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Dark French Fries And Salinity Go Hand In Hand

“It dictates that you just can’t grow potatoes in those soils.”


You know there’s something wrong with a potato if the tip of a french fry turns dark when it hits the oil.

It’s a phenomenon potato processors hate: a disorder called “sugar-end defect.” The sugar caramelizes, leaving the tip dark and unappealing. Also known as “dark ends,” it’s caused by plant stress during the growing season which predisposes potato tubers to accumulate excess sugar during storage.

French fry processors rate sugar-end defect as one of their main problems because it downgrades the quality of their finished product.

So it was a relief to Manitoba processing potato growers when a recent study offered a solution to it.

The three-year study funded by Keystone Potato Producers Association, Simplot Foods and McCain Foods found that maintaining available soil water (ASW) above 65 per cent reduced plant stress and lowered the incidence of sugar-end defect.


There was only one problem. Some commercial potato growers around Winkler and Portage la Prairie found maintaining ASW above 65 per cent still did not lower sugar end defect to an acceptable level.

There had to be another explanation. And there was. It was soil salinity. Credit Blair Geisel for discovering it.

Between 2008 and 2009, Geisel, president of Gaia Consulting Ltd., a private agronomic research company, took extensive soil samples from 128 locations in eight fields within the problem areas. He also harvested tubers to determine their quality and yield.

He analyzed the soil samples to determine their salinity and nutrient content. Then he did a statistical analysis comparing saline and non-saline areas to see if there was a relationship between soil factors and yield.

And there was. A strong relationship, too.


Geisel found that as soil salinity increased, yields went down and the incidence of sugar-end defect went up.

Besides explaining the high rate of sugar-end defect, the presence of salinity also helped to explain another factor. The potato land in question contained an excessive buildup of nitrogen. There was a positive correlation between residual nitrogen levels and sugar-end defect.

The reason? Producers were applying nitrogen fertilizer at a recommended crop removal rate. But the crop wasn’t removing it because soil salinity inhibited plant growth.

The fact that they had salinity wasn’t a big surprise to the producers in Geisel’s study. Although not a major problem in Manitoba, saline soils do occur, especially in the Winkler area, a major potato-producing region.

But the growers were surprised at the impact salinity had on yield and quality. Geisel said some saline areas produced fewer than 100 cwt of potatoes per acre – far below a producer’s usual break-even point of 230 to 250 cwt/acre.


Unfortunately, there is no easy solution to salinity. Salts in the soil are difficult to remove.

Sometimes saline areas can be improved with tile drainage. But Geisel said the best way to avoid poor production on saline soils is to avoid crops with low tolerance to salinity. That especially includes potatoes, as well as beans, which are often in the same rotation.

“It dictates that you just can’t grow potatoes in those soils,” said Geisel.

His take-home message is that farmers should select potato fields carefully. If portions of those fields are saline, they should select other crops to grow there.

In the case of Geisel’s co-operators, several of them changed their field selection and altered their crop rotations. They planted potatoes in places where salinity was low and salt-tolerant crops where salinity was high. Or they just looked for alternative fields to plant potatoes. It was more work but it was effective, Geisel said.

Times are tough these days for potato farmers because of oversupply, falling consumer demand and low prices. For that reason, growers need every edge they can find. That includes checking fields for salinity, said Geisel.

“They have to have good yields to achieve a profit margin. It’s hard to achieve on saline soils.” [email protected]

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