Top Crop Manager

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Optimizing nitrogen fertilizer response

Going into the 2007 crop year, growers are a bit more optimistic

November 28, 2007  By Top Crop Manager

Going into the 2007 crop year, growers are a bit more optimistic, as fertilizer
prices are reasonable and grain prices are going up. However, finding ways to
optimize fertilizer response and improve efficiency, particularly nitrogen (N)
fertilizer, remains a priority. Fertilizer use accounts for between 40 and 60
percent of crop yield, and is one of the primary inputs for crop production.
N fertilizer accounts for a major portion of fertilizer inputs.

Optimizing N fertilizer requirements is influenced by many soil, crop, climatic
and management factors. "Growers need to look at the management of their
entire cropping system, and have a good understanding of all aspects of their
system to make the best decisions on any inputs," says Ray Dowbenko, senior
agronomist with Agrium. "It's important to look at all of the variables
first, so you can get the full value out of any inputs applied."

Determining N fertilizer rates is an individual decision, based
on soil tests and yield goals.
Photo Courtesy Of Rick Taillieu, RTL.

Important management tools for optimizing N fertilizer are soil testing and
setting realistic yield goals. "Soil testing is money well invested on
the farm because it provides a baseline of what nutrient levels you are starting
with and what needs to be added to meet target yields," explains Dowbenko.
Soil testing aids in nutrient application decisions by providing information
to use in matching nutrient application rate with producer-set yield goals.
Soil testing provides valuable economic and environmental information.


"Conditions in the fall of 2006 are a good example of why tools such as
soil testing can be so important," says Dowbenko. For example, in some
areas of Manitoba, farmers are finding a few fields with 45 to 60 pounds of
nitrogen being carried over in the top 12 inches of soil. Without soil testing
information, growers could be spending a lot of money on fertilizer they may
not have needed. On the other hand, growers in other areas who assume they are
in the same situation and do not bother with soil testing could find themselves
short of nutrients in the coming crop year.

"We've also looked at a lot of soil tests in the high rainfall areas,
such as the northern fringe of the cropping area," adds Dr. Adrian Johnston,
Northern Great Plains Director, PPI/PPIC. "The soil tests are quite low
because there was a lot of rainfall during the growing season. Although we can
expect to see some mineralization this fall if the weather stays warm, it's
likely those areas will have quite low nitrogen levels going into the next crop

Johnston explains that some growers are also using crop nutrient removal measurements
as a means of estimating crop nutrient requirements. Nutrient removal refers
to only that portion of the crop actually hauled from the field and does not
account for other crop residues, such as leaves, stems and roots, left behind
after harvest.

"For many growers, the yield goals on their farm may exceed those recommended
on a soil test," says Johnston. "They tend to have a better understanding
of how their fields respond to nutrient additions, and often work closely with
industry crop advisors to develop field specific recommendations unique to their
own farm." These farm managers not only look at soil test results, but
also consider crops grown the year prior, and the yield goal of crops planned
for the upcoming year.

Managing fertilizer efficiency is key, remembering to apply fertilizers at
the right time, at the right rate in the right place using the right equipment.
Although spring application at time of seeding would seem to be the most efficient
means of application, in most areas N fertilizer can be applied prior to seeding
if field conditions are carefully assessed. With the early harvest and late
rainfall in 2006, many growers are looking at fall fertilizer as a good option
to be prepared for early seeding next spring.

"With fall fertilization, another important factor is soil temperature,"
explains Dowbenko. The soil temperature should be down to five to eight degrees
C to minimize fertilizer N losses. Researchers like to see soil temperatures
of three degrees C or lower, but that is not always practical. "Putting
fall fertilizer on too early when temperatures are too warm, or when soils become
saturated with water, can cause losses from 20 to as much as 70 percent if conditions
are right. Trying to get ahead and being prepared for seeding early the next
spring is a good idea, just make sure the conditions are right before rushing
out to fall fertilize."

Table 1. Canola N rates based on
N price and canola price, 50bu/ac yield goal.
50bu/ac Canola price $/bu
$5.00 $6.00 $7.00 $8.00
N price $/lb N Recommended nitrogen fertilizer
rate (lb/ac) returns presented based on a grain income : N cost ratio
of $1.00 : $1.00 or $1.25 : $1.00 or
$1.50 : $1.00
0.25 118/113/107 122/117/113 124/121/117 126/123/120
0.30 113/107/100 118/113/107 121/117/112 124/120/115
0.35 109/101/93 114/108/101 118/113/107 121/116/111
0.40 104/96/87 110/103/96 115/109/102 118/113/107
0.45 100/90/80 107/98/90 111/105/97 115/109/103
0.50 95/84/73 103/94/85 108/100/92 112/106/99
0.55 91/79/66 99/89/79 105/97/87 109/102/94
0.60 86/73/59 95/84/73 102/92/83 107/98/90

Dowbenko notes that fertilization and soil testing cannot account for farm
management skills alone and there are other management practices, such as early
seeding, that can have a big impact on yields. "In 2005, we had an early
frost around August 14 in many areas," says Dowbenko. "Late seeded
crops were not mature enough to withstand the frost and losses of 20 to 30 percent
of yields were common. Earlier seeded crops, just five to seven days earlier,
escaped those losses."

