An explosion at a Texas fertilizer plant April 18 killed at least five, injured 160 and forced locals to remain indoors as officials feared further explosions or leaks of ammonia from the plant's ruins.

Fertilizer is commonly used to help farm crops grow better and make lawns greener. But one of the ingredients often employed in the manufacturing process, ammonium nitrate, is highly fickle and can feed an explosion under the right circumstances.

Most plants require some combination of 16 different nutrients to grow properly. Some of these come from air and water, but plants derive most nutrients from soil, which doesn't always contain all the substances they need to thrive. Farmers compensate for poor soil by using either animal manure or fertilizer to replenish the nutrients.

Worldwide, fertilizer plants produce about 170,000,000 tonnes of fertilizer nutrients annually, according to the International Fertilizer Industry Association.

Plants like West Fertilizer Co. in Waco, Texas, where the April 18 explosion occured, manufacture fertilizer for commercial use. Producing fertilizer involves treating raw materials to either purify them or increase their concentration, changing them into plant-available forms, and frequently combining materials so they contain multiple nutrients, according to IFIA.

There have been other explosions at fertilizer plants, including one on Sept. 21, 2001, at an AZF plant about 3 kilometers outside the city centre of Toulouse, France. About 300 tonnes of ammonium nitrate exploded, killing 29 and injuring at least 2,500. It shattered windows up to 5 kilometres away, and resulted in about 40,000 insurance claims.

What is ammonium nitrate?

Ammonium nitrate, sometimes referred to by the initials AN, is a chemical compound that's most often seen in the form of white granules. It can be easily made in a lab by reacting nitric acid with ammonia.

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A common and nitrogen-rich fertilizer, ammonium nitrate is usually found in the form of white granules. ((CBC))

AN is used in fertilizers because of its high nitrogen content. It was once the top nitrogen fertilizer in the world, though it has since been supplanted by higher-concentration urea. In 2010, AN accounted for about 15 per cent of all nitrogen fertilizers used around the globe.

On its own, ammonium nitrate is benign, but when mixed with certain hydrocarbons, such as fuel oil, it can become a powerful explosive.

Such a mixture of ammonium nitrate and fuel oil is commonly called ANFO, and is often used as an explosive in industrial applications like mining and construction. About a sixth of global AN production is used in industrial explosives.

Western Europe, the former Soviet states and the U.S. are the planet's top consumers of ammonium nitrate.

How much does an average farmer use?

It's not unreasonable for a farmer to use up to 50 tonnes a year to fertilize fields. It greatly depends, though, on the size and type of crops under cultivation.

Usually farmers apply ammonium nitrate in the early spring, when it's spread on fruit fields, pasture lands and vineyards. But a number still use it on wheat and apply it after planting, typically sometime in June.

What else is AN used for?

Ammonium nitrate has also been used as an oxidizer in some solid fuel rockets and to help deploy airbags in cars.

Other uses include as a heat absorber in instant cold packs. Mixed with zinc and ammonium chloride, AN can be found in survival kits because it will ignite on contact with water.

Why is it used to make bombs?

Ammonium nitrate readily combines with a variety of other substances, including diesel, aluminum and even molasses, to make so-called fertilizer bombs. It's also widely available in large quantities because of its commercial applications in agriculture, and is relatively cheap.

A 25-kilogram bag of ammonium nitrate sells for around $20, according to farm-supply stores in southern Ontario.

A powerful explosion can be triggered by less than a tonne of AN.

Who has used it for bombs?

Anti-government extremist Timothy McVeigh used about 2,000 kilograms of ammonium nitrate, mixed with $3,000 worth of motor-racing fuel, to blow up a U.S. federal government building in Oklahoma City in 1995, killing 168 people.

The group allied with al-Qaeda that set off explosives in a Bali, Indonesia, nightclub district in 2002 made its main bomb with ammonium nitrate. More than 200 people were killed.

The U.S. army estimates that 95 per cent of all bombs built by militants in southern Afghanistan, including roadside improvised explosive devices, or IEDs, are made with ammonium nitrate.

Police allege the so-called Toronto 18 group of bomb plotters — of whom only 11 were ever prosecuted — planned to set off one-tonne ammonium nitrate bombs at three targets: the Toronto offices of Canada's spy agency, CSIS; the Toronto Stock Exchange; and a military base in eastern Ontario. The plot was foiled in 2006.

How is AN restricted?

It used to be common for farmers and landscapers to use moderate quantities of the chemical, in the form of ANFO, to blow up and remove stubborn tree stumps on open fields. But Canada now tightly restricts the use of ANFO to open pit mines and quarries, and then only with the permission of an official called the chief inspector of explosives.

The Canadian government passed tight regulations on the storage and sale of ammonium nitrate in February 2008. The rules require, among other things, that all storage areas be locked down when nobody's working in them, that a seller inform local police of all their storage areas, and that the seller conduct weekly checks to make sure their supply hasn't been tampered with.

Anyone purchasing the chemical has to show ID or a special licence and cannot buy more than is proportional to their needs. The seller has to record the buyer's info and must deny them the chemical where there's reason to believe it will be used for a crime.

Some other jurisdictions have bans on fertilizer-grade ammonium nitrate because of its potential use in bomb-making. Those countries include Afghanistan, Colombia, Denmark, the Philippines, parts of Pakistan, China and Algeria. In 2004, in the wake of the Bali bombing, Australia's largest fertilizer supplier declared it would no longer sell AN.