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At the coffee pot meeting today we had a long discussion about nitrogen fertilizers, fertilizer efficiency, and vine nitrogen demands. Many of you, as producers, apply nitrogen fertilizers in the form of urea or ammonium nitrate. The following article explains where those fertilizers come from, what the other sources of nitrogen in the soil are, and what happens to fertilizer nitrogen once it leaves your fertilizer spreader. The article does not tell you how much N to apply, when to apply it, or what the vine N demands are. Many of those questions have been answered in previous articles. Hopefully, understanding where plant available nitrogen comes from and where it ends up will help you make a more educated decision about your fertilizer practices.
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| Figure 1: The synthesis and agricultural cycling of three commonly used nitrogen fertilizers in the eastern, United States. |
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Nitrogen Fixation
Atmospheric nitrogen is by far the largest pool of nitrogen on the planet; however, this molecular form of nitrogen is unavailable for plant uptake. Ultimately, both natural and industrial nitrogen fertilizers are derived from the fixation of atmospheric nitrogen into forms usable by plants. In industrial nitrogen fixation, nitrogen and hydrogen are combined under high temperature and pressure to form ammonia, otherwise known as the Haber process. Hydrogen in this reaction is derived from natural gas, petroleum, or coal, which makes the fertilizer industry dependent on the availability and cost of fuel sources. Industrial nitrogen fixation accounts for only a small fraction of world-wide nitrogen fixation. Biological nitrogen fixation, the dominant fixation process, takes place though the action of microorganisms. Free-living bacteria and bacteria that have symbiotic associations with certain plant species contain enzymes that harvest molecular nitrogen into ammonia. Agriculturally, biological nitrogen fixation is important because it is difficult and expensive to satisfy vineyard nitrogen requirements through industrial fertilizers alone.
Formation of Nitrogen Fertilizers
Nitrogen sources can be supplied to vineyards through both inorganic and organic nitrogen fertilizers. There are several commercially available nitrogen sources that supply ammonium, nitrate, or both to the soil solution for plant uptake. When ammonia is combined with nitric acid under heat and pressure, ammonium nitrate fertilizer is formed. Similar reactions with sulfuric and phosphoric acids produce ammonium sulfate and ammonium phosphate, respectively. Urea, a common inorganic nitrogen fertilizer, is formed from the reaction of ammonia and carbon dioxide under heat and pressure. Since industrial nitrogen fertilizers require high temperatures during the formation of both ammonia and ammonium, the cost of fertilizers are dependent on the cost and availability of fuel sources. Therefore, inorganic nitrogen fertilizers that cost the least per unit of nitrogen are preferred.
There are many sources of organic fertilizers because once nitrogen is fixed by bacteria and incorporated into organic compounds; nitrogen can enter any number of biological pathways in microorganisms, plants, and animals. Organic nitrogen incorporation and organic matter decomposition are also energy intensive processes; however, the energy is derived from biological activity and not the burning of fossil fuels. Ultimately, the breakdown of organic matter releases free ammonium ions and the build up of humus acts as a soil reserve of nitrogen.
Decomposing organic matter and humus are the largest pools of nitrogen in most agricultural systems and represent slow release nitrogen sources given the correct biological and environmental conditions. During periods of rapid vine growth, the release of nitrogen from organic stores can be too slow to meet vine demand. Although inorganic nitrogen fertilizers are only supplemental to organic nitrogen sources, properly timed inorganic nitrogen fertilizers can be essential to desired vineyard production during periods of peak vine nitrogen demand.
Agricultural Nitrogen Cycling
Inorganic and organic fertilizers, through a variety of chemical and biochemical reactions, supply ammonium and nitrate ions to the soil solution for plant uptake. Plants assimilate nitrogen into organic compounds for growth and reproduction. Cane prunings, leaf litter, and dead root tissue are eventually recycled into an organic nitrogen source. Vineyard nitrogen cycling is dependent on several factors such as temperature, moisture, oxygen, organic matter, soil pH, and microbial activity.
Nitrogen fertilizer salts such as ammonium nitrate, ammonium phosphate, and calcium nitrate when applied to the vineyard floor are dissolved into the soil solution and dissociate into their component ions. For example, ammonium nitrate (NH4NO3) dissolves into the ammonium cation (NH4+) and nitrate anion (NO3-). Ammonium cations can absorb onto soil clay particles and the degree of absorption is dependent on the cation exchange capacity and the competition from other cations. Ammonium can be converted to nitrate through the process of nitrification (discussed later). Nitrate anions, preferentially absorbed by grapevines, are a quick source of nitrogen but they are also subject to leaching. Both ammonium and nitrate make up a small percentage of the total nitrogen in agricultural nitrogen cycles; however, they are the nitrogen forms taken up by grapevines. It is estimated that 70% of all mineral nutrient ions taken up by plant roots are in the form of ammonium or nitrate.
Urea is converted to ammonia and then to ammonium through hydrolysis with the urease enzyme. Urea hydrolysis is a biochemical reaction influenced by several factors such as temperature, moisture, and enzyme concentration. Strongly acidic soils and soils with low clay content slow the rate of urea hydrolysis. Urease activity is optimum between a soil pH of 6.5-7.0. The intermediate step in the conversion of urea to ammonium is the formation of ammonia which can be lost from the system through volatilization. Sandy, alkaline soils, high temperature, wet soils, as well as high and unincorporated urea applications increase ammonia volatilization.
Mineralization, the release of ammonium from decomposing organic matter, is also dependent on several environmental and biological factors. In general, warm, moist, well drained soil conditions with reasonable soil pH (4.5-9.0) and low C/N ratio substrate material increases the mineralization rate.
Dissolution of ammonium based fertilizers, hydrolysis of urea, and mineralization of organic matter all generate ammonium ions in the soil solution. Ammonium can be converted to nitrate through the process of nitrification. In nitrification, ammonium is oxidized to nitrite by one group of bacteria and then further oxidized to nitrate by a second group of bacteria. Hydrogen ions are released during nitrification which leads to potential soil acidification. If all the nitrate ions produced through nitrification were absorbed by plant roots, ion excretion by roots would neutralize the reaction. However, plant roots absorb only a fraction of the total nitrate produced and the leaching nitrate leads to soil acidification. Therefore, the addition of ammonium based fertilizers tends to acidify vineyard soils.
Nitrogen Loss
Nitrogen can be lost from the vineyard system through erosion, denitrification, harvesting plant tissues (grapes), and leaching. Erosion leads to the physical removal of organic nitrogen in the upper soil profile. Denitrification is the conversion of nitrate back to atmospheric nitrogen. Grapes and sometimes wood infected with disease removed from the vineyard also removes organic nitrogen from the system.
Nitrate leaching is an agricultural concern because excess leaching leads to soil acidification and potential groundwater pollution. Industrial and organic fertilizers both provide ammonium to the soil where the ammonium is oxidized to nitrate and potentially leached. Efforts should be made to make the most efficient use of nitrogen fertilizers by using the appropriate material, rate, and timing for the individual vineyard goals.