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Wet soil and nitrogen don’t mix, says expert

Annoyed that you have to fork over more cash to replace lost fertilizer nutrients from last year’s flood?

It could be worse, says Tom Jensen, Northern Great Plains regional director of the International Plant Nutrition Institute.

On Manitoba’s relatively youthful soils deposited by glacial activity about 9,000 years ago, potassium is abundant. Farmers here apply not more than a dozen pounds of potash per acre each year to supply crop needs, if at all.

But on the banana plantations of Ecuador, where the soil is old and tropical rains every year cause heavy leaching, farmers have to put on 600 pounds per acre, per year.

“If they don’t, they don’t produce any bananas. That’s why bananas are a good source of potassium source for us,” said Jensen, to bemused laughter from the assembled farmers at a recent Excess Moisture Meeting organized by the Canola Council of Canada.

Last year’s floods robbed farmers not only of their crops, but also soluble nutrients, mainly nitrogen and to a lesser extent, sulphur.

Some nutrients may have been lost due to erosion from run-off and leaching down through the root zone, but the biggest losses — up to 50 per cent — came in the form of the nitrogen returning to the atmosphere via denitrification.

“We used all this natural gas to make fertilizer, and if the rains come at the wrong time, it ends up going back where it came from,” said Jensen.

That’s bad enough. What makes it worse, is that some of it goes home in the form of nitrous oxide (N2O), a potent greenhouse gas.

In saturated soils, water drives out the oxygen in between the soil particles. The countless trillions of microbes that live there are then forced to seek out other sources of oxygen, and they do that mainly by stripping the “O” out of the nitrate (NO3) to make N2O, or out of the sulphate to make H2S, or hydrogen sulphide.

Under normal soil conditions, UAN gives an immediate boost to crops via its nitrate component. But if excess moisture appears, nitrate losses may be significant, although the urea and ammonium content has greater staying power. For this reason, ammonium-based fertilizers such as urea or anhydrous ammonia may be better choices where wet conditions are expected.

The good news

“We don’t have gaseous losses of phosphorus and potassium. It’s hard to lose P and K from our soils,” said Jensen. Of all the P put on soils over the years, some 90 per cent gets converted eventually into crops.

If elemental sulphur was applied, losses may not be that great under wet conditions because it must first be converted to sulphate by microbes before gaseous losses or leaching can occur. However, on sandy soils leaching can be a problem and deficiencies early in the season are common until the roots can reach down for it.

“Forced summerfallowing” occurred over much of the province last year. Some compensating gains may be found from the slow effect of microbial mineralization of organic matter, he added. On the other hand, where weeds were allowed to flourish, nitrogen losses to weeds may be significant until the residues are cycled back into the soil in coming years.

Nitrogen losses will depend on when the nutrient was applied, he said. Fall applications likely suffered more than spring.

If soil tests show lots of N at 18-24 inches of depth, don’t despair, because the roots of future crops can generally tap reserves as deep as four feet.

“Below four feet, it’s lost unless you grow sunflowers or sugar beets which can reach down six feet, or alfalfa. It can root down to 12 feet,” said Jensen.

On sandy soils, soil testing should be done down to four feet. Not going that far can lead to strange results.

In one case, a hobby farmer in Alberta bought an acreage that had been in alfalfa for years, then summerfallowed. Heavy rains leached the N down deep, but he sampled only the zero- to six-inch zone. The results recommended 90 pounds of N for the oat crop he planned to grow.

Post-harvest, a feed test found dangerous nitrate levels in the crop, and he ended up not being able to use or sell the greenfeed oats he baled off it, he said.

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