The explosion that rocked Beirut last month, killing at least 190, was a grim echo of previous accidents caused by the explosive synthetic fertiliser, ammonium nitrate.

Tianjin, China, 2015: 173 dead. West, Texas, 2013: 15 dead. Toulouse, France, 2001: 31 dead. Such incidents go back at least as far as 1947, when 581 people were killed in Texas City, Texas. The substance can also be used with destructive intent, as was the case with the anti-government terrorist Timothy McVeigh, who killed 168 people in Oklahoma City in 1995.

Yet ammonium nitrate’s deadly potential isn’t its most dire threat to human life. As a widely-used synthetic fertiliser, ammonium nitrate and its chemical cousins ammonium sulfate, sodium nitrate and potassium nitrate are significant contributors to climate change.

Chemical reactions

Its production is energy-intensive, requiring the burning of fossil fuels. After farmers apply these synthetic fertilisers to crops, chains of chemical reactions generate nitrous oxide, or N2O, a greenhouse gas. The International Fertilizer Association pegs the amount of anthropogenic GHG emissions for which the industry is responsible at 2.5 per cent, but all greenhouse gasses are not created equal. N2O has a far greater global warming potential than either methane or carbon dioxide.

“Even worse, nitrous oxide emissions are higher than previously thought, and going up faster than previously thought,” said Rona Thompson, senior scientist at the Norwegian Institute for Air Research.

‘We need to cut down on the use of land, water and fertilisers’

Green groups have been pushing for years to get farmers to reduce their fertiliser use, and many are starting to listen. Emissions in the US and Europe have stabilised and started to go down, and farmers maintained or slightly increased yield while using no more nitrogen fertiliser, Thompson said.

Market factors have also influenced the reduction. Still, there’s room for improvement, particularly in China, where a ‘more is better’ approach to ammonium nitrate persists.

Alternatives include organics such as manure, and deployment of cover crops like soya and other legumes that convert nitrogen in the air into plant food. But these methods will only take food production so far. Estimates of the percentage of humans who would not be alive today without synthetic fertiliser range from 40 per cent to 50 per cent.

With fertiliser one of agriculture’s biggest operating expenses, it also pays to find the optimal amount of ammonium nitrate to apply to soil. AgroCares, based in the Netherlands, has developed a scanning device about the size of a flashlight that translates spectral information into a nutrient value, determined through a large database of soil samples and a machine-learning algorithm.

Paired with a smartphone app, the device provides information similar to that produced by labs for US and European farmers, but more accessible to the developing world. In parts of Africa, many farmers use fertilisers without knowing the balance of nutrients in their soil, said AgroCares’ project manager ,Florent Mournetas.

More pandemics will happen due to climate change: Experts

In the last few years, US biotech firms from California to St. Louis to Boston have developed microbial solutions to replace at least some of the synthetic fertilisers used with hungry crops such as corn and wheat.

“Before use of ammonium nitrate became widespread, microbes existing naturally in the soil provided some nitrogen to plants. But with large amounts of ammonium nitrate poured into the soil around them, they don’t bother expending energy on nitrogen production. Legumes are able to circumvent this by chemically shielding the microbes in their roots, convincing them they’re in a nitrogen-poor environment.

“We go in and break the wiring in the microbe that connects their ability to sense nitrogen in the soil to their decision to become a source of nitrogen for a farmer,” said Karsten Temme, chief executive of Pivot Bio, based in Berkeley, California. The goal is less fertiliser required.

Now in its third commercial growing season, Pivot prices its product to produce yields equivalent to what the same dollar amount of ammonium nitrate would provide. The advantage, Temme said, is that the microbes do their job more consistently over the growing season, and don’t wash away with the heavy rains that are becoming more frequent in the Corn Belt.