Is Nitrogen a double-edged sword for our garden soils?By Darius Van d'Rhys (darius)
January 7, 2009
Here’s a look at just nitrogen, the “N” of NPK (without getting into what the “numbers” of each one mean in this article). Nitrogen makes up 78% of our Earth’s atmosphere as N2, a gas. As abundant as Nitrogen is, we living things (animals and plants) cannot use it without first changing it into some nitrogen compounds, which are essential for all living things. Nitrogen is there, we need it, and we cannot use it as is. So, how do we get it?
Nitrogen can be converted biologically, and chemically. When nitrogen is biologically converted to a useable form, it is considered “fixed”. Organic nitrogen (decaying vegetable matter) is converted by some microbes (nitrogen-fixing bacteria like Rhizobium trifolium) into an inorganic nitrogen like ammonium ion, which plants convert to compounds used to make proteins and other essential building blocks. The plants in turn secrete sugars/carbohydrates necessary to the microbes. The proteins made by plants from nitrogen compounds are found in all parts of the plant from roots to leaves, plus nitrogen is an essential component in the production of chlorophyll. Plants deficient in nitrogen often exhibit signs of “yellowing” leaves because their chlorophyll production is diminished.
Some nitrogen is present in soils as nitrates like saltpeter, and ammonium nitrate used as fertilizer. Currently, ammonia is widely produced chemically into fertilizers. The cost of natural gas makes up about 90% of the cost of producing ammonia. Plants are able take up fertilizing nitrogen in three forms: nitrate, ammonium and urea. How does each one work, and which is best?
Nitrates, commonly available fertilizers, are negatively charged, meaning they don’t bind to soil particles. This allows nitrates to wash away, contaminating waterways. Researchers at Kansas State University, “…found that freshwater pollution by phosphorous and nitrogen costs government agencies, drinking water facilities and individual Americans at least $4.3 billion annually. Of that, they calculated that $44 million a year is spent just protecting aquatic species from nutrient pollution.” In poorly drained soils, nitrates can also be converted by some bacteria into gases like carbon dioxide (a greenhouse gas) where it enters the atmosphere.
Ammonium carries a positive charge, meaning it can attach to soil particles, but it is also a reversible action. It can detach when conditions are right, like high soil temperatures or high pH, and then it releases gases into the atmosphere. This generally happens when fertilizer concentrations are applied in excess. “The use of ammonium nitrate in fertilizers is particularly damaging - plants absorb the ammonium ion preferentially to the nitrate ion - this means that the nitrate ions are not absorbed and therefore are free to be dissolved by rain leading to eutrophication.” (Eutrophication is generally considered to be excessive plant growth and decay.)
Urea is a man-made source of nitrogen chemically identical to the organic source known as urine. Urea is used as a nitrogen fertilizer, 45-0-0 (45% nitrogen, 55% inert), whereas urine as fertilizer is 2-5% nitrogen, coupled with minerals and microbes. Urea is the principal form of dry fertilizer N in the United States, approaching 16% of total N use. Urea application directions emphasize immediate incorporation it into the soil to minimize damage, and not to add it to or near tender plant growth.
Here is where diverse opinions begin to set in (the double-edged sword). Proponents of organic methods remind us that urea and heat are the two main ways to kill soil bacteria, and that urea causes so much immediate growth that it stresses the plants. Most of us have seen plants that are all lush foliage and bear little or no fruit, like tomatoes with too much fertilizer applied. Nitrate fertilizers also leach rapidly into our water systems causing untold damage. Ammonium fertilizers release gases that contribute to global warming. Never-the-less, plants need nitrogen.
There appear to be two somewhat conflicting pathways to get nitrogen to our plants. The first path (or choice) is to let the bacteria do the work via organic fertilizers. That could mean doing several things, such as planting nitrogen-fixing crops and cover crops (legumes and clovers) that utilize Rhizobium to fix nitrogen on root nodules. However, nitrogen-fixing legumes only supply nitrogen to the particular plant where the root nodules attach and “fix” the nitrogen. Those nodules do not supply nitrogen to other plants unless incorporated into the soil (tilled under) as green manure.
| Nitrogen-fixing peas|| Nitrogen-fixing nodules|| Red clover cover crop|
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| Nitrogen cycle|| Tilling in nitrogen biomass|
You could incorporate lots of plant and animal waste material into the soil continuously so the nitrogen-fixing bacteria always have something to convert into nitrogen as the plants use it up. You might also apply foliar sprays made from a nutrient-rich compost tea containing live beneficial microbes to convert atmospheric nitrogen.
An alternative is to use commercial fertilizers, choosing wisely with the planet ecology in mind, and applying in carefully recommended amounts. Neither way is necessarily the better since many other factors come into play, including our own personal circumstances, size of land to fertilize, existing soil fertility and pH, the balance of nitrogen to phosphorus, potassium, macro soil minerals and trace (micro) minerals.
“Soil is more than the mineral earth that comprises the majority of the weight of a soil; it is also this small world of animals and plants that convert the raw mineral earth into the rich soil upon which all life depends.” Therefore it behooves us to do the very best we can to feed the right stuff to the critters that feed the soil that feeds the plants that feed the animals, and feed us.
 Sawyer JE (2001). Natural gas prices affect nitrogen fertilizer costs. IC-486 1: 8.
 Roots, Nitrogen Transformations, and Ecosystem Services http://arjournals.annualreviews.org/doi/abs/10.1146/annurev.arplant.59.032607.092932
Nitrogen cycle, Photo by Johann Dréo, GNU Free Documentation License
Ammonium nitrate, Photo by Ondřej Mangl. Public domain
Salts of Potassium nitrate on a wall. Photo by Abujoy, Creative Commons Attribution ShareAlike 2.5 License
Urea, iStockPhoto # 5224260, © Jeffrey Heyden-Kaye, Used by Permission
Thanks to Melody for her PlantFiles photo of Red Clover and White Clover Roots (Nitrogen-fixing nodules)
Thanks to Farmerdill for his PlantFiles photo of Peas (nitrogen-fixing legume)