WITH input costs already under intense scrutiny, and nitrogen prices rising sharply following instability in the Middle East, potato growers are carefully examining every expenditure line.
Yet Tom Decamp, Crop Nutrition Business Manager at Yara, says evidence suggests growers targeting pre-pack quality are largely maintaining their traditional nutrition programmes and seeking savings elsewhere – in land rental decisions, machinery investment and contractor use – simply because of the financial prize at stake.
“Potato growers may cut back a little on total nitrogen,” Tom said. “But they’re still trying to achieve that premium over the general ware price, which can add £40–£50 a tonne to the return from a crop. If you are achieving a yield of 50 tonnes per ha, for example, this could potentially increase margins by around £2,500 per ha from achieving pre-pack quality.”
A nitrate fertiliser, for example Yara’s Tropicote, is therefore a sound investment as part of a potato nutrition strategy, Tom said, adding that soil-applied calcium nitrate remains the most effective route when it comes to getting calcium and plant-available nitrate nitrogen into the potato crop at tuber initiation.
The agronomic case for soil-applied calcium nitrate is rooted in fundamental plant science. Calcium is only mobile upwards within the potato plant, so the only pathway into the tuber is through the fine root hairs in the soil. Foliar calcium applied later to the canopy will stay within the canopy, enriching the foliage, but it cannot travel down into the developing tuber.
“When the tuber initiates, and it’s just hooking over and booting up, that’s when it’s critical to get the calcium in,” says Tom. “We’ve demonstrated this process using YaraLiva Tropicote and a red dye. You can see the calcium nitrate moving through the root hairs and flooding into the tuber, like blood through an artery.”
Another key advantage of a high-quality, granular calcium nitrate fertilizer is that it begins working immediately on contact with soil moisture, without requiring rain or irrigation to activate it, he said.
Applying something that works in cool, wet conditions as well as under drought stress conditions, without having to go through the nitrification process can be a valuable time-saver, with the calcium and nitrogen being available straight away.
“Cool or dry, you will get uptake,” Tom said.
Timing is crucial, he added.
“You don’t want to be moving way past tuber initiation because it becomes too late,” he said.
Once inside the tuber, calcium strengthens cell walls, providing the physical integrity needed to withstand the rapid growth spurts that occur during warm, moist conditions. Without adequate calcium, cells expanding too quickly simply burst.
“When the cell walls are not strong enough, they expand and burst — and that’s necrosis,” explained Tom. “They die, and that’s the internal browning (rust spot) that triggers rejections at the packhouse and strips pre-pack growers of their hard-won premium.”
The benefits of adequate calcium reach beyond internal quality. A stronger outer cuticle provides greater resistance to bacterial infections and skin disorders. Tom points to observed reductions in silver scurf and other skin complaints alongside improvements in storage performance, including reductions in soft rots. Calcium also builds stress tolerance: During dry periods where irrigation is unavailable, calcium-rich crops are better equipped to survive and recover.
“If they’re looking at a sample and they start cutting them in half and there’s internal rust spot, they’ll get rejected – they won’t get through for pre-pack and you lose your premium. It’s the same story if the skins are terrible, or they’re all irregular sizes. Supermarket pre-packs might be four in a pack — everything has to be absolutely right.”
The benefits of well-supplied calcium also carry through into seed potato production. A stronger, healthier mother tuber produces better quality seed for the following season, which is a consideration of particular importance in Scotland.
Although some potato varieties carry a lower rust spot risk, and some seasons are kinder than others, Tom says the unpredictability of which seasons and varieties will be affected is itself an argument for applying calcium nitrate to the soil early and treating it as a non-negotiable part of the programme rather than a discretionary extra.
Tom said the arithmetic is unambiguous. A 50-tonne pre-pack crop losing its quality premium of £40 to £50 per tonne represents up to £2,500 per hectare forfeited at the point of sale. Against an application cost of under £200 per hectare soil-applied calcium nitrate, even allowing for today’s elevated input prices, the potential return far outweighs the cost.
