There is a common misconception that liming causes potato scab. I’d like to dispel that idea. Many growers produce excellent potatoes in high pH soils and don’t have any issues with scab.
When I first started looking into potato scab and its control in the early ‘80s, I was working with Dr. George Lazarovits, who at the time was with the Southern Crop Protection and Food Research Centre (SCPFRC) in London, Ontario. He had done research that determined potato scab was worse in high production, high processing areas where growers tended to use high levels of potassium. That strategy may have given farmers increased yields, but it also threw off the balance between calcium, magnesium and potassium in their fields.
That balance is crucial. Many growers are afraid to lime their potato fields because they believe that doing so will lead to scabby potatoes, but it’s not the lime that’s the problem. It’s the imbalance that’s created when lime is applied to fields without considering how that will affect the potassium-to-magnesium ratio.
We’ve done extensive research into the cause of potato scab and have found no correlation between potato scab and calcium. What we’ve found instead is that there’s more scab in soils that are high in potassium and low in magnesium. In fact, our research shows that low-level liming is beneficial. The best yields, quality and storability come from fields that have adequate amounts of calcium.
To control potato scab, a comprehensive soil test that checks for imbalances is a must. Growers then need to take steps to balance fertility and restore soil health. As soil health improves, so does the overall quality of the potato crop, resulting in optimal microbial activity in the rhizosphere, suppression of soil-borne diseases and significant reductions in scab.
That’s why we recommend low-level liming. It keeps your calcium inputs low while you adjust your magnesium levels to restore the potassium-magnesium balance. Once you’ve done that, you can bump up your calcium without fear of producing scabby potatoes.
We’ve used this approach with potato growers around the world for the past 25 years and have seen excellent results. And yet I still find that growers sometimes have a hard time believing that the solution could be that simple.
I once worked with a Prince Edward Island producer who was having a terrible time with potato scab in his field. He wasn’t at all convinced that a potassium-magnesium imbalance was to blame and was adamant that liming would make the situation worse. I managed to convince him to apply 600 pounds of sulfate of potash magnesia (K-Mag) right up the middle of his field. At harvest, the strip where he’d applied the product saw a huge increase in yields with virtually no scab. On either side of that strip, where no product had been applied, yields were low and the potatoes were scabby. One simple treatment was all that was needed.
If you’d like further information on how to mitigate potato scab through balanced fertility, please reach out to Greg Patterson by contacting the lab at [email protected] or 1-(855)-837-8347.
