Cultivation/Production, Events, North America, Research, Studies/Reports

Agronomist shares insights on specific gravity of processing potatoes

Image result for Ryan Barrett, PEI Potato BoardDuring last week’s International Potato Technology Expo held on Prince Edward Island (PEI) in Canada, the Research Coordinator/Agronomy Lead at the PEI Potato Board, Ryan Barrett, presented on Factors Influencing Specific Gravity in Prince Edward Island Potato Production. “In recent years, achieving higher specific gravity scores has been a priority for PEI processing potato growers,” Barrett said. “Increased value has been placed on gravity/total solids by processing customers, and the shorter growing season in Prince Edward Island can have a negative impact on gravity scores.  Therefore, a priority project through the industry’s Agronomy Initiative for Marketable yield (AIM) has been to identify factors in Prince Edward Island identified with better specific gravity scores.” Examining scientific literature reinforces the fact that any cultural practice associated with maximizing the ability of plants to complete their natural life cycle will favour specific gravity. 

Planting early, using high quality seed of adequate size, and having healthy soils with lots of water holding capacity (high soil organic matter and low compaction) will enable the crop to emerge quickly and stay healthy through the growing season.

“This type of crop will then be ready to shut down naturally in the fall, allowing the plants to move sugars to the tubers efficiently without excess movement of water to the tubers”.  Preliminary examination of grower yield and quality data from the last few years does not show any relationship between yield and specific gravity, and there are a number of top grower achieving top scores for both 10 oz size bonus and specific gravity on their processing contracts. “There are some specific management practices that have significant impact on specific gravity in PEI,” Barrett said.

  • Nitrogen rate.  Higher than needed rates of N can lead to excessive vegetative growth, delaying vine maturity and senescence and lowering specific gravity, as well as potentially reducing both yield and size profile of tubers.  Growers should ensure that they are applying only the necessary amount of N for their crop, and look at split application of N where possible.
  • Heavy use of vine dessicants such as diquat (Reglone) are also associated with low specific gravity.  Grower Nathan Ching from Black Pond Farms in Souris talked about on-farm trials he performed that showed that one half-rate application of Reglone resulted in much higher specific gravity than two half-rate applications in the same field.  Allowing potato plants to die as naturally as possible is associated with higher gravities.
  • Numerous studies both in PEI and elsewhere in North America have shown that heavy use of KCl (muriate of potatsh) banded at planting is associated with depressed specific gravities.  In recent years, a number of Island growers have turned to spreading a higher percentage of their potassium in the fall before potatoes or as a pre-plant fertilizer application.  In addition, a number of growers have also replaced KCl in the planter blend with other lower-salt index products such as sulfate of potash or K-Mag.  Andrew Lawless of R&L Farms in Kinkora talked about changes he made in 2017 to reduce the amount of K banded at planting as well as reducing the amount of KCl in the planter blend.  This resulted in substantial improvements in specific gravity scores across four different varieties.

Ryan Barrett’s presentation can be downloaded as a pdf file. He can be reached at for further information.

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