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Supplies of by-product ammonium sulphate will expand

Can using cheap 21-0-0-24 be too much of a good thing?

November 19, 2007
By John Harapiak


Within western Canada, the benefit of using ammonium sulphate (AS) as a nitrogen
fertilizer was not truly appreciated until canola production had become well
established. The importance of sulphur (S) in the production of this crop has
been demonstrated widely. Prior to this development, AS was marketed primarily
as a nitrogen fertilizer. In that regard, the relatively low N content of this
granular fertilizer product was a hindrance to its market acceptance.

AS most acidifying N source
A problem can arise with the use of 21-0-0-24 as a primary N fertilizer, when
there is no requirement for S. Over a period of time, the unneeded S can have
a significant acidifying effect on the soil. While the use of any N fertilizers
can gradually reduce soil pH, Doug Penney, a senior Agri-Coach with Agri-Trend
Agrology, claims that based on a review of published scientific information,
the acidifying effect of AS is about three times as great as that of ammonium
nitrate (AN), urea or ammonia.

Expanding by-product AS
Initial supplies of AS became available as a by-product of metal refining activities
occurring within western Canada. However, increased volumes of AS are expected
to be produced as a direct result of environmentally mandated efforts to reduce
air pollution. The S-containing industrial gases that were normally vented into
the atmosphere will increasingly be combined with ammonia to produce AS. One
such example is the by-product AS (DakSul) produced at the ManDak coal based
power generating plant, located in North Dakota.

Tar sands activity means more AS
As a result of environmental activities associated with the production of heavy
oil from the tar sands, we expect relatively larger amounts of by-product AS
to be generated within western Canada in the near future. The first such by-product
AS could be available, starting this winter, from a Marsulex/Syncrude facility.
Domestic markets cannot easily absorb these additional supplies of AS, so most
of it will be channelled into the export markets.

Temptations of low priced AS
The problem with depending on export sales of AS is that the size of the off-shore
requirement could vary significantly from year-to-year. A temporary downsizing
of the export opportunities for AS could result in significant storage containment
problems for this by-product. Historically, such a development has resulted
in distress pricing of AS in order to move it at any cost. In such cases, the
resulting lower N prices could result in strong grower interest in using AS
as their main source of N. Over the long haul, such a decision could have a
negative impact on soil quality.

Excessive N rates assess acidification
Westco researchers established several long-term forage trials in order to assess
the impact of repeat applications of various sources of N fertilizer on soil
pH. The information summarized in Figure 1 illustrates the impact on soil pH,
after 15 years of applying N fertilizer at excessively high rates of N. The
elevated N rates were used in an attempt to simulate the impact of a much longer
period of fertilizer applications. The graph illustrates the impact of AS on
soil pH as it filters much further into the soil profile than in the case of
other N fertilizers. Keep in mind that a pH decline from 6.0 to 5.0 represents
a tenfold increase in soil acidity.

Less impact at normal N rates
The pH results in Figure 2 illustrate the impact of 23 years of AN and urea
applications at a rate of 100lb/ac N. In this study, AN decreased the soil pH
of the zero to two inch layer more than urea. This could have resulted from
the fact that urea was about 15 to 20 percent less effective in increasing forage
yields in this trial. This difference was attributed to volatile losses of ammonia-N
from the urea treatments. For that reason, the effective rate of urea application
was also 15 to 20 percent lower. It is therefore uncertain whether AN was actually
more acidifying than urea.

Less acidification on cereals
In forage trials where the bulk of the crop is removed from the field, the rate
of soil acidification will be greater than for cereal crops where the straw
remaining in the field has a buffering effect on soil acidification. Unfortunately,
there is no reliable long-term data on the soil acidification resulting from
the usage of N fertilizers in the cereal cropping systems.

AS caution warranted
Unless you believe that the decrease in soil pH has no relevance to your farming
operation because of the rather high rates of N fertilizer in these forage trials,
do not become complacent. During the 1950s and '60s, when excess by-product
AS was accumulating at a metal refining operation located at Fort Saskatchewan,
this product was distress priced in order to ensure it was moved from the plant
site to farm fields. In some of these years, this product was sold for $15 per
ton or less. After a decade or so, the Alberta Soil Testing Laboratory, located
in Edmonton, began to express concerns about the noticeable decline in soil
pH levels they were detecting around the region where most of the by-product
AS was applied.

John Harapiak has more than 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