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Fine-tuning foliar applications

Foliar applications of fertilizer, particularly micronutrients, can have a positive effect on the crop, but using them can also be an exercise in futility.

April 7, 2009  By Top Crop Manager

Foliar applications to improve potato crops can be a gamble since only small amounts are needed for improvement.


Foliar applications of fertilizer, particularly micronutrients, can
have a positive effect on the crop, but using them can also be an
exercise in futility. The challenge is knowing when to apply foliar and
when not. Certified crop advisors advocate confirming the necessity of
a foliar application with a tissue test because that is the only way to
know for sure if the plant is lacking nutrients. However, a better plan
would use a soil sample to set up a soil and foliar program and then
monitor its progress using tissue analysis, suggests one certified crop

John Lightle of Holmes Agro in Orangeville, Ontario, conducted some
field tests in 2008 using foliar applications to demonstrate the
effectiveness of the treatments. He used fields at four different
farms, putting a foliar spray on one portion of the field and leaving
another portion without. What he learned was that the foliar treatment
was not always helpful. In fact, in two fields the foliar treated
section did not have as high a yield as the check section. In the other
two fields the yield improvement was less than five percent.


“Foliar applications can be used at the critical time of plant
development when the demand for specific nutrients is higher,” Lightle
explains. “Foliar applications are a good way to apply micronutrients
because only small amounts are required and the plant can absorb it.”
He says that boron deficiencies can cause hollow heart, so a foliar
application of boron could prevent that. However, he stresses again
that growers need to know their soils and the individual variety
responses to micronutrients.

Another certified crop advisor in Mount Vernon, Washington, says his
company, Ag Tech Services, only recommends foliar applications after a
petiole analysis. “Each variety has a standard regarding what amount of
nutrient should be present in the plant,” explains Rudy Allen. “In the
case of phosphorus, you don’t want the level to drop below 0.2 percent
and, if it does, then a foliar application might be helpful. Phosphorus
will translocate, so it will get into the plant’s system.” He admits
that if tests indicate that phosphorus levels are low then a foliar
will be necessary to “help the plant out.”

Allen says that products with an acid component to them will allow
nutrition into the plants quicker than a product with neutral or high
pH components. He says the petiole test will indicate what the analysis
is and, if a foliar application is deemed necessary, he will take
another tissue analysis a couple of weeks after the application to see
if it was successful.

“Micronutrients will often have an impact in a foliar application,”
Allen continues. “But, again, you need to make sure the product is
acidic.” He says the micronutrient needs of the plant can be small, but
necessary. For example, he says, the lack of soluble calcium can
contribute to hollow heart, but calcium does not translocate, so a
foliar application of calcium would likely not be effective.

Using foliar applications to improve potato crops is somewhat of a
gamble as only small amounts may be necessary to cause improvement.
Without a tissue or petiole test, it is difficult to determine which
nutrients might benefit the plant. It can be a problem determining how
much of the formulation to use given that not all nutrients are
absorbed into the plant and at the same rate.  Both Allen and Lightle
believe that some of the problems that can appear in the crop during
the season could be minimized by applying balanced nutrition prior to
planting based on the recommendations of soil tests for all nutrients.

“Certainly foliar applications are necessary and beneficial under the
right conditions and in the right formulation,” admits Allen. However,
he points out, only careful analysis will determine if the benefit can
be achieved. “Using a ‘just in case’ approach just isn’t affordable any
more,” he adds.

“If you know your soils, do your sampling and apply balanced nutrition
in the fall, you will likely be okay,” says Lightle. “Foliar is only a
supplement and cannot be used successfully to correct problems.”

As Lightle’s in-field tests showed, an average six percent yield
reduction occurred in the foliar treated field section over the
untreated portion of the field. While the foliar did not cause the
reduction, the tests show that without it, the crop did just fine.
Therefore, if growers believe a foliar application is necessary, it
would be best to base that belief on the results of soil sampling and a
tissue monitoring program rather than a visual scan of the field. As
Lightle says, sometimes what appears to be a crop deficient in
potassium could be a crop that is suffering through a spell of cold
weather. Tissue tests can be misinterpreted, he adds, and nutrient
levels will vary in a plant depending on weather conditions, so it is
best to include tissue tests as part of a full program while monitoring
the crop’s progress throughout the season.

Getting nutrition right is often as much about consultation with
agronomists or crop advisors, soil and tissue test results, proper
application and years of experience as it is about a quick foliar fix.
Deficiencies can be corrected with a foliar application, according to
Lightle, but if a tissue test is being used to verify it, yield may
already be lost. Therefore, using foliar applications should be part of
an overall plan and not just as a quick fix.


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