Nitrogen—catch it if you can. It’s the component in manufactured (inorganic) fertilizer that is responsible for as much as 60% of crop yields in developed countries (Stewart et al., 2005), and is responsible for feeding 48% of the world population (Erisman et al., 2008). It is a critical component of amino acids and nucleotides, which form life’s hardware and software, respectively. Yet nitrogen can readily change from a form that plants utilize to one that is nonreactive, like nitrogen gas (78% of air we breathe), or a reactive, ozone-depleting, greenhouse gas, the ever-nefarious nitrous oxide. Nitrogen can also leach out of soils as nitrate, which can enter waterways and cause algal blooms that decompose into hypoxic dead zones in oceans, to the chagrin of aquatic life, tourists, and fishermen. Moreover, nitrogen fertilizer can be one of the most expensive input costs in crop production, and nitrogen losses are detrimental economically as well as environmentally. Since nitrogen (N) is usually in the wrong place, in the wrong form, at the wrong time, soil scientists and agronomists have been following nitrogen molecules for decades (literally, with isotopic labeling), trying to impede its losses and promote its capture by plants.
Organic farming systems use recycled N by fertilizing with composted organic waste and manure, or rely on legumes (alfalfa, clover, beans…) to draw in nitrogen gas from the air and convert it to plant-usable forms.
Conventional systems rely on compost, manure, and legumes also, but primarily fertilize soil with N by adding synthetic fertilizer. Fossil-fuel-generated heat converts nitrogen gas to ammonia nitrate and other N-containing fertilizers in the manufacturing process. This is why synthetic fertilizer prices are so sensitive to energy prices.
One system yields 7 – 35% more food than the other system (Alex Avery, Hudson Institute, 2009), being also responsible for reducing starvation rate from 1/3 in 1965 to 1/7 in 2010 (FAO), but we’ll open that can of worms next week!
Following the advent of industrial ammonification in the last century, it was generally thought that “more was more” when it came to fertilization and farming. Over-application of N exacerbates nitrification and denitrification losses (Watson et al., 1992). No one realized this until research like that performed by Stout et al. (1974) showed that “less is more” by graphing the points of diminishing return for fertilizer and crop yields. Beyond a certain point, applying N is simply wasteful and harmful. Even as N application rates are more precise and are performed as indicated by soil tests, N still tends to escape the field by way of natural processes. Here are the discoveries that have mitigated challenges by nitrification (nitrate leaching) and denitrification (gaseous N).
More ways to reduce nitrification (nitrate leaching):
Less N leaches out of clay than sandy soils (Macduff et al., 1990)
Development of nitrification inhibitors (Singh and Verma, 2007)
Splitting the total N application into several small applications for crops, while and not before they are growing (Alcoz et al., 1993)
Dry application of fertilizer is better than dissolved in water (Pratt et al., 1976)
Practice no-till or conservation tillage (Drury et al., 2004)
Crop rotation (studies by Wright and Hons, 2004-2005)
Low-intensity grazing (Wright et al., 2004)
High soil organic carbon (Williams et al., 2001)
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More ways to reduce denitrification (gaseous losses):
High soil organic carbon (especially from soils under conservation tillage) significantly reduces nitrous oxide emissions (Bijay-Singh et al., 1988)
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You may notice that no-till farming systems promote efficient capture of nitrogen…as well as sequester more carbon to reduce atmospheric CO2. Unfortunately, organic farming relies heavily on tillage to combat weeds, which of course frustrates this whole scheme of N and C capture. Then again, in organic systems, soil carbon accumulation by manuring, mulching, and cover-cropping is more common. Next topic will be much more consumer-friendly, since we’ll debate organic vs. conventional!
What do you Think about all this farm chemistry today?
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