Fertility and Nutrients
Does variable rate fertilizing pay
Beyond the first steps.
November 15, 2007 By Mike Hall
Crop production potential within a single field can vary substantially. Despite
this variability, fertilizer inputs are usually applied as blanket applications.
This often results in the over-application of fertilizer on some portions of
the field. Not only is this uneconomical, it increases nutrient loading of groundwater
and the release of greenhouse gases (like nitrous oxide).
Crop yield potential can vary substantially across a field for a number of
reasons. Usually changes in available soil moisture related to elevation accounts
for much of the variability. But yield potential can also vary on perfectly
flat land in response to other soil properties such as fertility, texture, salinity
and acidity. For many fields in Alberta, varying fertilizer rates so that every
part of a field receives the appropriate amount of fertility would increase
economic returns and address environmental concerns. However, variable rate
fertilizing does not make sense for every field. The purpose of this article
is to show a process which farmers can use to determine the benefits of variable
rate fertilizing on their fields.
The first step towards determining whether variable rate fertilizing makes
economic sense is to generate yield maps. This requires coupling a GPS receiver
with a yield monitor to provide spatial co-ordinates necessary for a computer
software program to create a yield map. The purpose of creating these maps is
to identify regions of variable production. Often farmers are surprised at how
much yield variation exists within fields they considered relatively uniform.
The next step is to scout these regions, soil test them and determine the factors
responsible for the variability in production. Often the yield variation is
based on changes in soil moisture related to elevation. If this is the case,
the variation in yield may be more prominent during a drought than during a
year with adequate precipitation. Such was the case for a field located near
La Crete in the Peace region of northern Alberta.
Figure 1 is a yield map from this field for wheat in 1998 and canola in 1999.
The darker blue areas represent greater crop yields. In 1998, wheat was grown
during a drought and the highest yields were associated with the moister lowland
located at the far east end of the field. In 1999, canola was grown and the
in-season precipitation was good. The difference in yield between the highland
and lowland field positions was negligible. It appears the variation in yield
potential in this field is highly correlated to in-season precipitation. Soil
tests taken from the two field positions revealed that there was more organic
matter present in the lowland (two versus seven percent). However, available
fertility was essentially low regardless of field position.
In conjunction with the North Peace Applied Research Association, an experiment
was conducted on this field in 2003. The objective was to determine the impact
of field position on crop yield response to added fertility. The crop was canola
and the treatments were zero, 30, 60 and 90 pounds per acre of actual N. Other
nutrients were applied generously so as not to be limiting. Each treatment was
30 feet wide and ran the length of the field (one mile) (see photo). The data
was separated between the two field positions and analyzed separately. The response
of canola to added fertility was found to be the same in the lowland and highland
positions (see Figure 2). In other words, there would have been no benefit to
variable application of fertilizer in 2003. This was not a huge surprise since
there had been adequate rainfall and the yield potential did not differ between
The experiment was repeated in 2004 on the same field with some changes. The
crop was oats and the fertility treatments were NPKS applied at 17-5-3-1, 34-9-6-3,
51-14-9-4 and 68-19-13-5 pounds per acre of actual nutrient. There was a severe
drought that year and yield response differed significantly between the highland
and lowland field positions (see Figure 3). The addition of NPKS from 17-5-3-1
to 68-19-13-5 pounds per acre of actual nutrient increased the yield of oats
by 13 bushels on the moister lowland, but by only three bushels on the drier
highland. Depending on the cost of fertilizer and commodity price, variable
rate fertilizing would have made economic sense in 2004.
For this particular field, variable rate fertilizing can provide greater economic
returns during droughty years. Of course, predicting a drought year can be difficult.
We can make some predictions based on soil moisture reserves using a Brown soil
probe. However, most of the yield potential of a crop is still determined by
in-season rainfall. Recently, this region has been going through a dry cycle.
Four out of the last seven years have been particularly dry. Perhaps the wisest
approach for this field is to fertilize assuming a dry season (ie: less fertilizer
on the dry highland positions). If rains do come, more nitrogen can always be
broadcasted on the highland positions.
The use of variable rate fertilizing is not an exact science. The economic
benefit of the approach depends on the field and other factors such as in-season
precipitation and the relative fertilizer cost and commodity price. The approach
is still somewhat of a gamble but over the long-term, variable application of
fertilizer can reduce risk and improve economic returns by improving the efficiency
of fertilizer use. -30-
*Mike Hall is an integrated crop
management specialist for Alberta Agriculture, Food and Rural Development at
The Bottom Line
The key factor in present economic times is measuring the economic
return of each input dollar invested. In years of better financial return, it
would be easier to justify the investment cost of variable rate equipment and
the cost could be spread over a couple of years. Now, with the high cost of
risk and the inability to ensure adequate returns per acre on most commodities,
covering the cost of the equipment would be difficult in most areas.
I believe the benefit of variable rate fertilizer application is applying fertilizer
after a rainfall occurrence. So often, our riskiest most limiting factor in
crop production on the prairies is dry weather at critical times. As the article
states, quite often in a dry cycle we are reluctant to add more nutrients than
what the crop is capable of using. Once there is more soil moisture, we can
feel more confident putting in more inputs. Variable rate application would
allow us to do that more efficiently. Warren Kaeding,
This technology is a great idea, but I wonder about the economics of it. Often,
we do not have the answers to what our problems in fertility are and our biggest
issue is still moisture availability.
We have recently purchased a GPS lightbar which we move from the sprayer to
the seeder and can see how this could eventually progress to yield monitoring
then mapping and variable rate technology. However, it is not for the novice
to jump right in! Dave Hegland, Wembley, Alberta.