The potential dual role of liming in potatoes

Fri 16 October 2026

Liming, far from being a no-no before potato crops, could play an important role in maximising nutrient uptake, improving skin quality and minimising incidence of key diseases.

Previous theories about liming in front of potato crops increasing the risk of common scab have largely been disproven by trials by British Sugar and others in the past decades, even if the last version of AHDB’s Fertiliser bible, RB209, for potatoes published in 2021 still suggests that liming should be avoided unless soil pH is very low.

One good reason for applying lime is to maximise nutrient availability, says independent potato agronomist John Sarup from Spud Agronomy.

“Lime is extremely important,” he says. “We’re now spending an increasing amount of money on fertiliser, so we have to make sure we are maximising its nutrient efficiency, and one way is to make sure pH is optimal.”

A number of the growers he advises are using LimeX from British Sugar, primarily for that reason, he says. “But they are also seeing a better skin finish and quality generally.”

Common scab could be one skin finish condition, LimeX is helping with, despite the misconception that it increases risk. British Sugar trials in 2012-2015 and repeated in 2024 found reductions of up to 50% in scabbed tubers where LimeX had been applied to fields before potato planting.

Maintaining adequate moisture around tubers for up to six weeks following tuber initiation remains the key focus for controlling common scab, John says. “But we’ve seen enough data to know that liming can help.”

Surprisingly in such a dry summer, John reports that common scab infections are lower than normal, at least on his grower’s farms. “I think that might have been because there was no moisture at tuber initiation at all and the Streptomyces scabiei bacteria that cause common scab literally couldn’t move in the soil to infect the plants.”

It’s not the first very dry year where common scab incidence is less common than in a more normal year, he suggests.

Where he has seen common scab this season, it’s been in processing crops where the timing of irrigation was not targeted at tuber initiation.

LimeX business manager, Glenn Carlisle from British Sugar explains that the fine particle size of LimeX rapidly increases available calcium in the root zone ahead of planting, with over 250kg of elemental calcium applied in every tonne.

That’s important as one of the roles of calcium within plant cells is to strengthen cell walls.

“When growing a scab-susceptible variety, such as Maris Piper, making sure the plant has an abundance of available calcium in the soil will help with common scab control,” he says, “as the plant will transport calcium to areas infected by the bacteria to help the plant defend itself by strengthening cell walls.”

That mechanism is perhaps one reason why there has been a historic disconnect between liming and common scab, he notes. “Historic analysis of potato skins near infected areas showed a higher calcium concentration as a result of this natural defence mechanism. This led to the incorrect assumption that high levels of calcium in the soil and in the crop increase the incidence of scab rather than reduce it.”

In the most recent trials in 2024, LimeX was spread in strips at 7.5t/ha and 10t/ha in a field of Maris Piper separated by an untreated 12m buffer, resulting in a 50% reduction in scab incidence in the plots receiving 10t/ha of LimeX compared with the untreated.

That mirrored trials done a decade before in three separate years, where scab was reduced by applications of LimeX, Glenn says.

“LimeX will also help reduce calcium deficiency symptoms, like internal rust spot, as well as supplying a large proportion of the crop’s requirements for nutrients such as phosphate and magnesium.”

Timing for applying LimeX is flexible. Ideally it would be post-ploughing, pre-planting and then incorporated, Glenn notes. “But it is better to apply when there is access to the field, rather than holding off for ideal conditions.”