Fertiliser availability in a resource-limited world: Production and recycling of nitrogen and phosphorus

► Nitrogen fertiliser inputs to agriculture are essential for global production of sufficient food. ► Methane is the most efficient hydrogen feedstock and energy source for nitrogen fixation, and reserves should be protected in the absence of alternative hydrogen sources. ► Current known and potential reserves of phosphate rock are quantified. ► The requisite aggregate annual intake of phosphorus in food by the global population is estimated. ► It is essential that phosphorus is recycled to avoid exhaustion of reserves of this unsubstitutable nutrient.