► 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. Without the input of fertiliser nitrogen it is estimated that only about half of the current global population can be supplied with sufficient food energy and protein. The anticipated increase in the population to 2050 will increase the dependency on fertiliser inputs. The paper examines the different potential sources of energy and hydrogen required for this essential fixation of atmospheric nitrogen into plant-available nitrogenous fertiliser and concludes that methane from natural gas is clearly the most suitable source. In the absence of a cost-effective alternative source of hydrogen it is recommended that an on-going requirement for methane is acknowledged and that consideration be given to strategic reserves for the production of food. Phosphorus is also an essential and unsubstitutable nutrient for plants and animals, but while the global reserves of atmospheric nitrogen are effectively unlimited, the reserves of phosphate rock are finite. Recent estimates of the reserve suggest that at the current rate of use this resource will become exhausted within some hundreds of years. The annual increment of phosphorus contained in the human population is estimated to be in the order of 1 Mt/yr, which is a small proportion of the quantity mined. There is a clear requirement to ensure that phosphorus is recycled to a large extent, so that the rate of exhaustion of the reserves of phosphate rock is significantly reduced. Legislation relating to the management of phosphorus appears entirely associated with its potential to upset natural ecosystems, with apparently no regulations yet requiring the efficient use and reuse of a scarce resource.