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Fertilizing with copper: Will your crops benefit? Part II

Assess the potential for an economic response.

November 15, 2007  By John Harapiak

There are almost five million acres of farmland within western Canada that
could respond positively to the application of copper fertilizer. On these soils
the response of wheat to this micronutrient can often be quite dramatic. The
potential for an economic response to copper on the rest of the acres is in
fact really quite low, so it is important you do some 'homework' before deciding
to apply copper fertilizer. The first step in correcting a copper deficiency
is to identify that a deficiency does in fact exist.

Table 1. Statistical and economic analysis
of soils that test 'deficient' and 'marginal' in extractable copper in zero
to six inch depth as influenced by soil texture category.
  Copper deficient soils
Soil texture Median yield increase $ return/
$1 cost
Sandy loam 18.5bu/ac 3.85
Loamy sand 11.5bu/ac 2.45
Fine sandy loam 7.5bu/ac 1.45
Loam 0.5bu/ac 0.15
  Copper marginal soils
Sandy loam 1.85bu/ac 0.24
Loam 0.45bu/ac 0.16
Clay loam minus 0.45bu/ac 0.01
Adapted from Karamanos.

Critical plant tissue levels
Collecting whole plant samples for small grains just prior to filling can provide
useful information for confirming whether or not a potential copper deficiency
exists in your wheat or barley. Plant tissue levels below 3.0ppm of copper are
considered to be indicative of a potential deficiency. Samples that fall within
the range of 3.0ppm to 4.5ppm are considered to be marginal, while samples testing
above 4.5ppm are considered to be adequate in copper. When diagnosing potential
field problems, sample the good and poor areas separately. Do not sample areas
of extremely poor growth, since such samples seldom yield meaningful or useful

Critical soil test levels
The critical level
of extractable copper defining the probable existence of a copper deficiency
appears to be 0.4ppm based on a zero to six inch soil sampling depth. Copper
levels that fall in the 0.4ppm to 0.6ppm range are considered to be only marginally
deficient and copper application in these situations should first be evaluated
in a test strip rather than on a whole field basis. In these marginal situations,
sampling to a greater depth becomes very important in assessing the need for
copper application. If copper levels increase with depth, the prospects of obtaining
an economic response to an application of copper fertilizer decrease very significantly.

Table 2. Statistical and economic analysis of response
to copper fertilization as influenced by soil extractable copper level category
in zero to six inch depth.
Extractable Cu range (ppm) Copper deficient soils
Median yield increase $ return/
$1 cost
0.2 or less 17.5bu/ac 2.35
0.21 to 0.3 6.5bu/ac 2.15
0.31 to 0.4 5.5bu/ac 1.12
  Copper marginal soils
0.41 to 0.6 1.2bu/ac 0.26
Loam 1.1bu/ac 0.17
Clay loam minus 0.3bu/ac minus 0.06
Adapted from Karamanos.

Westco copper response survey
Rigas Karamanos, a senior research agronomist with Westco, recently compiled
the results from 118 copper experiments from across the prairies. The data pool
that was assembled provides some very useful agronomic information that helps
to define the probability of obtaining an economic response to the application
of copper fertilizer. In that regard, Karamanos evaluated the impact of the
level of extractable copper present in the zero to six inch depth and the influence
of soil texture. This information is summarized in Tables 1 and 2.

Role of soil texture
In Table 1, information is summarized according to soil texture for trials that
were located on soils that tested either deficient (0.4ppm or less) or marginal
(0.41ppm to 0.6ppm). Within the copper deficient sites, the yield response to
the application of copper ranged from a high of 18.5bu/ac on the lightest textured
soils (i.e. sandy loams) to a low of 0.5bu/ac on the heaviest textured of the
deficient soils (i.e. loams). In the deficient category, the returns per dollar
invested in copper fertilization ranged from a high of $3.85 to a low of $0.15
(i.e. non-profitable). On the sites that tested marginal in extractable copper,
the yield response declined as the soil texture category increased in clay content
from a sandy loam to a clay loam. In this summary of prairie-wide results, based
on the average data collected, the application of copper was unprofitable for
all soil categories testing in the marginal range of copper availability.

Role of soil copper levels
When the survey results were categorized according to the range of soil copper
levels, within the deficient sites (see Table 2), the yield response to the
application of copper fertilizer increased from 5.5bu/ac to a high of 17.5bu/ac
as soil copper became more deficient. The profitability of copper fertilization
increased from $1.12 to $2.35 per dollar invested. However, based on average
results, copper fertilization was unprofitable on all of the soils that were
marginal or higher in soil copper levels.

Caution warranted
Soil fertility specialists agree that deficiencies of micronutrients are not
a widespread problem on the Canadian prairies, although some deficiencies do
exist. Copper is certainly the one to be most concerned about in terms of cereal
grain production. In that regard, the data compiled by Karamanos is significant
since it provides some very powerful information for helping growers to fine-tune
their decision-making as to whether or not the application of copper is likely
to be profitable.

Suggested copper guidelines
If your soils are quite light textured (i.e. sandy), and the soil test indicates
that soil copper levels are low, chances of getting an economic response to
copper appear to be quite good. However, if your field has at least a loam soil
texture, it appears that the chance of getting a profitable response is quite
low. On soils with a clay loam to clay texture, it would appear the chances
of obtaining an economic response are very slim indeed!

If your soils test in the marginal range of copper supply, insist on getting
your soils re-tested for copper levels at the six to 12 inch and 12 to 24 inch
depths before considering an application of copper fertilizer. If copper levels
decline with depth, application of a copper test strip could be well worth a
try. However, if the levels of copper increase with sampling depth, you have
virtually no chance of getting a profitable response to an application of copper

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:
Part I of this story appeared in the March issue of Top Crop Manager



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