The problems with manufactured nitrogen fertilisers

Isobel Tomlinson

01 November 2011

Too much nitrogen harms the environment and the economy was the key message of the recent European Nitrogen Assessment which reported a study carried out by 200 scientists investigating the unprecedented changes humans have made to the global nitrogen cycle over the last hundred years.1 Through industrial processes, the cultivation of crops and the burning of fossil fuels, the supply of reactive nitrogen into the global environment has doubled. This nitrogen is very mobile and can be lost to the environment, causing water and air pollution, and contributing to greenhouse gas (GHG) emissions. The biggest source of reactive nitrogen is from the industrial manufacture of nitrogen fertilisers for agriculture using the Haber-Bosch process. The discovery that ammonia (NH3) could be synthesized by taking nitrogen from the air (N2) and reacting it with hydrogen in the presence of iron at high pressures and temperatures, was first made by Fritz Haber, who was awarded the Nobel Prize in Chemistry in 1919. The process was developed on an industrial scale by Carl Bosch who was awarded a Nobel Prize in 1931.

The introduction of manufactured fertilisers to farms around the world during the 20th Century has led to a profound transformation of agriculture, as farming systems obtaining ‘biologically-fixed’ nitrogen from legumes such as clover have become less common and animal waste is no longer seen as a key source of nutrients, but as a waste to be disposed of. Manufactured fertilisers have contributed to the intensification of agriculture, and played a key role in increasing crop yields over the last fifty years, albeit at a decreasing output per tonne of nitrogen applied in many parts of the world.
Today, it is estimated that nearly 50% of the world’s population eat food produced with manufactured nitrogen. The growing consumption of intensively produced meat and dairy products, alongside the expansion of biofuels, has further increased the global use of manufactured nitrogen, and if these trends were to continue, its use is estimated to double or almost triple by 2050.2

Worrying dependency

Our dependency on manufactured nitrogen for our food supply is, however, deeply worrying. Manufactured nitrogen fertilizer production is reliant on non-renewable fossil fuels, mainly natural gas, as a source of hydrogen and as a fuel for the chemical process. Like oil, at some point in the future the supply of natural gas will ‘peak’, if it has not already done so. What this will mean for manufactured nitrogen fertiliser is an issue that urgently requires attention. This may have an impact on food prices and poses a long-term threat to food security. Price volatility for manufactured fertilisers has already become a reality for farmers in the UK. The price of fertilisers is determined by a range of factors, but the price of natural gas is a key one.

The production of manufactured fertilisers is very energy intensive and global scale fertiliser production uses approximately 1.2% of the world’s energy.3 On a farm scale, manufactured fertilisers are estimated to account for 68% of onfarm commercial energy use in the Global South, and 40% in the Global North.4 When more nitrogen is put on farmland than is taken up by the growing crop, a nitrogen surplus is created. This can occur because too much nitrogen is being added, or it is added at the wrong time, or in the wrong form, for it to be taken up by growing crops. This can lead to environmental problems as surplus nitrogen may be lost to the environment. Nitrate leaching is a problem because nitrate is soluble and does not bind to soil surfaces making it susceptible to loss in drainage. Nitrate leaching can lead to eutrophication and acidification in fresh waters, estuaries and coastal zones which can lead to biodiversity loss and toxic algal blooms. The production, transportation, storage and use of manufactured fertiliser contribute to GHG emissions. In terms of manufactured nitrogen use, of particular concern are emissions of nitrous oxide (N2O), a powerful GHG, with a global warming potential of 298 times that of carbon dioxide. N2O makes up 54% of the UK agricultural sector’s GHG emissions.5

Of course, all agricultural systems need a supply of nitrogen to replenish that lost when crops are harvested, and some loss of nitrogen is inevitable. So how should we best deal with the environmental consequences, N2O emissions and future food insecurity caused by our century-long love affair with manufactured nitrogen?

What action is being taken?

A variety of technological developments, policy initiatives and changes in farming practices are being suggested as ways of tackling the nitrate pollution, energy use and GHG emissions arising from manufactured nitrogen production and use. Reducing the amount of energy used and GHG produced in the manufacture of fertilisers is possible through installing best available technology in new plants and upgrading existing plants. However, representatives of the global fertilser industry, admit that it will take decades to get new technology up and running and the widespread revamps of older plants.6

Theoretically, it is of course possible to make manufactured fertilisers using energy and hydrogen derived from non-fossil fuels, such as nuclear, wind and solar, and such developments are anticipated in the future. There are some test plants, such as a wind-powered plant at the University of Minnesota. However, currently, there are no public signs that the fertiliser industry is considering renewable sources in its policy on tackling GHG emissions, and available global statistics show no sign of the development of any such plants currently being used for ammonia production.

