Biological function of N

• Protein (one or more N per amino acid)
• Base pairs for RNA/DNA
• Prosthetic groups for protein (ex.: heme group of chlorophyll)
• Hormones (ABA, cytokinins)
• Metal uptake (phytosiderophores) and transport in xylem & phloem (ex: Cu with amines)
• Osmoregulation (ex.: lettuce and spinach, which may accumulate 0.1 M NO₃^- in vacuoles!)
• Chemical defenses, alkaloids, misc. biochemicals (ex: mescaline, cocaine, morphine, nicotine, caffeine, quinine)

Note that plants do not use nitrate and ammonium, directly but must reduce nitrate and assimilate them into organic compounds (with the minor exception of osmoregulation using nitrate above.) Reduction of nitrate takes place in both the root and the shoot. Particularly at high rates of nitrate supply or low photosynthetic activity, physiological limits to root reduction of nitrate will mean that increasing amounts of nitrate will be in the xylem flow, where it will be end up in shoots and ultimately be reduced in the leaves.

Nitrate reduced in the roots will be reduced first to nitrite by nitrate reductase and then to ammonium by nitrite reductase. Ammonium will undergo reaction with glutamate to form glutamine by the action of glutamine synthase. Glutamine may then undergo additional transformations before entering the xylem flow as reduced N, depending upon the plant species.

Deficiency:

Mild N deficiency will restrict plant growth, but often in a subtle manner that can only be assessed by comparison to plants grown with an adequate N supply. Moderate N deficiency will cause leaves to be light green or yellowish. Severe symptoms include necrosis (tissue death) starting at the tips of older leaves, with the tissue death developing a V-pattern down the midrib toward the base of the leaf.

Sources:

In natural systems, N sources are:

• Mineralization of organic-N. Microbial decomposition of plant and animal residues in soil releases mineral nitrogen, nitrate and ammonium, forms of N that plants can absorb from the soil solution. Soil organic matter is also constantly slowly decomposing (even as new soil organic matter is being formed), releasing a slow trickle of nitrate and ammonium into the soil solution.
• Biological fixation of atmospheric N₂. Even though the atmosphere is mostly N₂, this N source is unavailable to plants without catalytic conversion to mineral N (NH₄⁺) by highly specified microorganisms. Some of these microorganisms live in association with plants, principally the legumes, in symbiotic relationships while other nonsymbiotic microorganisms fix N₂, which is released in mineral form only upon the decomposition of the microbial biomass upon its death.
• Rainfall. Precipitation contains both nitrate and ammonia. The total input from precipitation is small, A certain amount of the dissolved N is due to reactions of atmospheric N₂ during lightning events, although volatilization of ammonia and atmospheric pollution (as nitric acid) is a significant part of this input.

All systems, including natural systems, "leak" nitrogen because nutrient cycling is never absolutely perfect. Agricultural systems, because of their expectations of large biomass production and crop removal of N in the harvest, require N inputs greater than those provided by natural systems in order to be sustainable. These human inputs include:

• Addition of manures and organic matter. This is basically a redirection of the natural process, but in human hands includes the disposal of many of the wastes generated by agriculture.
• Synthetic fertilizers, among them ammonia, ammonium nitrate, and urea. These fertilizers have been proposed since the days of Justus von Liebig but have only become available since the development of the Haber-Bosch process of synthetic nitrogen fixation.

N Testing

Soil

• Nitrate: a simple water extract will extract nitrate from the soil since most soils have only a small anion exchange capacity and nitrate is not preferred on that anion exchanger.
• Nitrate+Ammonium (="mineral nitrogen"): strong salt solutions to displace exchangeable ammonium (NH₄⁺). Most commonly used, 2 M KCl (recall lab exercise).
• Organic N: either determined by acid digestion and Kjeldahl distillation or determined as fixed percentage of organic matter content
• "Mineralizable N": usually an incubation study that measures increase of mineral N over time.