Again in 2006, earlier seeded crops out-performed later seeded crops by taking
advantage of early spring moisture. Most areas across the prairies turned very
dry by the end of June. For example, canola crops seeded five to seven days
earlier yielded about five to seven bushels more per acre in a lot of areas.
"By seeding just a few days earlier, combined with optimizing fertilizer
N use and other best practices, growers can maximize returns and improve profitability,"
says Dowbenko.

Implementing all of the beneficial management practices (BMPs) to optimize
fertilizer response is also the same practices that reduce environmental risks
such as nitrate leaching, denitrification, greenhouse gases (N2O)
and phosphates in surface water. "We've been doing a lot of work on BMPs
for fertilizer use and find that if a grower is applying fertilizer, particularly
at seeding time with no-till equipment, using conservation tillage, maintaining
surface residues to minimize runoff and using soil testing to guide their fertilizer
practices, there really isn't much more they can do to improve their system,"
says Johnston. "Those growers who are combining all those practices together
and managing their nutrients for both economic and environmental benefit need
to be congratulated for doing such a good job."

Fertilizer N economics – the challenge
When looking at fertilizer N economics, many growers immediately look at the
cost of N fertilizer and expected grain prices. "However, one of the most
important things growers need to know before making any decisions is their history
of crop yields, and the total number of bushels for any crop they need to break
even on the cost of fertilizer," says Dowbenko. "The reality is that
the influence of grain prices can far outweigh the influence of N fertilizer
prices. There is an opportunity cost to reducing N rates in the face of higher
N prices – you have fewer bushels to sell into potentially higher priced

When calculating the economic rate of fertilizer N use, it is generally accepted
that the recommended rate would be that which generates $1.00 in crop return
for the last $1.00 spent on fertilizer. For some growers this works well, but
others want less risk and more potential return. As a result, they may select
a grain income to N cost ratio such as $1.25 in crop income for every $1.00
or even $1.50 : $1.00 for example.

Table 2. Canola N rates based on
N price and canola price, 30bu/ac yield goal.
30bu/ac Canola price $/bu
$5.00 $6.00 $7.00 $8.00
N price $/lb N Recommended nitrogen fertilizer
rate (lb/ac) returns presented based on a grain income : N cost ratio
of $1.00 : $1.00 or $1.25 : $1.00 or
$1.50 : $1.00
0.25 83/78/72 86/82/78 89/85/82 90/87/84
0.30 78/72/66 83/78/73 86/81/77 88/84/80
0.35 74/67/60 79/74/67 83/78/73 85/81/76
0.40 70/62/54 76/69/62 80/74/68 83/78/73
0.45 66/57/48 72/64/57 77/70/64 80/75/69
0.50 62/52/41 69/61/52 74/67/59 77/71/65
0.55 58/47/35 65/56/47 71/63/55 75/68/61
0.60 53/41/29 62/52/42 68/59/50 72/65/57

Table 1 shows the recommended rate of N for canola grown in an area with a
yield goal of 50bu/ac and residual soil N of 25lb/ac. The three numbers in each
cell of Table 1 show the economic N rate based on a $1.00 : $1.00 rate of return,
$1.25 : $1.00 rate of return and $1.5 : $1.00 rate of return. However, for growers
in areas where canola yields are more commonly 30bu/ac, this is not something
worth considering and Table 2 shows a very different set of recommendations
for lower yield goals.

This data clearly shows that both low crop prices, and high fertilizer N costs,
are going to affect the N rate that provides an economic return. For example,
using $6.00 per bushel canola, N at $0.50 per pound (approximately $500 per
tonne) and a return of $1.25 crop : $1.00 in fertilizer N, we would have a recommended
rate of 61lb/ac N for the 30bu/ac yield goal, and 94lb/ac N for the 50bu/ac
yield goal (see Tables 1 and 2).

Johnston and Dowbenko remind growers, when looking at fertilizer economics
data, to ask for the source of the data. For example, working with 50bu/ac yield
goals, with common yields of 30bu/ac, would be a costly mistake. Also taking
a soil test becomes more important in years of a cost to price squeeze, so growers
should be sure to get an accurate reading on residual soil nutrients. 


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