In part one of this series on how nitrogen gets into our soil, I talked about the ability of certain bacteria to fix nitrogen nodules on the roots of certain plants, mostly legumes. In part two, I explained how organic nitrogen — typically in the form of protein in compost, manure and sewage — is converted by different sets of microorganisms into a form that is usable by plants. This month we look at the third and final way nitrogen gets into soil: as industrially-fixed inorganic nitrogen, better known as synthetic fertilizer.

Here’s what we know so far: Plants can use nitrogen — that is, take it in through their roots — in three main forms: ammonium (NH4+), nitrite (NO2-) and nitrate (NO3-). (The nitrogen in the air we breath — primarily N2 — is in its elemental form and hence unavailable for plant use.) This means that plants will use the nitrogen in these forms, and it doesn’t matter one iota to them whether the source of the nitrogen is legumes, manure, compost ... or synthetic fertilizer. The source of nitrogen does matter greatly to the soil organisms around the plants, but that has been covered in previous columns and will undoubtedly come up again! Here is a very brief history of fertilizers and how nitrogen can be converted to ammonium and then nitrate without microorganisms.

Natural fertilizers such as manure and ground-up animal bones have been used since ancient times. Early farmers probably didn’t know WHY these materials worked - only that they did. The study of chemical requirements for plant growth (the familiar N-P-K, or nitrogen-phosphorous-potassium, on fertilizer bags) that started in the 19th century led to the production of fertilizers from inorganic chemical sources. Various deposits of rock phosphate and potash were found to provide adequate sources of phosphorous and potassium, but there was really only one source of inorganic nitrogen: Chilean saltpeter (sodium nitrate). Other sources of nitrogen such as bat guano were discovered periodically, but these supplies usually were depleted within just a few years.

As world population increased, so did the use of fertilizers. Using fertilizer with high nitrogen content provided good crop yield, but in the 1890s some scientists begin to worry about the depletion of the sources of nitrogen for fertilizer. Because plants could not directly convert atmospheric nitrogen to soluble nitrogen compounds, the search for a solution was on.

Two German scientists developed a system of converting gaseous nitrogen to ammonia, and the process still bears their names today: the Haber-Bosch process. Both of these men were awarded the Nobel Prize in chemistry — Fritz Haber in 1918, and Carl Bosch in 1931. In a nutshell, the process they developed involves the industrial fixation of nitrogen by combining atmospheric nitrogen and hydrogen under high temperatures and high pressure to form ammonia, which is the basis for all synthetic fertilizers. And just where does the hydrogen come from? Most often it comes from natural gas — an expensive, non-renewable resource. The resulting synthetic nitrogen made the so-called “Green Revolution,” starting in the 1960s, possible. And although, over the years, the fertilizer likely has saved the lives of millions of people who otherwise would have starved, ammonia is also used to make explosives (which the Germans did in World War I). Technological advances are seldom single-edged.

Can the plants tell the difference if your nitrogen fertilizer started out as natural gas or cow manure? Nope. But there are many disadvantages to using synthetic nitrogen. Because it is made with natural gas, its production is unsustainable. And nitrate — whether synthetic or natural — is highly water soluable, which means that whatever nitrate is not taken up by plant roots quickly moves down past the root zone into ground water; epidemiological studies have linked nitrates in drinking water to reproductive problems and bladder and ovarian cancer. Also, the oxidized forms of nitrogen that are often released when fertilizer is applied contribute to the formation of smog, and acid rain, when the nitrogen oxides react with water in the atmosphere to form acidic compounds. Given all this, it’s no wonder that industrially-fixed (synthetic) nitrogen is not allowed for use on certified organic farms and gardens!

Before we conclude this look at nitrogen and soil, I’ve just got to tell you that there is one other way that nitrogen can be added to soil, and that is through the action of lightning. Lightning splits the nitrogen molecule, which can then react with oxygen or hydrogen to form nitrate (and ozone), which is carried to the earth in rain. But the amount of nitrogen “fixed” in this way amounts to only about five percent of the total amount of naturally fixed nitrogen.

Hmmm ... lightning plus nitrogen equals plants. Could this be a recipe for the beginning of primitive life on a planet or two? Scientists — or science fiction writers — may someday be able to tell us. Glen Andresen hosts “The Dirtbag” heard every second Monday at 10:30 a.m. on KBOO 90.7 FM. He tends his bees on a three-quarter acre organic garden at a retreat in Eagle Creek. He also coordinates Metro’s Natural Gardening Program. Comments and questions may be sent to glen {(*at*)}pacifier.com or c/o The Portland Allinace.