Fertilisers: Enriching the world's soilone of the biggest drivers of the Anthropocene-the age of man-is muck. More precisely, artificial muck, in the form of fertiliser. It's used to feed half of the world's people, but in order to provide food for the constantly escalating number of mouths we are running the risk of irreversibly damaging the planet. Directly or indirectly, plants are the making of us. We rely on plants because we can't metabolise the nitrogen that makes up four-fifths of the air we breathe. Nitrogen is a vital constituent of all proteins as well as other important molecules including DNA, but we can only use nitrogen once it has been broken down and combined into an  organic  molecule, such as an amino  acid, for example. So for thousands of years, humans have come up with ingenious ways of replacing the nitrogen oe and other essential nutrients such as phosphorous oe they have taken from the soil. Farmers left stalks and silage in the fields to rot down, and added whatever other organic material they could, including animal and human excrement. As populations grew in Europe and the US, nineteenth-century scientists found precious useable nitrogen supplies in South america, in the form of vast quantities of guano oe bird droppings oe which indigenous people had been using for centuries as a soil enricher. This discovery, and of the nearby saltpetre (potassium nitrate) mines, generated enormous interest in Europe and the United states. Trainlines were constructed at great expense through the desert to export the valuable materials, and the War of the Pacific kicked off between the guano-and saltpetre-rich nations of Peru, Bolivia and Chile. Britain supported Chile, enabling it to win most of the guano and saltpetre area and Bolivia's entire coastline in the process. Come the turn of the last century though, the need for guano was replaced by a revolutionary idea. The German chemist Fritz Haber invented a way of converting the nitrogen in air into liquid ammonia (NH3. The era of artificial fertilisers was born. The effect on crop production and hence population growth was immediate. The number of humans that could be fed from 1 hectare of land (2. 47 acres) rose from 1. 9 to 4. 3. Half of the protein in our bodies now comes from ammonia made in the Haber process. Unfortunately, the same reaction also leads to the production of powerful explosives, which have been responsible for the deaths of some 150 million people.)Billions of people owe their daily bread, rice or potatoes to artificial fertilisers. And fertilisers formed the backbone of the Green revolution across Asia and South america, which dramatically improved yields and has lifted millions out of starvation over the past 40 years. Global impactbut only 17%of the nitrogen used in fertilisers ends up in our food; the rest ends up in soils and water. And that's where the greatest problem lies, because nitrogen is also a superb fertiliser of algae and bacteria. Fertiliser pollution in lakes and the ocean causes massive blooms of algae, which use up the oxygen dissolved in the water, suffocating other species. The vast blooms of red or green algae cause dead zones for kilometres, with the associated stench. Ten times more nitrogen is used to produce food than humans consume as protein, and not all the nitrogen in the food we eat is used even by our bodies oe the excess enters the environment through human waste. Most people require only 2g (0. 07 ounces) of nitrogen a day but the average American consumes 13g (0. 46 ounces) daily, mainly in animal products, which are fed on fertilised crops. Overuse of fertilisers is a particular problem in China, where whole river systems are polluted and the soils degraded by intensive farming. But safeguards in Europe, including using only a small amount of fertiliser where needed, and providing catchment reed beds that filter out any runoff before it enters the river, have reduced greatly the problem there. However, local fertiliser use has a global impact. Producing fertilisers also pollutes the atmosphere with climate-altering greenhouse gases. The Haber reaction requires burning fossil fuels which emit carbon dioxide. And other potent greenhouse gases, including nitrous oxide, are released also while making or using fertiliser. Transporting fertiliser also releases significant carbon emissions, making it a dirty industry. So what's the answer? One school of thought advocates organic methods that use preindustrial methods of soil enrichment, such as muck-spreading, for example, They are useful, environmentally sensitive and cheap, if time-consuming ways of improving depleted soils across the world, but they have their disadvantages. Artificial fertilisers cannot replace lost organic matter (carbon) in soils oe mulch is a useful way of restoring structural integrity and preventing fertilisers from washing out of soil immediately, but it uses valuable animal fodder or cooking fuel. As with all fertilisers, manure runoff can also pollute rivers and oceans with eutrophication (death by oxygen starvation. Making manure releases