Poor Man’s Fertilizer

Illustration by Adelaide Tyrol

Remember the old adage, “snow is poor man’s fertilizer?” It turns out to be true. Snow and rain, sleet and hail, and just the dust settling out of cooling air carry trace amounts of nitrogen compounds – the stuff found in the fertilizers farmers use to make corn grow as high as an elephant’s eye. In fact there’s more nitrogen than ever in snow these days, but not everyone is happy about it.

Until about 100 years ago, human populations were, like other natural populations, limited by their capacity to make use of nitrogen. Though we live in a sea of it – nitrogen gas makes up 78 percent of our atmosphere – plants and animals can only use nitrogen when nitrogen gas becomes “fixed” into the usable form of nitrate (when oxygen is bonded with the nitrogen) or ammonium (when hydrogen is bonded with the nitrogen.) Most of this nitrogen fixing is carried out by specialized anaerobic bacteria living cooperatively inside the root nodules of legume plants – peas, beans, and locust trees – and other pioneer species like the speckled alder. Once it’s fixed, nitrogen is taken up quickly by plants and cycled through the biosphere from air to soil to living organisms and back. As a result, there isn’t much available nitrogen left in soil – it’s too valuable.

In the early years of the twentieth century, German scientist Fritz Haber began researching ways to make synthetic ammonia for use in explosives for the Kaiser’s impending war effort. Initially Haber ran hydrogen and nitrogen gas over iron filings, using uranium as a catalyst to create temperatures of 1,000 degrees Celsius and pressures more than 150 times that of the atmosphere. But he nearly blew himself up handling the ammonia, which is easily combustible. Eventually, he found he could safely produce 250 grams of ammonium an hour using the catalyst osmium. Within a year of his discovery, World War I was underway.

Haber’s invention fueled both the war and a revolution in agriculture. Synthetic nitrogen fertilizer applied to crops boosted their yields to previously unimaginable levels. Corn yields jumped from 22 bushels/acre in 1930 to 80 bushels/acre by 1968 in the United States. This exponential increase in food supply helped the human population to grow from 1.5 billion people in 1900 to 6 billion in the year 2000.

But the development of synthetic nitrogen has come at a price. The nitrogen compounds in fertilizer are highly volatile in air – they’re the source of the sharpness you can smell beside a recently plowed and fertilized field in spring. Anywhere from 20 to 50 percent of all fertilizer spread on fields evaporates or is washed out of the upper soil horizons by rain. But farming isn’t the only source of excess fixed nitrogen in the atmosphere. A small amount is fixed by lightning and drops to earth bonded to microscopic dust or absorbed in rain and snow. Burning fossil fuels also fixes nitrogen by incorporating nitrogen gas into new compounds that eventually settle back to earth.

This excess nitrogen then shows up in other parts of the biosphere. Though scientists aren’t exactly sure why, many studies suggest that too much nitrogen can actually lead to forest decline and increased tree mortality, changes in ecosystem composition, and loss of forest biodiversity. On land, excess nitrate in drinking water leads to human health problems, including methemoglobinemia “blue baby syndrome” – fatal in children and found worldwide where levels of nitrate in drinking water exceed 10 mg/l. In the water, nitrates end up in coastal estuaries, where they cause they growth of too much algae and degrade water quality in fisheries. Currently, 70 percent of the U.S. coastline is influenced by nitrate enrichment.

So, what to do about nitrogen enrichment? An important part of the solution is to use fertilizer as efficiently as possible, whether you are a farmer, a home gardener, or someone simply trying to green up your lawn. But we won’t be kicking our nitrogen habit anytime soon. The human population is projected to grow by another 2 billion in 50 years, and synthetic nitrogen fertilizer is here to stay. But we can become a lot smarter about how we use it. Otherwise, the expression “poor man’s fertilizer” will become synonymous not with snow but with synthetic fertilizer itself, since its misuse over time impoverishes the natural systems that support life on Earth.

Tim Traver is a science writer living in Woodstock, Vermont.

 
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