Plant

• Tissue test: usually a quick test for nitrate, as in lab
• Plant analysis: either whole plant or specific plant portions at specified level of maturity. Digestion and total N analysis, as by Kjeldahl distillation in lab

Nitrogen Fertilizers

The Haber-Bosch process produces NH₃ from atmospheric N₂ and H₂ produced from fossil fuel source, usually natural gas. It is a high pressure/high temperature reaction (130-680 atm, 340 -420^oC) using an iron catalyst. (Compare with biological nitrogen fixation that operates at 1 atm and ~20^oC with nodules as reactors using nitrogenase, a MoFe protein, as an enzyme for N₂ fixation! Visit the interactive 3-D computer model of the nitrogenase enzyme that "fixes" nitrogen at the "Virtual Museum of Minerals and Molecules" - a free browser plug-in must be installed to see the displays...instructions on site!)

(An essay on "The Present-Day Significance of Fritz Haber" by Morris Goran, published in 1947, is available on-line at this website, with the kind permission of the American Scientist.)

Ammonia from the industrial fixation of N₂ is the starting material for almost all synthetic N fertilizers.

1. Anhydrous ammonia, NH₃: First among N fertilizers in terms of synthesis path is anhydrous ammonia, since it is the product of the Haber-Bosch process. It contains 82% N and is a volatile gas at atmospheric pressure and ambient temperature but can be easily liquified with pressure and refrigeration. Agronomic use of anhydrous ammonia therefore requires proper storage infrastructure. Application must be subsurface (to avoid immediate volatilization), preferably into a soil that is not too dry so that the ammonia gas can dissolve into the soil water.
2. Aqua ammonia, ammonium hydroxide, NH₄OH: Ammonia gas dissolved in water to give a 33% solution (saturated). Ammonia still volatile if not contained. Bulk of fertilizer weight is water, not N.
3. Ammonium nitrate, NH₄NO₃: Solid, white powder usually pelletized and conditioned to improve handling, 33 or 34-0-0. Potentially explosive because of presence of both reducing and oxidizing agents (NH₄ and NO₃) in same compound.
4. Urea, CO(NH₂)₂: White powder, usually pelletized, 45 or 46-0-0. First organic compound ever synthesized inorganically. Soil microorganisms cause urea to hydrolyze to ammonium carbonate, which may then spontaneously reform as (volatile) ammonia gas and ammonium bicarbonate if pH remains high, by the following reaction:
   CO(NH₂)₂==> 2 NH₄⁺ + CO₃^2- <==> NH₃ + NH₄⁺ + HCO₃^-
   Incorporation of urea into the soil so that the pH buffer capacity reduces the potential volatilization loss of ammonia is essential for efficient use of urea. Most common fertilizer in many regions with infrastructure inadequate to transport and store liquified ammonia. Urea is produced industrially by high temp/ high pressure reaction of ammonia and CO₂:
   2 NH₃ + CO₂==> CO(NH₂)₂
5. Ammonium sulfate, (NH₄)₂SO₄: 21-0-0, produced either by addition of ammonia to sulfuric acid using virgin materials or as by product of coke industry in which the flue gases containing ammonia from the organic N in coal are reacted with sulfuric acid solutions.
6. Calcium nitrate, Ca(NO₃)₂: 15-0-0. Not particularly popular in U.S. as fertilizer except for nutrient solutions/greenhouses, but more popular in Europe, perhaps because of its suitability for soils in danger of acidification.
7. Potassium nitrate, KNO₃: 14-0-44. [=Saltpeter!! Remember Glauber's "Principle of Vegetation"?] Commonly used in greenhouses and sometimes for high value crops. Produced by reaction of KCl with nitric acid, followed by volatilization of HCl:
   KCl + HNO₃ <=> KNO₃ + HCl(g)
8. Ammonium bicarbonate, NH₄HCO₃: 17-0-0. Relatively unstable N fertilizer source that spontaneously reforms as ammonia, carbon dioxide, and water, causing major volatilization losses:
   NH₄HCO₃ <==>NH₃ + CO₂ + H₂O
   Overall N fertilizer recovery rates for this source are low ~30%, but this is the major synthetic N source used in China.
9. Ammonium phosphates: MAP, monoammonium phosphate, NH₄H₂PO₄, typically 12-52-0, and DAP, diammonium phosphate, (NH₄)₂HPO₄, typically 20-50-0. Formulated by reacting various quantities of ammonia with phosphoric acid. More often regarded as P fertilizer than N fertilizer since fertilizer application of DAP to achieve desired N rates exceed usual P rates, in most cases.
10. Sodium nitrate: NaNO₃, Chilean saltpeter". Salt deposits of natural origin, accumulated in high desert valley of Chile, deposits discovered in 1809. Mined by stripping off overburden, blasting the ore, and refining to 96% pure product. Principal N fertilizer of commerce until replaced by Haber-Bosch products. Sodium content generally considered to be negative influence on soil structure. Is this an "organic" fertilizer?

