Sulphur plays a crucial role in plant growth and health and, without it, crops cannot reach their full potential and nitrogen use efficiency drops.

“Sulphur is a life-essential nutrient for plants and is required for protein formation, physiological functions including photosynthesis and the formation of oils and glucosinolates,” says Origin Fertilisers’ technical director, Peter Scott. 

Despite its fundamental importance, the agriculture industry fails to give sufficient attention to sulphur nutrition. This is reflected in the lack of detailed sulphur recommendations in the RB209.

See also: How fulvic acid can cut glysosphate rates and improve foliar N

Applications of polysulphate fertiliser could be one way of addressing the sulphur shortfall in crops with yield benefits of 0.3t/ha in winter wheat.

Polysulphate benefits

Most soils in the UK are deficient in sulphur, and no matter how much N you apply, without sulphur you will never get the most out of it, says Scott Garnett, senior agronomist at fertiliser firm ICL.

Applications of a polysulphate fertiliser can be an effective way to improve crop yield.

UK trials consistently show yield lifts of 5-8% in winter wheat, and as much as 33% in oilseed rape. In legumes, polysulphate has been shown to increase yields by up to 40%.

“The high sulphur content boosts nitrogen use efficiency [NUE] and improves crop quality,” says Scott.

Polysulphate is a multinutritional natural mineral comprising 48% sulphur trioxide, alongside potassium (14% potassium oxide), calcium (17% calcium oxide), and magnesium (6% magnesium oxide).

It is mined more than 1,000m below the North Sea off the North Yorkshire coast and processed nearby.

“Following the wet winter, rooting potential in crops is limited. Without a well-developed root system, plants will be stressed, resulting in lower quality and yields.

“Farmers considering how to improve their nutrient strategy to mitigate this risk, may want to consider sulphur,” he adds.

“But to develop a good root system, you need calcium, magnesium and phosphate – these are the main components driving the cell growth required to build a strong and healthy root system.”

© ICL

The trial

A three-year trial in Suffolk, carried out by ICL and independent field trialist Envirofield, revealed polysulphate fertiliser increased winter wheat yields by 0.3t/ha.

“This is due to better nutrient utilisation efficiency and root development,” says Scott.

“Previous winter wheat trials have shown a 5-8% yield increase from spring-applied polysulphate, but this latest work shows a further response.”

Using the variety Skyfall on loam soil, a single 100kg/ha application of polysulphate in the spring – providing about 50kg/ha of sulphur trioxide – resulted in a three-year average yield of 10.49t/ha.

It greatly decreased leaching of N and K. A supplementary 100kg/ha the preceding autumn boosted yields to 10.79t/ha.

“At current values, the cost of the extra polysulphate more than covers the additional income. There are significant environmental benefits to factor in too,” adds Scott.

Polysulphate rapidly builds soil sulphur levels to an optimum, with its prolonged release supplying plants as they develop through the autumn. Its 14% potassium oxide works in synergy to maximise NUE.

Available sulphur

“University of Nottingham trials found that more than 50% of the sulphur content is available in the first 12 days post-application, with the remainder released over the following six to eight weeks,” says Scott.

In contrast, traditional nitrogen sulphur products release 100% of the sulphate within five to six days of application.

This results in low crop uptake of many nutrients, as well as increased risk of leaching, ammonia volatilisation, and acidification of soils.

Polysulphate differs to the rapid nutrient release of many sulphur and nitrogen sulphur inputs, with its prolonged release of nutrients.

\Unlike a lot of nitrogen sulphur products, polysulphate does not contain N, which gives farmers more control over their nutrient planning.

“Polysulphate can be applied from February to April at a recommended rate of 100-150kg/ha,” advises Scott.

“And from early October to before Christmas, either straight or blended with an N fertiliser, it can be part of a grower’s autumn fertiliser programme.”

It can be broadcast using conventional fertiliser spreaders – achieving 36m-plus spreading widths – or direct-drilled with the seed.

Environment

Trials have also shown a 36% additional N uptake from autumn-applied polysulphate, equivalent to 10kg N/ha more than controls.

“Combined with increased phosphate uptake, loss of key nutrients from soils over the winter period is significantly reduced.

“Not only does this mean crops receive and uptake the correct amounts of nutrients for their requirements, there are also environmental benefits.

“It is evident in our work that the calcium and magnesium components encourage greater phosphate uptake and root development,” explains Scott.

Autumn-applied polysulphate to winter cereals shows 34% greater phosphate uptake by plants, which is directly proportional to extra root mass.

Polysulphate has the lowest carbon footprint of equivalent fertilisers, at just 0.034kg carbon dioxide equivalent – and 6% is ammonium sulphate.

Decoupling N and sulphur, and using polysulphate over nitrogen sulphur fertilisers, could decrease ammonia emissions by 90%.

ICL Growing Solutions is exhibiting at this year’s Cereals event at stand 608a.

Case study: Graeme Mackie, Turriff, Aberdeenshire

A transition from seed potatoes to cereals saw a change in Graeme Mackie’s nutrient strategy, with the introduction of polysulphate as a means of more effective sulphur application and positive crop response.

Based in Turriff, Aberdeenshire, Graeme and Agrii agronomist Charlie Catto are currently trialling a new rotation, comprising oilseed rape, winter wheat and three to four spring malting barleys.

Graeme Mackie and Charlie Catto (left) © Graeme Mackie

Graeme switched from applying straight potash to his potatoes to polysulphate in 2015, to improve tuber quality and skin finish.

“We wanted to capitalise on the benefits of calcium and magnesium, which helps with plant cell wall strength and increases root mass, and because we were uncomfortable with the toxicity of the chloride within the potash.”

The short growing season and cold winter temperatures means the farm’s winter crops tend to shut down and reawaken in the spring.

“We have high rainfall, which leaches potash and sulphur in our lighter sandy loam soils, so applying large amounts in the autumn doesn’t make agronomic sense,” notes Graeme.

He now applies 125-200kg/haof polysulphate to all of his crops in late February to early March – coinciding with 80% of his fertiliser applications, to maximise nitrogen use efficiency.

About half of the 48% sulphur content is released over a six- to eight-week period, giving plants more time to absorb it, along with the other elements it contains.

Soil tests reveal a progressively better-balanced soil, with improved sulphur, potassium, calcium and magnesium levels and no nutrient deficiencies in the root zone.

By switching 80% of Graeme’s potassium application from late autumn to spring, the plants now have available potassium and sulphur in the root zone for early season growth.”

Oilseed rape yields have increased by 0.3t/ha and not dropped below 5t/ha in the past three years. Wheat and spring barley specific weights have increased by 5% over the five-year trial.

“The polysulphate has certainly improved nitrogen uptake and efficiency, which has en-abled us to reduce our overall nitrogen costs each season,” adds Graeme.

“Our cereals now look healthier, have stronger root structures and have shown more vigour in the early spring.”