It has been suggested that non-legume crops such as wheat could be genetically-engineered to fix their own nitrogen. However, a crop’s ability to fix nitrogen is not determined by just one gene but appears to rely on a complex relationship between soil bacterium and the crop, making genetic modification difficult or impossible.7

Under European legislation, the EC Nitrates Directive, land that drains into fresh or ground waters polluted by nitrates has to be designated as a Nitrate Vulnerable Zone (NVZ) and farmers with land in NVZs have to follow mandatory rules that limit application of nitrogen and modify timings. This has had variable results. Nitrification inhibitors have the potential to reduce emissions of N2O. They have not been widely used in the UK, but are now being considered as a way to reduce N2O emissions from animal urine and dung, but also from manufactured fertilisers. However, they are expensive and significant reductions in manufactured nitrogen use would be needed to make them cost-effective.8

The most commonly recommended ‘cost-effective’ solution to reducing the climate change impact of manufactured fertiliser use focuses on increasing nitrogen use efficiency in crop and animal production. However, the UK Government’s Committee on Climate Change has said that more efficient use of nitrogen fertilisers, as well as other minor changes in management practices, will not be enough for the agricultural sector to meet its share of emissions reductions under the Climate Change Act, and that post-2030, more radical measures should be considered.9

The need for a different agricultural system

These suggestions for reducing nitrogen losses and the GHG impact of our dependence on manufactured fertilisers clearly have either economic limitations or technological impracticalities, will have only a limited impact on the problem, or still leave agriculture reliant on scarce fossil fuels, in which case, the price of food is likely to continue to increase as fossil fuels get scarcer and more expensive. Instead, as the Climate Change Committee has said, more radical changes are needed. The Soil Association believes the solution lies in changing how we farm towards agricultural systems that do not need manufactured nitrogen fertilisers, but use nitrogen-fixing crops, such as legumes.

Over 78% of the atmosphere is composed of nitrogen, but in this gaseous form (N2), it is not usable by plants. Some plants, such as legumes, can ‘fix’ N2 from the atmosphere themselves. They do this by forming a symbiotic relationship with nitrogen-fixing bacteria (Rhizobium genus) that are found in the soil and form nodules on the roots of the plants. In exchange for fixed nitrogen from the bacteria, the plants provide the bacteria with energy (via photosynthesis) and other nutrients. Nitrogen fixing crops are well known as sources of nitrogen for agriculture and legumes are generally used in organic agriculture as a source of nitrogen for the farming system.

As an agricultural system that does not allow the use of manufactured nitrogen, but makes use of legumes to ‘biologically-fix’ nitrogen, we have reviewed the current evidence on the extent to which organic systems can meet this double challenge of reducing nitrogen pollution, especially N2O emissions, and building stores of soil organic nitrogen.

How well do organic systems perform?

‘Biologically-fixed’ nitrogen can also cause unnecessary N2O emissions and pollution if not managed properly, as can animal wastes that return nitrogen to the soil. Therefore, correctly timed farming practices are required to minimise the amount of newly-fixed nitrogen needed as an input in the first place, and prevent nitrogen being lost to the environment through ensuring that the release of nitrogen synchronises with demand for nitrogen by crops.

Making the most efficient use of limited nitrogen inputs will become a key driver for agricultural systems in the future. Research has found smaller nitrogen surpluses on organic farms than non-organic, due to the ban on manufactured fertilisers and limited livestock densities. Research published in the journal Science found that nutrient input including nitrogen in the organic systems to be 34-51% lower than in non-organic systems, whereas mean crop yield was only 20% lower over a period of 21 years.10 Scientific evidence shows that the lower nitrogen inputs in organic farming systems can lead to lower N2O emissions compared to non-organic farms on an area basis, although research comparing the N2O emissions from the two farming systems is limited. Most of the existing studies do not include the GHG emissions from producing manufactured nitrogen that is used on non-organic farms. Potential N2O hotspots on organic farms include the decomposition of manure and the incorporation of legumes.

It is argued by some scientists that biologically-fixed and manufactured nitrogen are indistinguishable once in organic form and equally become potential sources for N2O emissions and other nitrogen losses.11 However, there is now evidence that farming systems using these different types of nitrogen do function differently in terms of nitrogen retention and loss.

Recent research from the University of Illinois challenges conventional wisdom by indicating that in some circumstances the use of manufactured nitrogen can cause the loss of soil organic matter by stimulating the activity of soil microorganisms.12 Where soils are not managed carefully with appropriate levels of organic matter inputs, this can reduce the ability of soils to store carbon, to hold water, as well as to store organic nitrogen and thus lead to higher nitrogen losses to the environment.

Research published in the journal Nature in 1998 found that the level of organic nitrogen in soils is not just controlled by the net input of nitrogen, but that the type of nitrogen is also important. On research plots, legume-based systems had a higher retention of nitrogen in the soil in the long-term and less nitrate leaching, than the system using the same quantity of nitrogen from manufactured fertilisers.13

More research urgently needed

Whilst there are some known benefits of the organic system to reducing nitrogen surpluses, nitrate leaching, and N2O emissions, as outlined here, it is clear that specific understanding of how nitrogen behaves within legume-based systems is currently limited. Given the imperative to reduce our reliance on manufactured nitrogen and improve the efficiency of nitrogen use, we are asking the Government to fund further research into these issues.

