The following is part of a new series I am writing to help introduce some greater understanding of plant needs and how soil affects these needs. Feedback is always greatly appreciated!
Plant Nutrients and Soil Interactions
Nitrogen is a structural component of several essential plant parts and compounds and the one element used in the greatest quantities by terrestrial plants. Nitrogen uses include the structural component of chlorophyll, the structural component of the nucleic acids (DNA, RNA) in each cell, and is structural component of all proteins. Due to the critical nature of N, correct for low volumes in soil usually results in much higher gains of vegetative growth, fruit and vegetable yields, and higher protein levels of food crops. As N is essential for all life, low soil levels have an adverse effect on soil microbial conditions.
What is often not considered is that our atmosphere is primarily nitrogen, equating to 79.09% of our atmosphere with oxygen accounting for only 20.95%. With every breath of life giving oxygen we take in, we take in ~ 3.73 times more nitrogen. So with N being so abundant, why do we even is it the element plants need most and the most abundantly manufactured and applied fertilizer? This is very much a case of Coleridge’s line “water, water, everywhere, nor any a drop to drink” from the Rime of the Ancient Mariner. While we might be swimming in N, this is a gaseous for of diatomic nitrogen (dinitrogen), or N2, which for those who remember their college chemistry is a molecule that contains one alpha and two sigma bonds. This tri-bond has magnificent energy associated with it, far too much for much for most plants to break and then form nitrogen into new usable molecules. Those molecules that are usually taken-up by plants are nitrates (NO3-), ammonium (NH4+), and urea (carbamide) (CO[NH2]2). This does mean that most organic N is not immediately available to plants and must be converted, primarily though bacterial actions but also through ionization during lightning storms, into a suitable form before use.
Factors that affect nitrogen loss can be immobilization, where the nitrogen is tied up within the plant, soil microbes, and to a lesser extent soil macro organisms. Microbes will tie up available N when the C:N ratio is high. Usually, the C:N ratio of around 25:1 is the point where one would expect to see nitrates become in excess and therefore available to plant needs. Other losses of nitrogen within the soil are leaching due to excess soil moisture, volatilization where the NH4+ converts to NH3 turning gaseous and escaping into the atmosphere (one of many good reasons not to frequently turn your soil), and denitrification which is what happens when we pick our fruits and vegetables. When we remove our produce (e.g. bag and remove leaves and lawn clipping) all the N that was pulled from the soil is fixed into the fruit, and instead of rotting back into the soil, it provides our bodies with a source of nitrogen. Another form of denitrification is seen when we have over saturated soil which drives out gaseous oxygen. This leads to the soil bacteria removing the oxygen molecule from NO3-, converting it into gaseous nitrogen N2.
As a result of these functions, corrections of N shortages result in large gains in vegetative growth, much higher protein levels, and much higher yields of grain, fruit, and vegetative plant organs. While these gains are normally desirable, excess amounts of N, either in absolute terms or sometimes in the ratio of N to other elements, can have a negative impact on some aspects of various yield components.
Nitrogen, in the form of nitrates, is one of the few mobile elements in plants, along with phosphorus, potassium, magnesium, molybdenum, chlorine, and nickel. These mobile nutrients allow a plant that has low soil N available to cannibalize nitrogen from older plant parts and provide it with new vegetative growth. Often N deficiencies can be seen by the presentation of healthy new growth coupled with chlorotic old growth.
Often overlooked, nitrogen has a significant impact on the acidification of soil. Nitrogen, in the form of ammonium gets converted to nitrate in the presence of oxygen by nitrifying bacteria releasing hydrogen ions. The basic equation is: 2NH4+ (+) 3O2 ——- nitrifying bacteria—-à 2NO3- (+) 8H+
That hydrogen is, by chemical definition, an (or really the) acidifying agent as the pH scale is simply a measure of hydrogen, the more reactive hydrogen, the lower the pH. Of course hydrogen is not the only means of acidification N is involved in. Nitrates (NO3-) can ionize with potassium, calcium, and magnesium within the soil. Once the nitrate and associated base cycle, the base is removed and replaced by hydrogen, further acidifying the soil.
It is important to discuss the role nitrogen plays in plant health, that role is everything. Nitrogen makes up the base of all proteins, all genetic material, many important signal molecules, one could go on and one, but it cannot be stressed enough that without nitrogen, there is no life on earth. However, this is not a call to pump loads of nitrogen into one’s soil. Excessive N can be very detrimental to plants and soil healthy unless it has sufficient nutrient balance, especially potassium. When excess N is provided to plants they tend to have unhealthy growth that leaves them primed for disease and infection.
The overuse of nitrogen, especially inorganic (fast release) nitrogen fertilizers is a powerful poison to our native lands and waterways. The overabundance of N in soils leads to easy leaching into our waterways, promoting excessive growth (eutrophication) of algae and aquatic plants. These leads to dissolved oxygen being pulled from the water, creating anaerobic conditions. This has an impact that continues to reverberate throughout the food chain, natural system, and is determinate to everyone’s health via numerous vectors. The EPA called nutrient pollution via phosphorus and nitrogen run off from fertilizers and manufacturing “America’s most widespread, costly and challenging environmental problem”, https://www.epa.gov/nutrientpollution/problem, whose impact can easily been by the breadth of the anaerobic “dead zone” at the mouth of the Mississippi River. Nitrogen pollutes drinking water and the Haber process, the primary process for producing inorganic nitrogen, is a huge energy sink. Requiring significant amounts of fossil fuels to produce, releasing CO2 into the atmosphere and emitting oxides of nitrogen (e.g NO, NO2, N2O, NO5) which act as a main precursor for tropospheric ozone as well as being a threat to human health in its own right. So for those who care about global warming, the health of our planet, and conservation of our natural and built world, be kind to your soil as a healthy soil makes healthy plants naturally.