Fertility and Nutrients
Getting the most out of fertilizer
By Bruce Barker
Fertilizer best management practices.
Does the term best, or beneficial, management practices for nutrient use sound
familiar? If so, it was likely associated with manure management. While manure
is applied to two to five percent of lands on the prairies, what about fertilizer
management when it is used on 60 to 70 percent of the seeded fields?
"We came up with fertilizer BMPs for fertilizer application during all
the discussions around manure BMPs. We realized that we really didn't have a
document that summarized fertilizer BMPs," says Adrian Johnston, Northern
Great Plains Region Director with the Potash and Phosphate Institute of Canada
(PPIC) at Saskatoon, Saskatchewan.
Johnston says that BMPs need to focus on field specific recommendations that
growers can implement, since soils, environment and productivity can vary from
field to field, farm to farm and region to region.
Matching nutrient supply with crop requirements
One of most important BMPs is to ensure that crop nutrients are supplied at
a level that meets crop growth, but does not over or under-supply nutrients
to the land. That is easier said than done, since crop response to fertilizer
is generally governed by moisture. If growers predict the weather accurately,
they can more accurately apply fertilizer to meet crop needs. However, Johnston
says there are tools available to guide optimum fertilization.
Soil testing is the main science-based tool available for estimating soil nutrient
supply on agricultural lands. Soil test recommendations are based on how a particular
crop responds to a nutrient, using the average response from a multi-year and
multi-site data set. Given that a number of non-fertility factors impact final
crop yield (environmental conditions, pests and so on), Johnston says it is
important to remember that fertilizer recommendations based on correlation with
a field response database may account for only 50 percent of the yield variation
in the field. For this reason, fertilizer recommendations are often made based
on yield potential, a reflection of soil water at seeding, and past management
conditions for a specific field.
Periodic soil testing helps gauge nutrient sustainability for crop production.
Record keeping that includes prior soil test results, fertilizer and manure
applications, and crop nutrient removal will help indicate whether soil fertility
is increasing, decreasing or remaining the same.
Johnston says that soil testing for phosphorus (P), potassium (K) and soil
characteristics like pH and organic matter can take place every three to five
years at the zero to six inch depth. For nitrogen (N), he says sampling more
frequently to a depth of 24 inches is recommended, ideally, every year. Nitrogen
sampling becomes especially important in years following abnormal growing conditions,
such as in the drought years of 2002/03, or high yield years such as 2005.
|Table 1. Effect of landscape position on wheat grain yield
response to time of N banded application in Manitoba.
|Grain yield (bu/ac)|
|Early fall (mid September)||37.9||29.1|
|Mid fall (early October)||38.0||28.7|
|Late fall (mid October)||37.2||32.4|
|Spring (at seeding)||36.7||32.5|
|Early fall + inhibitors||36.7||29.4|
|Check (no N)||28.8||18.5|
|Source: Tiessen et al. 2005. Can. J. Soil Sci. 85:649-666.|
Understanding the soil test laboratory's recommendation philosophy is also
important. The big variable is estimating how much mineralized nitrogen might
be released from soil organic matter. Some laboratories do not take mineralized
N into account, while others provide estimates of how much mineralized N could
be released from the soil in a normal growing year. "Some labs take soil
test N and tell you how much N fertilizer to add based on the total N required
for your set yield goal," explains Johnston. "It's a very good idea
for growers to know the soil testing lab's fertilizer recommendation philosophy,
so they can understand how to apply those recommendations on their farm."
A benefit of well-managed no-till fields is the build up of soil organic matter
and the potential for higher mineralized N. While growers may not want to depend
on that N source for achieving target yields, it does allow a bank of organic
N that can be drawn on when growing conditions are above average. For growers
in the Black soil zone with high organic matter content, that means a 55 bushel
wheat crop might be grown when a 40 bushel fertility program was planned.
"When you get rain and warm temperatures, there will be a flush of mineralized
N that will contribute to higher yield, and possibly protein," explains
Johnston. He cautions, though, that the nitrogen will need to be replaced in
subsequent years, or the soil will be mined slowly of organic nitrogen supply
and mineralized N capability.
Establish realistic yield goals
Establishing realistic yield goals also becomes critical to developing fertilizer
recommendations. A common approach has been to select a target somewhere between
an above average crop and a past maximum yield obtained on that specific field.
Another way is to set a target 10 percent above the three to five year yield
average for crops not suffering from severe drought or pests. While these targets
may be short of maximum yield, they do provide a grower with the opportunity
to manage fertility programs for high yielding crops.
