Fertilizers are responsible for over half of global food production, but there are areas in world with nutrient deficiency and other areas of nutrient excess.
Managing mineral plant nutrients requires careful application of science and skill to meet production, environmental, and social goals.
Saturday, September 29, 2012
Urea-Ammonium Nitrate Fertilizer
solutions or fluid fertilizers are popular in many areas because they are safe
to handle, convenient to mix with other nutrients and chemicals, and are easily
applied. A solution of urea [CO(NH2)2] and ammonium nitrate [NH4NO3] containing between 28 and 32% N is the most popular
N fluid fertilizer.
nitrate (UAN) fertilizer is relatively simple to produce. A heated solution
containing dissolved urea is mixed with a heated solution of ammonium nitrate
to make a clear liquid fertilizer. Half of the total N comes from the urea
solution and half from the ammonium nitrate solution. UAN is made in batches in
some facilities or in a continual process in others. No emissions or waste
products occur during mixing.
Since UAN is a
concentrated N solution, its solubility increases as the temperature rises. To
prevent the N components from precipitating as crystals, UAN solutions are made
more dilute in regions with cold winter temperatures. Therefore, the N
concentration in commercial UAN fertilizers will vary from 28% N to 32% N
depending on geography. A corrosion inhibitor is usually added to the final
solution to protect the steel in storage tanks.
UAN fluid fertilizer
Properties: 28% N30% N32%
Composition (% by
Ammonium Nitrate 40 42
Urea 30 33 35
Water 30 25 20
Salt-out temp (ºC) -18 -10 -2
Solution pH - - - - approximately 7 - - - -
Solutions of UAN are widely
used as a source of N for plant nutrition. The NO3-
portion (25% of the total N) is immediately available for plant uptake. The NH4+
fraction (25% of the total N) can also be assimilated directly by most plants,
but is rapidly oxidized by soil bacteria to form NO3-.
The remaining urea portion (50% of the total N) is hydrolyzed by soil enzymes
to form NH4+, which is subsequently transformed to NO3-
in most soil conditions.
Fluid fertilizer application
Solutions of UAN are extremely
versatile as a source of plant nutrition. Due to its chemical properties,
UAN is compatible with many other nutrients and agricultural
chemicals, and is frequently mixed with solutions containing P, K, and
other essential plant nutrients. Fluid fertilizers can be blended to
precisely meet the specific needs of a soil or crop.
UAN solutions are commonly injected
into the soil beneath the surface, sprayed onto the soil surface, dribbled as a
band onto the surface, added to irrigation water, or sprayed onto plant leaves
as a source of foliar nutrition. However, UAN may damage foliage if
sprayed directly on some plants, so dilution with water may be needed.
UAN applied with fertigation
makes an excellent source of N nutrition for plants. However, since half of the
total N is present as urea, extra management may be required to avoid volatile
losses. When UAN remains on the surface of the soil for extended periods (a few
days), soil enzymes will convert the urea to NH4+, a portion of which can be lost as
ammonia gas. Therefore, UAN should not remain on the soil surface for more than
a few days in order to avoid significant loss. Inhibitors that slow these N
transformations are sometimes added. When UAN is first applied to soil, the
urea and the NO3- molecules will move freely
with water in the soil. The NH4+ will be retained in the soil where it first contacts
cation exchange sites on clay or organic matter. Within 2 to 10 days, most of
the urea will be converted to NH4+ and no longer be mobile. The originally added NH4+ plus the NH4+ coming from urea will
eventually be converted to NO3- by soil microorganisms.