its own potent greenhouse gas oe methane. Planting legumes that fix nitrogen can take up space in the field that many farmers would prefer to dedicate to cash crops. But, perhaps most damningly, in the world's poorest areas, the majority of farmers do not have a choice about whether to go organic or not oe most farmers are working with poor soils, bad seeds and uncertain water supplies. The truth is that it would be feed impossible to a growing global population using purely organic farming methods. And because organic farming is relatively inefficient (yields are on average 25%lower) compared to modern technological methods vast new tracts of land would need to be used, which would further impact our forests and other ecological spaces. Priceless subsidieswe're already seeing the effect of this Africa, where 75%of the continent's farmland is degraded. While Asia and Latin america farmers increased yields by increasing productivity of existing fields, in Africa, they have extended their cultivated land through slash and burn. As a result, grain yields in most of Africa are around 1 tonne per hectare, whereas South Asia achieves 2. 5 tonnes per hectare and East asia makes 4. 5 tonnes per hectare. With Africa's population set to double by 2050 to two billion this yield gap must be overcome. Which leads to the other main school of thought oe one that advocates industrial-scale methods with the efficiencies of synthetic tools. Africans use less than one-tenth of the global average of fertiliser use oe less than 10kg (22 lbs) per hectare compared to 140kg (309 lbs) per hectare in Latin america and South Asia. Very few African countries produce fertilisers, the majority are imported, and the cost of transportation is very high on the continent because of high fuel prices and terrible roads. As a result, African farmers pay as much as six times the global average price for fertiliser oe when they can find it at all. In the 1980s the IMF and World bank imposed conditions on development aid, preventing African governments from subsidising fertilisers. The cost of fertilisers have risen fourfold in the past few years because petrol prices have soared. Millions of African subsistence farmers survive on less than a dollar a day, so fertiliser is simply unaffordable. Without subsidies, farmers can't afford bad years, they plant less and are crippled by debts from loans just to buy seeds. After decades in which the population was continuously on the edge of famine, and following a disastrous harvest in 2005 when more than a third of the population needed emergency food aid, the Malawi President Bingu wa Mutharika said that enough was reintroduced enough, and fertiliser subsidies oe the first sub-Saharan country to do so. As a result, agricultural production more than doubled in 2006, and trebled by 2009 as fertiliser use doubled. By 2007, Malawi was exporting its surplus corn to Zimbabwe and Kenya, unthinkable just two years earlier. When I visited Malawi in 2010, the country had closed the grain gap from eight months (time during which the granary stores are empty) to two months, and some had two months'surplus. In going against the World bank Malawi created a production level that dragged its people out of starvation. The programme was curtailed in 2011, but a dozen countries are now following Malawi's lead. Balancing actthere is no doubt that African farmers need access to synthetic fertilisers if they are going to catch up with the rest of the world in crop production (that is, close the yield gap) and preserve their wild spaces from slash and burn. But a truly successful outcome for Africa, and the world in general, would be to see agricultural yield per hectare increased with minimal environmental impact and in a way that can be sustained as the climate becomes less conducive. So when I hear the passionate views about whether farming should be entirely organic and GM-free, or entirely industrial-scale with the efficiencies of synthetic tools, my response is that it will be all of these things. The answer is to use fertilisers in a far more efficient and better targeted way, so that just the right dose is given to each plant with minimal run off or waste oe a concept known as micro-dosing. And to eat less meat so that the protein in the crops we fertilise is used in the most efficient way. Meanwhile, traditional methods should be embraced. Farmers reap multiple benefits from planting nitrogen-fixing trees and hedgerows around their fields for example, while also sowing the most effective crops for the area, climate and soil type oe whether that be GM rice or conventionally bred cassava-rather than vast grain monocultures. With an enormous variety of crops, fields and farmers, there will be a variety of different practices used (including the investigation of wild-type relatives of staple crops), and we will need this variety in the coming decades. If you would like to comment on this article or anything else you have seen on Future, head over to our Facebook page or message us on Twitter

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