"Farmers may want to temper those above-average yield targets depending
on the year. Economics can play a role in deciding how much fertilizer to apply,
and soil moisture would as well," explains Johnston.
Another critical factor in establishing yield goals is making sure that the
goals are field specific. Johnston says the yield goal should be based on the
field's ability to produce, not the farm's. In addition, he says that setting
high yield goals for fertilizer application must also go hand-in-hand with optimum
management of other agronomic practices. There is no use putting on an optimum
fertilizer rate if a wheat crop is seeded in early June, because maximum yields
most commonly come from early May seeded crops.
Johnston says there are a number of situations where crop advisors and farmers
find they can make fairly good estimates of crop nutrient requirements based
on what was grown and what was applied in a specific field. Information such
as crop yield, grain protein concentration and straw management can all be used
to establish the status of a nutrient such as N. For P and K, the year-to-year
variation in plant-available supply is minor, and annual application based on
a balance between soil test levels and crop requirements can avoid depletion
This approach to a balanced nutrient budget, cautions Johnston, is in no way
an appropriate replacement for soil testing, given the absolute need to use
soil testing to establish a nutrient supply starting point. Often this type
of input/removal assessment is carried out in the years between which comprehensive
soil sampling is conducted.
|Table 2. Winter wheat response to fertilizer
N and P.
|N rate (lb/ac)||–|
|Grain yield (bu/ac)||Pounds grain/lb N|
|Source: Grant et al. 1985. Can. J.
Soil Sci. 65:621-628.
Apply a balanced fertility program
Liebig's Law of the Minimum states that the yield of a crop will be determined
by the element present in the most limiting quantity. In other words, says Johnston,
the deficiency of one nutrient cannot be overcome by the excess of another.
For example, a winter wheat study conducted at Agriculture and Agri-Food Canada
at Brandon, Manitoba, found that while application of N alone increased yields
more than P alone, it was the balance of N + P that optimized the crop response.
To maximize yields using the highest rate of N, the highest rate of P was also
required. Similar examples can be shown with N and sulphur (S) on canola.
Fertilizer placement also improves fertilizer use efficiency while reducing
losses to the environment. The mobility of a nutrient in the soil plays the
biggest role in fertilizer BMPs. For example, low mobility of P in calcareous
soils means that short-term crop utilization of the P is improved considerably
when it is placed close to the germinating seed.
Placement can also be a powerful management tool to minimize N losses, says
Johnston. Under ideal conditions, the goal is to apply the N so that it is in
the plant-available form and in close proximity to the plant roots. Deep banding
of fertilizer N is a very important BMP, widely used on the prairies. It has
been shown to reduce per-unit production costs by increasing fertilizer efficiency.
A seeding system also plays an important role on the impact of fertilizer placement.
When incorporated with tillage, barley showed a similar response to broadcast
and in-soil band application of nitrogen. However, when the broadcast urea is
applied onto the residue-covered surface of a zero-tillage field and not incorporated,
grain yield was reduced by 31 percent relative to an in-soil band.
Time of application
Timing fertilizer applications to provide a plant-available supply of nutrients
when the crop needs them is another BMP. Plants subject to a deficiency during
specific periods of growth may not recover to achieve full yield potential.
Johnston says the research showed the impact of an early season P deficiency
on wheat, where an absence of adequate P during the first four to six weeks
of growth, limited tiller and root formation by the crop. Even when the P deficiency
was corrected after four to six weeks, the negative impact on the crop had already
occurred, illustrating the critical early season response.
Application timing can play a critical role in optimizing crop nutrient response,
especially where fertilizers are subject to transformation in the soil. Nitrogen
is likely the nutrient that is influenced most by soil moisture and temperature
conditions. A University of Manitoba project on heavy clay soils in the province
found that fall N application timing had less of an impact on crop yield response
in well drained upland landscape positions than water accumulating in lowland
areas. Even though all of the urea N treatments were banded in this study, delaying
the fall N application timing improved the crop response with the wetter soil
conditions in the lowland areas of the field.
Johnston says that implementing these fertilizer BMPs does not need to be difficult
or expensive. In fact, many producers are meeting and exceeding these basic
management practices already which have proven to improve nutrient use efficiency.
In other cases, growers simply require a modification of application timing
or application of knowledge-based recommendations. In the end, the use of fertilizer
BMPs will result in greater fertilizer use efficiencies, reduced environmental
impact and ultimately, optimum yield and crop quality. -30-
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