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Fertility damage control after heavy rains

What do you do about potential nitrogen loss?


November 27, 2007
By Rosalie I. Tennison

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June was a rainy month in many parts of western Canada in 2005. In the Lethbridge
area alone, 12 inches of rain fell in June, which is almost the annual expected
rainfall. In Manitoba, 25 percent of the land did not get seeded because the
soil was too wet. For farmers fortunate enough to get a crop in the ground,
the rain may have caused nitrogen losses, especially if the fertilizer had been
put on in the fall. For the growers who waited until spring to fertilize, there
may have been little they could do to ensure their potential crop got the fertility
it needed. There are solutions to this challenge; in fact, some would just call
it 'damage control'.

"Let's start back on June 1 when the soil temperature was warming up,
the fertilizer was applied, the crop was starting to emerge and then the rain
came," begins Ross McKenzie, an agronomic research scientist with Alberta
Agriculture, Food and Rural Development in Lethbridge. "The rain saturated
the soil and the crop turned yellow. There was concern that the problem was
nitrogen loss, but, in fact, it was lack of oxygen reaching the plant. Nitrogen
loss comes much later."

McKenzie continues describing the scenario by pointing out that if the rain
had stopped at a reasonable time and the soil dried, the plant would likely
recover, but in many areas that was not the case and fertility loss eventually
occurred. He qualifies his description by adding that in sandy soils, nitrate
leaching can be a problem much earlier than in heavier soils.

After a few days without oxygen, bacteria in the soil will start stripping
oxygen from the nitrate. The process, called denitrification, becomes a serious
issue when soils are warm and have been saturated for a number of days. This
was a significant problem in June of 2005, according to McKenzie. "It's
still possible to do fertility corrections in June," he reassures growers.
"Until this year, the best option was to do an in-crop broadcast application
using ammonium nitrate (34-0-0). It was by far the best product to use to quickly
apply N to a growing crop and not have to worry about N losses or availability.
However, as of June 2005, the product is no longer manufactured in Canada."

There are other options, he continues, but more management may be needed. "You
can broadcast urea (46-0-0) and, if you get rain within a few days, the plants
will get the fertility they need over the next few weeks. But, if no rain comes
to carry the nitrogen down to the roots of the plant and the soil remains moist,
you can get another problem of volatilization of the urea nitrogen fertilizer
into the air as ammonia gas." Using a urease inhibitor will slow down the
conversion to ammonia and volatilization for 10 to 12 days, but the soil still
has to be dry enough to accomplish a broadcast operation.

McKenzie says UAN (28-0-0) dribble banded on the soil either with or without
an urease inhibitor is also a possibility. A quarter of the product is already
in a nitrate form and a quarter is in the ammonium form, which are not subject
to volatilization. The urea in UAN is subject to volatilization. He says ammonium
sulphate (21-0-24) would work well, too, particularly if both nitrate and sulphate
leaching is a concern. The ammonium in the fertilizer is not subject to volatilization
losses. The downside is a high rate of applied sulphur which may not be needed.
In areas where there is irrigation, fertigation is an option, but if the soil
is already very wet more moisture is being added unnecessarily.

For many growers knowing when a fertility problem is occurring, or about to
occur is like playing pin the tail on the donkey – if you are lucky, you
get close to the target. McKenzie suggests consulting an agronomist from the
local fertilizer retailer, and even taking a soil sample to confirm your suspicions.
"If a farmer has doubts, a confirmation from a soil test taken to the depth
of two feet is wise," he says. "In 48 hours, the farmer will have
a confirmation of the crop's nitrogen needs. It's always a good idea to double
check and get a second opinion."

In the future, problems could be reduced by using a coated urea product, according
to McKenzie, although the availability of this option could still be months
away. The coated product promises to release slowly over a 30 to 50 day period.
This would reduce or prevent denitrification losses where the soil was saturated.

There is no 'quick fix' for nitrogen loss, admits McKenzie. In fact, all a
grower can hope to achieve is damage control or loss reduction, if the rain
will not stop. "In wet conditions, loss is always possible," he says.

The best advice he can give growers is to try to minimize denitrification and
leaching problems when the soil is saturated for a long period. Getting good
advice with the diagnosis of a fertility problem and choosing the correct product
to solve it is a wise move. If the wrong product is chosen, you may be adding
to your woes with volatilization.

It is a tough decision to make if, like for many growers in spring 2005, a
crop is in the ground and excessive rain threatens its success. Before writing
off such a crop, get advice, consider solutions and take action.