Top Crop Manager

Features Agronomy Fertility and Nutrients
High yield canola uses 40 pounds of phosphorus

Hitting 40 bushels per acre of canola requires good phosphorus fertility.


November 15, 2007
By Bruce Barker

Topics

Hitting that 40 to 50 bushel per acre home-run crop in canola takes some luck,
but today's top growers in western Canada do it. Part of the secret, says Adrian
Johnston, northern Great Plains director with the Potash and Phosphate Institute
(PPI) at Saskatoon, Saskatchewan, is ensuring adequate phosphorus (P) fertility
from a combination of soil and fertilizer P.

"Farmers targetting high yields are using high rates of P," says
Johnston. "If high rates are used, they also look at a balance of nutrients
including nitrogen and sulphur."

The P requirement of canola shows the large nutrient demand of the high protein,
oilseed crop. Johnston says canola uses considerably more P than spring wheat,
for example.

Advertisment

Canola has a total uptake of about 1.5lb P2O5
per bushel in the seed and straw. Harvesting just the seed removes 0.7lb to
0.9lb P2O5 per bushel. A 40 bushel
crop, then, requires 60lb P2O5
per acre and removes 28lb to 36lb P2O5
per acre.

Built for efficient P uptake
Canola seedlings feed off the P in the seed for only one week, compared to wheat,
which can support two weeks of growth from the P in the seed. With such small
seed reserves, canola requires P from an external source, either soil reserves
or added fertilizer, very early in the growing season.

"Canola is very susceptible to early season P deficiency," says Johnston.
"We saw a lot of that in 2004 with the cold soils. The canola really struggled."

Uptake of P by canola is relatively uniform throughout the growing season.
Canola is also particularly suited to extracting large amounts of P from the
soil. Johnston says that canola releases large amounts of organic acids from
an area just behind the root tip. These acids increase the availability of soil
P in a similar way to the release of plant-available P from mined phosphate
rock when treated with sulphuric acid.

Nutrient uptake by canola growth stage
Root surface area is also critical to P uptake, and canola also produces longer
root hairs than other plants. In addition, these root hairs respond to a deficiency
of P by increasing in length and density. The canola plant, then, is able to
explore a greater amount of soil volume to acquire P.

When it comes to fertilizer P, plot studies have shown that canola takes up
a greater proportion of fertilizer P early in the growing season with the role
of fertilizer P declining in proportion to soil P as the season moves on. Canola
also shows the greatest root response to fertilizer P in a P-deficient soil
compared to oats, soybeans and flax when banded 0.5in below the seed at planting.

Early season uptake of fertilizer P by canola is particularly important, with
12 percent of applied fertilizer P found in the canola plant at the first rosette
stage. That compares to one percent for flax, the least efficient in fertilizer-P
recovery.

Soil test research indicates optimum P fertility
Soil test calibration research shows the optimum levels of soil P for high yield.
The research gathers data on soil-P fertility and compares that to yield responses
in canola. On the northern Great Plains, these studies primarily deal with seedrow
and band applications of P. The research shows that canola yield is optimized
when soils have 20ppm to 25ppm (Kelowna test) in the top six inches, which equates
to 40lb to 50lb P per acre.

Johnston says there are several strategies for meeting this high P requirement.
If soil fertility is at these levels, then only continued applications of P
at a rate equal to crop removal is required. For a 40 bushel crop, that would
be 28lb P2O5 per acre. Subsequently,
soil tests should be conducted to ensure that the P fertility level is staying
in the desired range.

However, even with high levels of soil P, early season P must be accessible
to young canola seedlings. For this reason, P placed near the seed at planting
(starter P) has proven effective, especially in cold soils. The response to
starter P is often referred to as the 'pop-up effect' and is marked by improved
leaf and root growth, and seed yield.

If soil testing shows lower than optimum levels of P, then fertilizer P should
be applied at rates recommended in the soil test report.

Johnston says a well-balanced fertility program is also necessary to maximize
yield. He says research has shown that nitrogen (N) alone, or P alone provides
limited canola yield responses. Increasing N rates increased the crop P requirements
and in the absence of supplemental P fertilizer, canola response to N was very
limited. In the research, yields were optimized when the added N was balanced
with 36lb P2O5 per acre.

High rates of fertilizer, though, often cause problems for growers at seeding.
Seed-placed rates are restrictive and handling large volumes while side-banding
can be time-consuming. "A lot of top growers find that if they can't apply
high rates of all the nutrients with the crop at seeding, they look at putting
nutrients down somewhere else in the rotation," explains Johnston. "That
strategy provides an accumulation of nutrients that canola can then use."

That strategy is especially useful for P, since it is immobile within the soil
and will not leach through the soil like N and sulphur. With this strategy,
starter P fertilizer should be applied while seeding canola to ensure the crop
does not run short early in the growing season.

Recognizing that canola requires such high P fertility, Johnston says that
P nutrition must be considered as part of the overall fertility program. And
that will help increase the fertilizer use efficiency of other nutrients as
well. -30-

The Bottom Line
As canola has become the number one cash crop on the prairies, producers
need to squeeze every bushel out of their crop. Although we have been growing
the crop on our farm since the mid 1940s, fertility management is still very
important. We are now working with different varieties with different growth
habits, oil complexes. We now have access to phosphate inoculants and research
in sulphur inoculants and nitrogen fixation in non-pulse plants on the horizon.
Fertilizer utilization research will need to be ongoing to ensure all these
variables are taken into account.
Warren Kaeding, Churchbridge, Saskatchewan.

We've always been told that too much P near the seed can burn the seed and
reduce germination. However, with new drills and openers, this should now be
do-able: I might have a look at this myself.

It's worth remembering the saying: 'The barrel is only as good as the shortest
plank in the barrel'. There are more factors than just how much P there is,
like P availability: we typically apply more P than soil tests in our sandy
soils indicate is needed. We're also careful about seed burn because we have
reduced our canola seed rate to 5lb/ac from 7lb/ac and we want to be sure we
do not burn the seed.
Dave Hegland, Wembley, Alberta.