Features Agronomy Soil
No yield boost from micronutrients in peas

Specialists suggest that rather than making blanket applications of micronutrients,
apply them in test strips where you can assess their real impact on the crop.

In theory, peas should benefit from additional micronutrients and major nutrients
when soils are deficient. Alberta Agriculture, Food and Rural Development (AAFRD)
pulse specialists Mark Olson and Ken Lopetinsky have tried to find whether adding
micronutrients to seed-placed fertilizer has any impact on pea yields.

"For some reason, productivity on the prairies is a long way short of
the maximum potential yield," says Olson. "The average yield in France
is around 72 bushels, and their top yields are 125 to 130 bushels per acre.
We don't know what it is that limits our yields to 60 or 70 percent of that."

Micronutrient deficiencies were one potential cause, especially when greenhouse
studies showed a response to molybdenum and manganese. Over four years, Olson
and Lopetinsky put out field trials across central Alberta, supplying molybdenum,
iron, manganese, zinc, boron and copper, alone or in combinations along with
a complete package of major nutrients. They chose light sandy land where micronutrient
deficiencies are more common and they used both seedrow and foliar applications
at two to six node stages. They grew Espace, a short variety with small green
seed, and the taller Integra, which has large yellow seeds, to highlight varietal
differences.

Over nine site years of the trials where they were able to harvest, providing
micronutrients individually or in combination made a difference just once. At
that site, manganese and zinc produced a small increase in yield.

Ross McKenzie had similar results during the 1980s. The AAFRD crop nutrition
researcher tested micronutrients on various crops and found higher bean yields
at only one site in over 100 site years. But banding two pounds per acre of
boron reduced yields of peas, beans and canola by 10 to 15 percent.

"That doesn't necessarily mean micronutrients are of no value for peas,"
says Olson. "Sometimes our plots with micronutrients looked a little better
than those without, but it didn't show in the yields. But if the benefit of
a micronutrient is to have the crop flower three or four days earlier, that
might allow the crop to mature a few days earlier and enable it to escape damage
from an early frost."

Olson does not advise treating a whole field with micronutrients based on results
of a soil analysis or tissue testing. The availability of micronutrients in
soil is not well understood, he says. Plant and rhizobia uptake of these essential
nutrients is complicated by interactions among them, and with other soil materials.
Sandy soils often have lower levels of plant nutrients including micros. Most
nutrients become less available in cool, wet, high pH or low organic matter
soils, as well as in very acid soils, or peat soils with very high organic matter.

Even tissue testing does not always reflect the nutrient status of plants.
"In my experience, tissue testing is not an exact science," says Olson.
"Nobody really knows the ideal numbers for nutrient levels in leaf tissue.
It has helped me identify some problems, but we use the tests as one of several
clues to a diagnosis. Whole plants are particularly unreliable indicators from
tillering to flag leaf stages.

"Copper, for example, accumulates in older tissues, at least in cereals.
The youngest leaves may be indicators for micronutrients. But the concentration
of copper in the whole plant increases as the deficiency worsens from marginal
to severe. You'd need to look at identical plant parts of the same variety at
identical physiological ages from good and deficient areas to find real differences."

Try test strips if deficiencies suspected
Olson suggests testing micronutrients in a strip the full length of the field,
perhaps even two strips, to ensure all the variations in nutrient availability
across the field are accounted for. Then, watch closely for any differences
in the crop and if possible, use a yield monitor to measure yields with and
without the micronutrients.

"Applying micronutrients is costly, especially if you use foliar applications,
and we've only seen a response in very few fields," says Olson. "Even
where we've been able to measure a response, it's been small. I wouldn't rule
out micronutrients as a cause of problems, but a lot of things can lead to a
crop that doesn't look as good as it should. I'd advise looking at those things
first."

Cool, wet soils, late seeding, inadequate inoculation, poor seed-to-soil contact,
a lack of major nutrients or pests, or a combination of these things could be
at the root of poor growth. Olson advises making sure all the agronomics are
right before looking to micronutrients to improve field pea performance.

McKenzie echoes those thoughts. "Micronutrients play a vital role in crop
nutrition," he says. "But micronutrient fertilizers are often emphasized
far beyond their true significance."

Response to macros not always high, either
Peas do not always respond to major nutrients as might be expected. The uptake
of phosphate by peas and other legumes is by different mechanisms from those
in other crops, says McKenzie. He has not seen a response to fertilizer phosphate
as long as total soil test P levels are greater than 30 pounds per acre. Even
where soil phosphate was below that, he saw a yield benefit at only half his
sites.

Sulphur response has been equally difficult to demonstrate. Even with more
than 40 sites across the province over four years, McKenzie only saw a measurable
benefit at one site in one year.

Researchers are still not sure what it is that keeps prairie yields so far
below the potential of field peas. McKenzie suspects environmental conditions:
soil temperature, air temperatures, low humidity and wind. Olson is more optimistic.
"It may be genetics, some element of nutrition, seed treatments, climate,
fungicides or something else entirely," he says. "We're fairly sure
it's not a lack of micronutrients. We'll keep working to achieve the sort of
yields French growers harvest." -30-