No-till thatch build-up aggravates urea concerns.
November 26, 2007 By John Harapiak
Applying spring top dress nitrogen fertilizer as ammonium nitrate (AN) was
often a key component of a successful winter wheat production program on the
Canadian prairies. Of course, with the loss of this N fertilizer, this is no
longer an option. Some of these same growers are now extremely upset about this
development, and are questioning the motives of the fertilizer industry for
undertaking this decision.
AN set standard for top dress applications
Ammonium nitrate had a long history of usage in western Canada. It was known
for reliability and predictability as a broadcast and seedrow applied nitrogen
fertilizer. Because of the stability of AN as a broadcast fertilizer, growers
preferred to use this product to provide N fertilizer to hay fields, pastures
and for top dress applications to annual crops. Alas, that is no longer a possibility.
|Evidence of the presence of a significant amount of partially decomposed
crop residue from two crops (i.e. mustard and winter wheat) located below
the residue of the durum wheat crop.
Dominant fertilizer N source is urea
Like it or not, growers must now adapt to the new N supply paradigm that has
developed on the prairies. The bulk of the N fertilizer used within western
Canada is now supplied by urea and that is unlikely to change for a long time.
The next most important source of N is anhydrous ammonia. Both of these products
require special care in their application in order to achieve maximum value.
Be aware of urea's shortcomings
Urea does not behave like AN, especially when it is broadcast applied. The fact
that urea is converted by an enzyme called urease, to an unstable product called
ammonium carbonate, can predispose this source of N to atmospheric losses of
gaseous ammonia. In terms of the risk of these losses, this conversion is not
an important issue when the urea is in good contact with the soil, since the
ammonia is attracted to, and held by the soil particles.
Surface residues create challenge
The adoption of reduced tillage and no-till practices can result in the accumulation
of significant amounts of crop residues on the soil surface, which is desirable
from a moisture conservation point of view, given the nature of the region's
semi-arid climate. However, a serious potential problem can arise if the broadcast
applied urea remains in contact with crop residues, rather than penetrating
the thatch layer to establish contact with the soil.
Conversion in thatch is problematic
The thatch layer contains an abundant supply of the urease enzyme that converts
the urea into an unstable form. In the presence of a moist thatch layer, or
as a result of a light shower, this conversion will began to take place. Unfortunately,
the ability of the thatch to hold ammonia is quite limited, especially if a
light shower is followed by drying conditions which reduces the ability of the
thatch to retain ammonia.
Rating of urea vs. ammonium nitrate
While practical experience has demonstrated that AN is superior to urea for
top dress applications for winter wheat, in terms of relative performance rating,
there is limited research available on this issue. However, there have been
plenty of comparisons conducted with grass forage crops. The relative efficiency
of urea compared to AN, as a grassland fertilizer, was reviewed by Bill Toews
of Alberta Agriculture in 1971. He found that urea was about equal to AN in
only one-fifth of the trials. In about one-tenth of the trials, urea was 10
percent more effective than AN, and in nine-tenths of the trials it was about
70 percent as effective as AN. A large number of forage trials carried out by
Westco indicated that, on average, urea was rated as being between 80 and 85
percent as effective as AN as a grass forage fertilizer.
Relative surface thatch cover
The presence of a thatch layer on the surface of grass stands creates a greater
risk of volatile ammonia losses from surface applied urea. However, since the
bulk of a grass hay crop is removed, the opportunity for a large build-up of
a thatch layer is limited. This is not the case under no-till production of
crops where only the seed is harvested. As illustrated in some of the accompanying
photos, the amount of thatch that can accumulate on the soil surface in no-till
is really quite impressive.
Soil contact difficult to achieve
Under these conditions, it is easy to understand why no-till winter wheat producers
preferred using AN for their spring top dress application. The lack of immediate
penetration of the fertilizer granules, through this type of thatch layer, can
create a serious performance issue for urea. However, since AN is not impacted
by the presence of the urease enzyme, some short-term stranding within the thatch
layer is much less of a concern than is the case with urea.
Rainfall can solve problem
One possible option would be to delay the application of urea until a day or
two prior to a period of rain. It is assumed that 0.5 inch of rain should dissolve
the urea and move it into contact with the soil. Given the difficulty in predicting
rainfall, this approach is not that practical, at least not on a large-scale
basis. "Treating the urea with Agrotain can significantly widen the window
of opportunity for the urea to be moved into soil contact by rain."
Limited options available
Growers have inquired about using a polymer-coated urea in these situations.
However, these products have performed poorly in forage trials and their performance
will likely be even more erratic where significant accumulations of thatch exist
in no-till fields. Dribble-band applied UAN has worked well in winter wheat,
but heavy layers of thatch could pose a serious challenge for achieving effective
Switch to 'at seeding' option
In terms of being able to contend with higher levels of thatch accumulation
under no-till, I believe the most practical solution for getting the required
urea-based N effectively applied, is for growers to switch to some type of side-band,
or mid-row band application of all their nitrogen requirements at the time of
John Harapiak has approximately 40 years of western Canadian based
fertilizer related experience. He will continue to contribute stories to
Top Crop Manager. He can be contacted by e-mail at: firstname.lastname@example.org