Regardless of soil type N rates shouldn’t change too much with the increased prices, says Brian Chambers of ADAS.

Higher N prices shouldn’t have a big impact on use rates in potato crops next year, Brian Chambers of ADAS told growers at a Potato Council-organised West Midlands Potato day.

Two years ago N was costing 30p/kg, now at an ammonium nitrate price of £280/t it was nearer 80p/kg, he said. “Even that price is now on the low side, at £320-330/t, it is more like £1/kg.”

Despite that, and a doubling of the break-even ratio from 4:1 to 8:1 (ie, 8kg of ware potatoes are required to cover the cost of 1kg of N), ADAS studies showed N inputs shouldn’t change much next season, he said. “N is fairly insensitive.”

For example, eight trials indicated the optimum N rate at a 4:1 ratio on irrigated sandy soils was 290kg/ha N, he said. At an 8:1 ratio it dropped to 280kg/ha N. Similarly on non-irrigated silt soils it dropped by 20kg/ha N to 170kg/ha N, and by 30kg/ha N on shallow soils.

He also pointed out that more profit was lost from under dosing than being over the optimum dose. “You don’t want to be over, because it is not good for the environment to waste N, but going under is worse [for profit].”

Applying 40kg/ha N more than an optimum 200kg/ha N dose costs £14/ha in profit compared with £48/ha for being 40kg/ha N under, he explained.