FACT SHEET
foliar uptake of nitrogen
Foliar application and absorption of nitrogen has been
explored since the 1950s, with many experiments
showing that various plant species can rapidly absorb
nitrogen (and other elements) through the leaf. Foliar
application of nitrogen has potential benefits compared
to soil applications; in particular, it may overcome plant
nitrogen stress more rapidly, especially when soils are
overly wet or dry, or when root function is impaired.
Foliar applications also have the potential to reduce
nitrogen losses associated with soil application.
Phloem
Xylem
Upper leaf
surface
Mode of uptake
Foliar-applied nitrogen is thought to enter the plant through
stomata, microscopic pores in the leaf cuticle. These are
more numerous on the underside of the leaf (see Figure
1), which may explain why nitrogen uptake is more rapid
when it’s applied to the leaf underside, compared to the leaf
surface.
The form of nitrogen also influences the rate of uptake, with
urea being absorbed more rapidly than either ammonium or
nitrate. This may be due to the negative charge on stomata,
which would slow uptake of nitrate through electrostatic
repulsion (nitrate carries a negative charge) and slow
ammonium through electrostatic attraction (ammonium
has a positive charge). Urea is neutral (i.e. it does not carry a
charge) and so is not affected by the charge on stomata.
Nitrogen is absorbed rapidly once applied: in one New
Zealand study, 30-40% of the nitrogen could be found in the
plant within a few hours of urea being applied to perennial
ryegrass.1 This is consistent with research results achieved
elsewhere in the world.
In the field, the rate of absorption will be influenced by the
rate of nitrogen applied, the degree of dilution used, the plant
species and the climatic conditions.
Underside
of leaf
Stomata
FIGURE 1
A stylised leaf, showing the presence of numerous stomata on
the underside of the leaf
negative
charge
NO3-
Stomata
NH4+
C0(NH2)2
Underside
of leaf
FIGURE 2
Stomata carry a negative charge, which repels nitrate and
holds ammonium by electrostatic attraction. Urea, having no
charge, is not affected by the negative charge on stomata
Plant damage
Foliar urea is less likely to cause plant tissue damage (leaf
scorch) than other forms of nitrogen, but it is not risk free.
Leaf scorch is believed to be caused by the dessication of
plant cells as water moves out of them to equilibrate the
osmotic pressure and/or phytotoxic effects from the build-up
of urea or aqueous ammonia in the plant tissue.
The risk of leaf scorch increases with the rate and
concentration of nitrogen application: 1-2% nitrogen has been
recommended as the maximum concentration. Leaf scorch
risk also increases if nitrogen is applied when leaves are
damp or when humidity is high, since under these conditions
urea stays in solution on the leaf for longer, promoting
greater and more rapid absorption, and so increasing the
osmotic stress on plant cells.2
NH3
NH3
Nitrogen losses
Foliar application of nitrogen is sometimes proposed as a way
to reduce the losses of nitrogen that occur when solid urea
is applied to soil. These include microbial immobilisation,
ammonia volatilisation, denitrification and leaching. Of these,
only ammonia volatilisation is of significant concern.
NH4+
Leaching and denitrification losses will be low if solid urea
is applied using good practice. Microbial immobilisation is
in effect a temporary store for nitrogen, which will later be
released through mineralisation.
Ammonia volatilisation is well documented, with typical
losses from solid urea applied to pasture being in the order
of 10-15% of the nitrogen applied. The urease enzyme that
facilitates this loss is found in and on leaves, as well as in soil,
so it is not surprising that ammonia volatilisation losses also
occur when urea is foliar applied. Measured losses on a range
of grasses are between <1% and 16%, although most work has
been carried out on turf grass species, rather than ryegrass/
clover pastures.
It is also pertinant to note that foliar applied urea may still
enter the soil, particularly if too much liquid is used to
carry the urea. This urea will be at increased risk of loss by
volatilisation, unless there is sufficient moisture (5-10 mm
applied within 8 hours) to wash it into the soil. Nonetheless,
some of this urea will still be absorbed by the plant, via the
root system.
NO3-
FIGURE 3
Foliar-applied urea can enter the plant through the leaf, be
lost to the air by volatilisation or fall onto the soil, where it
may still be taken up by the plant via the roots.
References
1
Dawar, K, Zaman, M, Rowarth, JS & Turnbull, MH (2012) Applying
urea with the urease inhibitor N-(n-butyl) thiophosphoric
triamide) in fine particle application improves nitrogen uptake in
ryegrass (Lolium perenne L.), Soil Science and Plant Nutrition, 58:
309-18
Gooding, MJ & Davies, WP (1992) Foliar urea fertilization of
cereals: a review, Fertilizer Research, 32: 209-22
2
Yield gains
The gains that result from the reduction of loss by using
foliar urea instead of surface-applied solid urea are
generally relatively small. In addition, any leaf scorch
of plants may negate some of the benefit of increased
nitrogen use efficiency. As a result, reported yield response
advantages from using foliar nitrogen in preference to soil
nitrogen are generally extremely variable.
In a series of Ballance-funded trials in 2009, the effect of
foliar and soil-applied nitrogen on yields of pasture, kale
and wheat found no significant difference between the
two approaches. However, when rates of foliar application
were 40 kg N/ha or greater, the risk of scorch increased,
particularly when the liquid urea was applied as a fine
spray (i.e., using herbicide spray nozzles).
A review of a large number of trials examining the
differences in cereal grain yield between crops receiving
either foliar or soil-applied nitrogen revealed examples of
a signficant increase in grain yield, others of a significant
decrease in yield, and yet others in which there was no
significant difference between the crops.2
In summary, current evidence indicates that plant
responses to foliar and solid urea are equivalent,
provided the same amount of nitrogen is applied in both
circumstances, and that best practice is followed in both
situations.
www.ballance.co.nz • 0800 222 090
Created 04 April 2014
ONYX 15130