Mineralization | While weather brought down 2012 yields, growers should see long-term benefits
Canola growers who fertilized last year in anticipation of a 50 bushel crop, only to watch disease and wind diminish yields, shouldn’t despair.
Jeff Schoenau, a soil science professor at the University of Sask-atchewan, says they should get use out of those inputs in the year ahead depending on how much mineralization takes place in the soil.
Schoenau told the first Grain Expo held during Canadian Western Agribition in Regina Nov. 21 that it’s hard to predict how much of the nitrogen will be recovered at any one time.
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“You can expect to get some of that nitrogen in the straw, and especially those canola seeds that are laying on the ground,” he said.
Micro-organisms in the soil are responsible for breaking down organic matter and turning the nutrients into plant available forms. This process of mineralization depends largely on the weather. The colder it is, the less activity there is.
Schoenau estimated 20 to 30 percent of the nitrogen left behind would be available in the first year as a result of mineralization. Eventually it will all become available, but growers might consider applying nitrogen this spring.
Sulfur and phosphorus will be available very quickly, he said.
“That’s because typically about half of the sulfur and phosphorus in crop residues is in a water-soluble form, meaning that it readily leaches out of that straw,” he said.
Potassium will be available almost immediately because it isn’t bound in organic matter.
“The toughest one to get out of there is going to be nitrogen,” Schoenau said.
Environmental conditions will be the biggest determinant.
“Sometimes we see unexpectedly high mineralization because there’s been a series of wet-dry cycles that’s enhanced that release of available N,” he said.
Freeze-thaw cycles can also in-crease mineralization, which is usually what happens in most spring seasons.
Mineralization slows in dry summers but continues in wet summers, which results in unexpectedly high protein levels, Schoenau said.
Measuring just how much nitrogen is provided by mineralization is not easy.
Schoenau estimated that as little as 10 to 15 pounds per acre could be released in the top 15 centimetres of soil, but it could also be as high as 50 lb.
“I’ve seen even amounts as high as 80 under some circumstances,” he said.
“If you think of canola that requires 120 lb. of nitrogen per acre, this mineralization process supplies as much as half or even more of what that crop needs.”
For sulfur, Schoenau estimated that one to three percent of the nutrient in the soil, or five to 15 lb. per acre, is converted for crop use.
Modern hybrid canola varieties with high yield potential require 30 to 40 lb. of sulfur per acre, and farmers will need to add fertilizer to meet those requirements.
Schoenau said a good rule of thumb is that high soil organic matter content generally results in high mineralization.
Farmers who are considering renting or buying land should measure its organic matter content to get an idea of how much to expect from mineralization, he said.
Mineralization is also greater in the black soil zone, which has more organic matter, compared to the brown soil zones.
However, there can be differences within fields.
Schoenau has done research on his own farm near Central Butte, Sask., in the brown soil zone, in which he looked at nitrogen gain over a fallow period on an undulating field with knolls and low slopes.
“A fallow period kind of gives you an idea of mineralization because without any plants growing, any nitrogen that’s released from that organic matter converted into available form usually simply accumulates there as nitrates,” he said.
The nitrogen gain ranged from three lb. per acre at the high points to 26 lb. per acre in the low spots.
However, there are always exceptions.
Schoenau said the wet conditions of 2010 and 2011 that flooded low points in fields actually slowed mineralization and increased nitrogen loss.
He also said more work needs to be done to develop more accurate, reliable ways to measure how much they can expect from mineralization.
