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Heat, sun and moisture – striking a balance

Too much, too little or just enough?

November 12, 2007
By Ralph Pearce


When it comes to the elements, there may be no truer statement than, 'Everything
in moderation'. To some, too much of a good thing can be more harmful than too

Weather conditions provide multiple stresses, some being extreme, at varying
points of the growing season. But the question that comes to mind is, which
is more important to crop growth and development: heat, sun or moisture? For
Dr. Dave Hooker of Ridgetown College, the answer comes by shifting the consideration
of the factors: heat, sun and moisture from 'requirements' to what factor is
'limiting' or a stress factor. From there, the question he poses is, Which stress
is more harmful? For Hooker, the key for assessing limitations to crop growth
and development is to acknowledge the impact of different stresses. "And
the impact of the stress depends on the magnitude of the stress, the presence
and magnitude of other stresses and the stage of crop development," he

6aOne idea that he and other extension personnel have tried to convey in presentations
is that the effect of one stress factor cannot be evaluated without taking other
stress factors into consideration. For instance, says Hooker, during the seedling
stage, cool temperatures limit growth and development, but the seedling's development
also depends on whether the ground is saturated or excessively dry. Moreover,
it may not be an additive impact of the two, like some mathematical equation.
"If cool temperatures reduced crop development or crop growth by 10 percent,
and saturated soil comes in at the same percentage of impact, even though they
might add up to 20 percent, it might be a 30 percent drop, and we call that
a synergistic effect," says Hooker. "But the key point is that the
impact of one stress depends on the presence of other stresses."

Time of growing season also important
Hooker agrees that the particular stage of crop development is also a key. Warm
soils and adequate moisture is the desired combination at planting, warm and
dry with timely rains is best for the prime growing months while clear skies
and warm temperatures are best at harvest. Hooker maintains that as a stress,
moisture is the most important throughout the year. "Moisture is at the
top of my list; either too little or too much, it's the most common and influential
factor reducing crop yield," he says. "Of all of them, it's the hardest
stress to overcome."

Albert Tenuta tends to agree with that assessment. As field crop pathologist
with the Ontario Ministry of Agriculture and Food at Ridgetown College, he acknowledges
the impact of multiple stresses hitting crops from 1999 to 2004. "The case
can be made that the profile of soybeans has changed to where we have far more
choice in varieties and how to grow and market them, beyond the parameters of
agronomics and scouting," says Tenuta, adding that such time-sensitive
issues might shift focus away from monitoring stress. "We can make all
of our decisions on what we think will happen according to weed pressure, disease
pressure, insect pressure, but ultimately how the weather picture in a year
unfolds is the great unknown factor in all of this."

All growers can do, suggests Tenuta, is minimize or manage potential risks
that are controllable. "There are so many other risks we can't manage,
and when you start compounding them on top of other things, it's even more of
an issue," he says.

A definitive answer
As for a clear cut, take-home message on heat, sun and moisture, Tenuta echoes
Hooker in that there is no definitive answer that covers the entire year. In
2004, with its cool, wet planting and growing seasons, the answer might have
appeared obvious: poor yields were going to result from cool temperatures and
excessive moisture. "But as we found out, it didn't take a lot of warmer
temperatures and better growing conditions towards the end of August and September
to make a difference in many cases," says Tenuta. The key is taking things
in stages. "In the early season, cold and wet are the most important stresses
that have the greatest impact on planting and emergence, resulting in poor stands
and weakened seedlings. As the plant progresses and the plants are growing,
moisture likely becomes the most limiting factor."