Simple conventional fertilizers containing nitrogen, phospho-
rous, and/or potassium can be applied using drip irrigation if they
are soluble in water. Due to the lower solubility of phosphate fer-
tilizers, the practice of using fertigation for phosphorous is less
common. Potash fertilizers are better soluble than phosphorous,and
therefore
farmers
use
fertigation
for
potash
fertilizers
to
some
extent. The chief source of nitrogenous fertilizers is urea. Urea is
quite soluble in water, and thus fertigation with urea is very pop-
ular among farmers. Urea is a highly reactive fertilizer and begins
reacting
immediately
after
its
dissolution
in
water.
First,
urea
is
nitrified into ammonium, and subsequently, into nitrate (Fig. 1).
While urea is electrically neutral, ammonium ions are positively
charged
and
hence
adsorb
to
the
negatively
charged
clay
particles
well. Consequently, the leaching of ammonium ions from the soil
is significantly reduced. In the soil, ammonium is converted into
nitrate, which is a negatively charged ion. Therefore, nitrate does
not
adsorb
to
clay
particles
and
hence
the
possibility
of
nitrate
leaching to groundwater is very high. Plant roots absorb nitrogen
in all three nitrogen forms: urea, ammonium, and nitrate. Some
quantity of ammonium also gets volatilized into ammonia gas.
Ammonia volatilization is highly dependent on soil pH and soil wet-
ness. Under anaerobic conditions, nitrate can also be denitrified into
nitrous oxide. However, in the case of drip irrigation, the process
of
denitrification
can
be
neglected,
because
most
of
the
time
soil
is
not under saturation.