Keywords: agriculture, fertiliser, population, protein supply, energy efficiency, greenhouse gases, phosphorus recycling.

The development of agriculture from hunter-gatherer times to the present day is examined. The removal of the restriction on potential population growth resulting from the introduction of the use of mineral fertilisers and particularly the development of the Haber Bosch process as a cost-effective method of providing almost half the protein nitrogen required by mankind is discussed. It is acknowledged that the availability of a relatively unrestricted supply of carbohydrate and protein through the use of mineral fertiliser has enabled the recent and projected growth in the world population.

The use of energy in the production of fertilisers, particularly nitrogen, is analysed, together with associated emissions of the greenhouse gases carbon dioxide and nitrous oxide. Some historic and potential improvements in energy use efficiency and reductions in emissions are described.

The energy use and capture by agricultural activity is discussed. The essential use of fertilisers for the provision of the quantity of food required is established, and the necessary trade-offs are examined, acknowledging that capture and emission of greenhouse gases are an intrinsic part of natural biological cycles.

Concern is expressed over the long-term supply of the essential nutrient phosphorus, with analysis of opportunities for the minimisation of waste of this element.

Chris J Dawson, Westover, Ox Carr Lane, Strensall, York YO32 5TD, UK.

39 pages, 17 figures, 10 tables, 14 references.