Molecule of the Month: Explosive stuff of life: John Emsley profiles a chemical that can feed - or kill

AMMONIUM nitrate explodes in our cities, blights our rivers and contaminates our drinking water. Police said their discovery on 10 March of ammonium nitrate in a north London garage foiled a huge IRA bomb. But without this awesome chemical, the world would be a quieter place because there would be far fewer of us: as a fertiliser, ammonium nitrate provides most of the nitrogen needed to grow our food.

About 75 million tonnes of the chemical are manufactured worldwide each year, and at present there is a glut. Britain makes 1.5 million tonnes, more than 90 per cent of which is used as fertiliser; the rest goes into explosives. Only a tiny amount ends up in terrorist bombs. The IRA's most spectacular nitrate explosion, caused by the chemical packed in a van, wrought destruction in the City of London last year on the scale of a Second World War flying bomb (these too were armed with ammonium nitrate).

In one of the German chemical industry's triumphs at the turn of the century, Fritz Haber (1868-1934) discovered how to turn air into ammonium nitrate, and was awarded the 1918 Nobel Prize for Chemistry. The discovery changed the course of history by undermining the Royal Navy's blockade of Chilean nitrate supplies to Germany.

Haber's process turns nitrogen in the air into ammonia, NH3 , some of which is then converted to nitric acid, HNO3 . Mix ammonia and nitric acid and the result is that you get ammonium nitrate, NH4 NO3 .

Earlier this century some tragic accidents involving ammonium nitrate occurred. On 21 September 1921, a German chemical store at Oppau, containing more than 4,000 tonnes of the stuff, exploded, killing 430 people. Workmen had used dynamite to break up a pile of ammonium nitrate that had become caked solid. And on 15 April 1947, a ship in Texas City harbour caught fire, igniting its 5,000-tonne load of ammonium nitrate, killing 552 people and injuring 3,000. Every building within a mile was demolished.

Today, ammonium nitrate is treated so that it cannot cake solid, and stores are carefully regulated.

Britain can easily make all the ammonium nitrate she needs, but 500,000 tonnes a year is imported from Eastern Europe.

Crops need nitrogen, often scarce in heavily farmed soil. Ammonium nitrate, which can be applied easily, has more than doubled crop yields in the past 20 years.

As fertiliser use has increased, however, so has the nitrate level in rivers and streams, polluting drinking water and encouraging unwanted algae and weeds. But there appears to be little scientific support for the claim by environmentalists that nitrate in drinking water might be a hazard to human health.

Plants absorb nitrogen as ammonium ions that they can use straight away, or as nitrate ions that they reduce to ammonia. Nature supplies nitrogen to the soil through bacteria and microbes, which produce the equivalent of 140 million tonnes of ammonium nitrate a year worldwide. This so-called organic nitrogen, however, is far from sufficient to supply the body-building protein for 5 billion humans, hence the need for inorganic ammonium nitrate.

Crops receive most of their nitrogen from inorganic fertilisers, with manures, soil residues and the atmosphere providing smaller amounts. However, very little nitrate leached from farmland into water supplies comes from inorganic fertilisers; most of it is due to soil residues and manures.

'There is no relationship between ammonium nitrate applied as fertiliser in spring and the nitrate run-off into rivers the following winter,' says Dr Keith Goulding, a soil chemist at Rothamsted, the agricultural research station in Hertfordshire, which has been monitoring fertilisers and crop yields for more than 150 years.

Researchers there have used ammonium nitrate tagged with the 'heavy' nitrogen isotope N-15 to show that 70 per cent of ammonium nitrate fertiliser ends up in the crop and 15 per cent in soil microbes, while soil bacteria return 10 per cent to the atmosphere as nitrous oxide or nitrogen. Therefore, they say, only about 5 per cent is washed into rivers: much less than the 'organic' nitrate leached from the land, although anything that improves fertility will be leached in the long run.

The EC limit for nitrate in drinking water is 50 parts per million (ppm) or 50mg per litre. But to drink water containing twice this amount would not even harm a baby.

Nor do we get nitrate only from drinking water; vegetables contain it (beetroot and spinach have more than 1,000ppm) and we also make about 50mg in our own bodies every day. 'The present levels of nitrates in drinking water are perfectly safe,' says Dr Goulding, who says the last time a British baby went blue from too much nitrate was in 1972.

Blue-baby syndrome is caused by bacteria in saliva, stomach and intestines, reducing nitrate (NO3) to nitrite (NO2). The latter molecule is the cause for concern. Nitrite turns haemoglobin into inactive methaemoglobin, which cannot then pick up oxygen. In the middle years of this century, some mothers used water from wells that contained more than 200ppm of nitrate, and their bottle-fed babies turned blue through lack of oxygen. One or two babies died.

More alarming is the claim that nitrite can react with other components of our food to form the dangerous free radicals called nitrosamines, which are known to trigger cancer. However the incidence of stomach cancer is actually lower in those areas of Britain where nitrate levels in water are above average. One way to neutralise free radicals is to eat foods rich in Vitamin E, a natural free radical scavenger. But be careful about which foods you choose; lettuce, for example, contains not only lots of Vitamin E, but also 1,000ppm of nitrate.

The author is science writer in residence at the Department of Chemistry, Imperial College, London.

(Photograph omitted)