One 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 – and other essential nutrients such as phosphorous – 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 – bird droppings – 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.
But 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 even used by our bodies – 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.