Prolonged release fertiliser brings benefits to potato production

Matching key nutrient supply more closely to plant requirements through the growing season can not only increase potato yields, it can improve tuber size and skin quality, says ICL Growing Solutions agronomist Scott Garnett.

Grower experience is now supporting trials evidence that applications of the prolonged-release fertiliser Polysulphate can help potato producers increase uptake of vital nutrients in their crops and improve their marketability, he says.

“Potatoes have requirements for specific nutrients, including  nitrogen, sulphur, phosphorus, potassium, and several micronutrients such as magnesium and calcium, which are critical for their growth and yield.

“Sulphur, for example, is essential in helping reduce storage sugars and promotes optimum nitrogen use efficiency while Calcium is important in the development of roots and shoots and also helps reduce storage losses.

“Balancing these nutrients is vital throughout different growth stages to ensure healthy plant development, tuber formation, and disease resistance.

“Unlike most other fertilisers, Polysulphate is a natural multi-nutrient fertiliser that has a proven prolonged release action that delivers a gradual and sustained nutrient release, ensuring a consistent supply of vital elements throughout the potato plant’s growth stages.

“This prolonged nutrient delivery not only maximises nitrogen use efficiency, it also minimises nutrient leaching, reducing environmental impact.

“Combined with a unique 48% SO₃, 14% K₂O, 6% MgO and 17% CaO analysis for Polysulphate, this action is highly effective in delivering essential nutrients for healthy foliage, robust potato growth and tuber development.

“All in all, it means potato producers can achieve improved crop health, enhanced yields, and sustainable agricultural practices, contributing to a more productive and environmentally conscious farming approach.”

Significant benefits

Aberdeenshire grower Graham Twatt, of Alan Twatt Potatoes Limited, has seen many of these benefits in recent years.

Managing two successful businesses, as a seed potato producer and a potato merchant, Graham has developed new export markets for seed potatoes in countries including Egypt, Morocco, Bangladesh, Thailand, Indonesia and Brazil.

Based in Gamrie near Banff, he attributes much of the growth in exports to a good number of excellent and very reliable farms who supply him with high quality seed potatoes.

“As a potato merchant we’ve been trading for about 50 years now, but we also have an established arable farming business where we annually grow 150ha of seed potatoes.

“Based on a 10-year rotation for our seed potato crop we also grow spring barley, winter wheat and oilseed rape on the farm. Prior to 2018, we applied our nitrogen as a starter fertiliser with a later application of straight potash.

“However, the problem with the potassium chloride elements within the potash is that, although the potassium was beneficial to the soil, the chloride element had no agronomic benefit to our potatoes.

“Neither product was improving nutrient levels in our soil, and we weren’t consistently achieving the yields or dry matter content in our potato crops.”

In 2018, on advice from Agrii agronomist Charlie Catto, the potash applications were stopped and Polysulphate was introduced, primarily to put more sulphur into the soil as well as improving existing levels of calcium, magnesium, and potassium, he explains.

“At the same time, we began trial work to determine the effectiveness of Polysulphate in helping our crops to use the N we were applying more efficiently.

“The switch to Polysulphate also offered the additional benefits of calcium and magnesium which help the potato plants to build cell wall strength and increase root mass.”

Regular soil testing

Charlie Catto says regular soil testing has also been part of the initiative.

“Since advising Graham to make the change to Polysulphate, we have tested annually and recently completed our fifth year of soil and plant tissue sampling enabling us to monitor the health of Graham’s soil and his crops so we can see how they react to Polysulphate right through the growing season.

“The key to the tests was to determine how efficiently his potato crops were absorbing the N and other key nutrients during their growth cycle, and how we could optimise the efficiency of that process with the timing of Polysulphate applications.

“The five-year trial has shown progressively better-balanced soils with improved sulphur, potassium, calcium and magnesium levels and no nutrient deficiencies in the root zone of the potato plants.

“During the five years the overall quality of his potatoes has improved, particularly in terms of improved root structures, better skin finish and more uniform tuber sizes, all critical factors for the export side of the business.

“Graham’s N and Polysulphate is all spring applied. On application rates, the amount of Polysulphate is decided after checking the soil analysis, the variety being grown, and any yield offtake agreement.

“Rates can therefore vary from 135kg/ha – 400kg/ha, with a higher yielding variety steering us towards applying a higher rate.

“The key to the success of Polysulphate is the slow release of its 48% sulphur content, about half of which is released over a 6–8-week period across March and April as he begins planting.

“This gives the young potato plants much more time to absorb it along with the other nutrients as they are released into the soil.”

Improvements in gradable yield seen

According to Graham Twatt plant late summer tissue samples taken to check levels of magnesium, calcium and sulphur present in the potatoes confirm the Polysulphate has more than done its job.

“It’s certainly given us the return on investment (ROI) with a noticeable improvement on ‘gradable yield’. This means we now have a higher percentage of lifted potatoes going successfully through our grader, and not being rejected on skin finish.

“As gradable yield links directly to profit margin/ha, I’d argue that it is even more important than field yield given we’re looking for a good uniform size for export, not necessarily more individually bigger, heavier potatoes.”

On the environmental benefits of Polysulphate, both Graham and Charlie recognise the importance of a naturally occurring product, produced less than 200 miles from Aberdeenshire, which has the lowest carbon footprint of any equivalent fertiliser.

“As farmers we are always going to be judged on how effectively we can reduce our carbon use and as a business we can see that new carbon footprint targets are being introduced into payment schemes with incentives for lower carbon audits,” Graham points out.

Agrii’s Charlie Catto agrees, saying such thinking is very close to the heart of the company and its customer support in the future.

“With current fertiliser practices accounting for 75% of the carbon footprint of all UK crop production, products like Polysulphate have an important role to play in helping farmers reduce their carbon use and meet these green targets.

“At Agrii, as part of our Green Horizons project, we are currently doing a lot of research in this area, enabling growers to successfully reduce their carbon use and produce more accurate carbon audits.”