First of all, we are asking that the likelihood that legume-based organic systems and those using manufactured nitrogen behave differently in terms of nitrogen cycling, retention and loss, be further investigated. There is an urgent need to understand the consequence of this for long-term soil fertility, reducing GHG emissions and storing carbon in soil.

Second, we would like to see more research that looks in detail at N2O emissions from organic systems to bring scientific understanding to the same level as willbe provided by the Government’s new ‘InvenN2Ory’ project that is measuring N2O emissions from non-organic farming practices.

Third, we are asking the Government to fund research into best practice for organic farms and other agro-ecological farming systems on how N2O emissions and other nitrogen losses can be minimised. Research is also needed into innovative methods already being trialled on organic farms. For example, alternatives to ploughing in legumes such as crops direct-drilled into clover, the use of perennially-based cropping systems and agroforestry.
Fourth, the European Nitrogen Assessment called for a lowering of the human consumption of animal protein as a way of also tackling nitrogen excesses. Research into the impact of nitrogen use and pollution as a consequence of a shift in diets in the UK to lower consumption of meat and dairy products, especially from animals fed on grain rather than grass, should be commissioned to accompany existing evidence of the climate change and health benefits.

Finally, using clover on grassland to fix new nitrogen, rather than manufactured nitrogen is a practice that can be readily adopted by non-organic farmers. The use of winter cover crops by those growing spring sown crops should also be encouraged. The Government should provide financial incentives to help farmers implement such measures, and these could be included as part of the ‘greening’ of Pillar 1 of the Common Agriculture Policy.

Isobel Tomlinson is a policy and campaigns officer at the Soil Association

This article was first published in Mother Earth, the Soil Association's journal of organic thought and policy. We hope you enjoyed this article, please feel free to share this with your contacts. If you wish to support the production of Mother Earth in future, and receive the latest issue direct to your door, then please subscribe to Mother Earth, for just £12 a year.


1 Sutton, M., et al (eds) (2011) European Nitrogen Assessment: Sources, Effects and Policy Perspectives, Cambridge University Press, Cambridge.

2 Gomiero, T., Pimentel, D., and Paoletti, M.G. (2011) Environmental Impact of Different Agricultural Management Practices: Conventional vs. Organic Agriculture, Critical Reviews in Plant Sciences, p.102.

3 Jenssen, T.K. and Kongshaug, G. (2003) Energy Consumption and Greenhouse Gas Emissions in Fertiliser Production, Paper presented to The International Fertiliser Society at a meeting in London, on 3rd April 2003.

4 Crews, T.E. and Peoples, M.B. (2004) Legume versus fertiliser sources of nitrogen: ecological tradeoff and human needs, Agriculture, Ecosystems and Environment, 102, p.279-297.

5 Committee on Climate Change (2010) The Fourth Carbon Budget: Reducing emissions through the 2020s, December 2010.

6 International Industry Fertiliser Association (2009) Fertilisers, Climate Change and Enhancing Agricultural Productivity Sustainably.

7 FOE Europe (2010) GM crops: A false solution to climate change.

8 Moran, D., et al (2008) UK Marginal Abatement Cost Curves for the Agriculture and Land Use, Land Use Change and Forestry Sectors out to 2022, with Qualitative Analysis to 2050. Final report to the Committee on Climate Change, Edinburgh, SAC Commercial Ltd.

9 Committee on Climate Change (2010) The Fourth Carbon Budget: Reducing emissions through the 2020s, December 2010.

10 Mader, P., et al (2002) Soil Fertility and Biodiversity in Organic Farming, Science, 296, p.1694- 1697.

11 Bouwman, A.F, Stehfest, E., and van Kessel, C. (2010) Nitrous Oxide Emissions from the Nitrogen Cycle in Arable Agriculture: Estimation and Mitigation, in Smith, K. (ed) (2010) Nitrous Oxide and Climate Change, Earthscan, London.

12 Mulvaney, R.L., Khan, S.A., and Ellsworth, T.R. (2009) Synthetic Nitrogen Fertilisers Deplete Soil Nitrogen: A Global Dilemma for Sustainable Cereal Production, Journal of Environmental Quality, 38, November- December 2009, p.2295-2314; Khan, S.A., et al (2007) The myth of nitrogen fertilisation for soil carbon sequestration, Journal of Environmental Quality, 36, Nov-Dec 2007, p.1821-1832.

13 Drinkwater, L.E., Wagoner, P., and Sarrantonio, M. (1998) Legume-based cropping systems have reduced carbon and nitrogen losses, Nature, 396, 19th Nov 1998, p.262-265.


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