Essential Elements for Plant Growth
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.
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.
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
- 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
- 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.
- 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.
- 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
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
-420oC) using an iron catalyst. (Compare with biological nitrogen
fixation that operates at 1 atm and ~20oC 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
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
Ammonia from the industrial fixation of N2 is the starting
material for almost all synthetic N fertilizers.
- 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.
- 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.
- 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.
- 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+ +
CO32- <==> 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
- 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.
- 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.
- 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)
- 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 +
Overall N fertilizer recovery rates for this source are low ~30%, but this is
the major synthetic N source used in China.
- 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
- 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"
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
- 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 (1oF), 30-0-0
(15oF), and 32-0-0 (32oF), 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
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
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.