The "Big Three" N fertilizers in the U.S. are anhydrous ammonia, ammonium nitrate, and urea. Anhydrous ammonia is a gas that is liquified for ease of handling by low temp and/or high pressure. Ammonium nitrate and urea are both solids, but soluble in water, thereby making a liquid N fertilizer. Liquid N fertilizers may be subdivided as:

• High pressure: Anhydrous ammonia. Pressure required to keep liquified.
• Low pressure: Aqua ammonia. Pressure required to retain ammonia gas dissolved in water.
• No pressure: Concentrated mixtures of urea and ammonium nitrate in water, UAN or URAN. Analyses are typically 28-0-0 (1^oF), 30-0-0 (15^oF), and 32-0-0 (32^oF), where temperature is the temperature below which the fertilizer salts fall out of solution to form a hard-to-redissolve mass at the bottom and sides of tank.

Controlled release (Slow-release) N fertilizers

These fertilizers are high value-added products, that is a relatively cheap material (fertilizer N), is remanufactured into a more expensive product for a specific purpose--fertilizer availability timed to more closely match the gradual and continual plant nutrient needs, also minimizing sudden leaching of excess fertilizer. These products are typically used for high-value agricultural needs such as turf and greenhouses, rather than row crops. The basic strategy is to either make the N insoluble or trap it with some sort of barrier.

Ureaform fertilizers are manufactured by the reaction of formaldehyde, CH₂O, with urea, CO(NH₂)₂, to produce compounds such as methylene diurea:

or dimethylene triurea:

Using urea with isobutyraldehyde, (CH₂)₂CHCHO, instead of with formaldehyde, yields isobutylidene diurea (IBDU):

About 90% of the N in IBDU is not water-soluble.

Note: These fertilizers are organic in the strict chemical sense, i.e., C-containing. Indeed, urea was the first organic compound chemically synthesized, but organic farmers (and the nonscientific public) would not recognize the urea that comes out of the fertilizer factory gates as of organic origin. Neither would the ureaform fertilizers be considered organic except for a scientific audience: The public at large would be deceived by such a claim.

Another technique in the manufacture of controlled-release fertilizers is coating the fertilizer. Sulfur-coated urea (SCU) typically consists of 30-40%N (as urea), 10-30% S, 2-3% conditioner, and 2-3% sealant. Soil microorganisms oxidize the sulfur to sulfate, breaching the coating and allowing the urea to be released. Other coatings are formulated with micropores and cracks to permit slow diffusion of the fertilizer out of the fertilizer granule. Yet another group of techniques "trap" the N fertilizer in a matrix, either a gel or fritted solid or a cation exchanger such as zeolites or vermiculite, to slow the release of N.

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This page was last modified by Phillip Barak, Univ. of Wisconsin, on 5 Jan 1999. All rights reserved.