Fertilisers are applied to supply nutrients essential for plant growth, which have been removed through production and harvesting of crops or grass.
The use of fertilisers
Much modern agriculture is reliant on inorganic fertilisers, with one estimate being that almost half of the global population are fed as a result of synthetic nitrogen fertiliser inputs.*
Manufactured nitrogen fertilisers are not allowed on organic farms so essential nutrients and minerals are provided by processes such as applying manures, composts, green manures and using legumes in rotations which can ‘fix’ nitrogen from the atmosphere.
These measures have the benefits of reducing reliance on manufactured fertilisers and the addition of organic matter plays an important role in building and maintenance of soil structure. However, manures and green manures including legumes are potential sources of nitrous oxide emissions and other nitrogen losses so use of best practice is essential to minimise pollution.
* Erisman, J.W., Sutton, M.A., Galloway, J., Klimont, Z. & Winiwarter, W., 2008 How a century of ammonia synthesis changed the world. Nature Geoscience 1:636-639
Fertilisers in Europe
In Europe, manufactured fertiliser adds 11 million tonnes of reactive nitrogen annually to fields.
However, around half the nitrogen in fertilisers and manures is lost to the surrounding environment, threatening air, water and soil quality. Pollution from excessive nutrient enrichment is one of the major causes of environmental damage, with nitrate pollution being one of the five greatest threats to global biodiversity.*
In Europe, the environmental and economic costs of nitrogen problems are very high, with a major study (the European Nitrogen Assessment) estimating that nitrogen pollution is costing each person in Europe around £130-£650 a year. Indeed, the cost of this damage is more than double the value that nitrogen fertilisers add to European farm income.**
The assessment (authored by 200 experts) found nitrogen pollution is also causing toxic algal blooms and dead zones in the sea, and around 80 per cent of European freshwaters exceed a nitrogen threshold for high risk to biodiversity.
Modern agriculture is dependent on phosphorus derived from phosphate rock, a non-renewable resource, which may be depleted in 50-100 years and for which production may peak around 2030. Furthermore, the production of fertilisers from rock phosphate is associated with environmental costs including carbon emissions, radioactive by-products and heavy metal pollutants.***
Eutrophication of waterways due to excess nutrients, including phosphorus, is having a major impact on biodiversity.
* Convention on Biological Diversity ,2010 Global Biodiversity Outlook 3.
** Sutton et al, 2011 The European Nitrogen Assessment
*** Cordell et al, 2009 The story of phosphorus: Global food security and food for thought Global Environmental Change 19 292–305
Fertilisers and grassland
On grassland, there can be a number of nutrient sources.
Inorganic fertiliser may be used in addition to farmyard manure or slurry, which may be deposited by grazing livestock or applied. Dung and urine patches from grazing animals can lead to nutrient pollution where stocking rates are high as nutrients are supplied at a rate much higher than the grass requirement.
Grassland on dairy farms tend to be intensively managed, typically receiving twice the rate of inorganic fertilisers used on grassland on the average beef or sheep farm (mean rates of c 120 kg/N/ha on dairy swards, though this is nearly half the amount used in the mid-1990s).*
There can also be issues with other nutrients causing pollution, although data is often lacking or difficult to access, for example, on phosphorus balance on more intensive livestock farms.
* Hopkins and Lobley, 2009 A scientific review of the impact of UK ruminant livestock on greenhouse gas emissions. University of Exeter CRPR Research Report 27.
To reduce pollution, more efficient use of fertilisers is needed but to make farming more sustainable. We also need to reduce reliance on manufacturer fertilisers, which are produced by an energy intensive process using natural gas, a non-renewable fossil fuel.
More widespread uptake of typical organic practices such as effective rotations, using legumes and incorporating green manures can cut the greenhouse gas emissions associated with fertiliser production, while providing other benefits.
The European Nitrogen Assessment also highlighted the need for less meat consumption as part of reducing nitrogen pollution. Livestock consume around 85 per cent of the nitrogen in crops harvested or imported into the EU so European nitrogen use is not primarily an issue of food security but one of current patterns of consumption.*
Influencing diets is also needed as part of an integrated approach to addressing phosphate scarcity and pollution associated with its leakage, this is because livestock products require relatively much higher phosphorus inputs than vegetable-based products.**
* Sutton et al, 2011 Too much of a good thing, Nature 472 159-161 http://www.nature.com/nature/journal/v472/n7342/full/472159a.html
** Cordell et al, 2009 The story of phosphorus: Global food security and food for thought Global Environmental Change 19 (2009) 292–305