovered. As a consequence, two diametrically opposed viewpoints on whether there is a looming crisis due to imminent PR depletion have emerged. The pessimists predict that the PR deposits will be depleted in the next few decades and that unless serious efforts are invested in the efficient utilization of the current deposits, the world could be faced with serious food and political crisis. There are others however who view such predictions as scaremongering with an aim of arm-twisting the donors to invest more in phosphorus research. They argue that the existing PR deposits can last for several hundreds of years. However, what is not in doubt among these view point is the finiteness of PR reserves. An approach that encourages society to adopt a sustainable utilization of phosphate resources should therefore be advocated.

The  authors  have  not  declared  any  conflict  of interests.