More important than TV, apparently

Did inorganic nitrogen produce our 6 billion-strong globe, Jules Pretty asks.

Nitrogen is vital. It is all around us, as the most abundant element in the atmosphere, and in us, as a building block of proteins. But nitrogen in the air cannot be used by plants, and thence by animals, until it is chemically converted into nitrate and ammonium compounds. This happens naturally through the action of some bacteria, particularly those associated with the roots of legumes, and by lightning. But these processes are not common in natural ecosystems, and so nitrogen is generally a limiting element in plant growth. This is important for food production, as a growing world population has set continuing challenges for farmers to increase crop and animal productivity.

Modern agricultural methods have raised productivity spectacularly in industrialised countries - more cereals and animals per hectare, more meat and milk per animal, more food output per person employed. In the UK, wheat yields were largely unchanged between the 1880s and the 1940s, about 2-2.5 tonnes per hectare. But since then, there has been a rapid increase to reach an average of 8 tonnes per hectare. What has driven these changes? One factor has been the increased use of inorganic nitrogen fertiliser, produced by the Haber-Bosch process. In the late 1940s, about 1 million tonnes were applied to agricultural fields worldwide - now that has grown to more than 80 million tonnes.

The story begins with an account of how the German chemist Fritz Haber discovered how to synthesise liquid ammonia, and then how Carl Bosch, working for the chemical company BASF, commercialised this invention. The author, agricultural historian Vaclav Smil, gives us a detailed narrative on a century of struggle to develop the process, and then takes us through the chance discoveries that led to the invention in 1908. He also makes an extraordinary claim: "Industrial synthesis of ammonia from nitrogen and hydrogen has been of greater fundamental importance to the modern world than the invention of the airplane, nuclear energy, space flight or television." Even worse, and I suspect publisher's hyperbole: "The expansion of the world's population from 1.6 billion in 1900 to 6 billion would not have been possible without the synthesis of ammonia." We will never know, and it could equally be argued that a lack of such an industrial process may have prompted scientific endeavour and innovation in other areas of agriculture, such as in improvements in the capacities of legumes to fix nitrogen from the air for free.

However, Smil presents a balanced story, as the latter half of the book discusses the problems arising from using too much nitrogen. About half the inorganic nitrogen in fertiliser applied to crops is not used, and so is lost to the environment. Some leaches into ground and surface water, where it can provoke excessive plant growth. In the worst circumstances, such growth can remove all the oxygen from the water, killing plants and animals. Off the south coast of the US, the so-called dead zone in the Gulf of Mexico is expanding every year as nitrogen continues to be brought to the sea from farmers' fields by the Mississippi River. Nitrogen is also emitted into the atmosphere in the form of nitrous oxide, a potent greenhouse gas. The manufacture of nitrogen itself is highly energy intensive - each kilogramme of nitrogen applied to a crop indirectly results in the emission of 1-1.5kg of carbon, again contributing to climate change. High levels of nitrate in water can also be a health hazard to infants, playing a role in blue-baby syndrome when water is also heavily contaminated with bacteria.

An important question not addressed in this book is: what are the alternatives to Haber-Bosch and the use of inorganic nitrogen? The conventional wisdom is that redoubled efforts are needed to industrialise agriculture to increase food supply to meet the challenge of persistent hunger. But there are doubts about the capacity of such systems to produce the food where the poor and hungry people live. They need low-cost and readily available technologies and practices to increase food production.

A more sustainable agriculture therefore seeks to make the best use of nature's goods and services. It does this by integrating regenerative processes (such as nutrient cycling and natural enemies of pests) into food production. It minimises the use of inputs that damage the environment or harm human health, and builds on the skills of farmers. There is growing evidence that such agricultural systems can be economically, environmentally and socially sustainable, and that they can increase productivity.

Haber earned a Nobel prize in 1918 for his invention - but to stop there is to omit a dark side to his character. He also pioneered the use of chlorine gas as a weapon in the first world war. During the first gas attack in Ypres in April 1915, he oversaw the release of 168 tonnes of chlorine gas, which resulted in 5,000 French fatalities in a single day. With the announcement of his Nobel prize came the popular appellation that he was "the inventor of gas war".

Jules Pretty is professor of environment and society, University of Essex.