Decades of conventional wisdom about how to fertilise milling wheat for yield and grain protein is set to be challenged after initial trials by Agrii found that the use of Polysulphate enabled nitrogen rates to be cut by more than a third without compromising performance.
The trials, performed by Agrii’s R&D team, were initially conceived to consider how nitrogen use efficiency could be improved in response to tightening environmental regulations. This gained an economic impetus following Russia’s invasion of Ukraine which sent wholesale gas prices soaring. Natural gas is the principal component in the production of ammonium nitrate which at £650/t and applied at 250 kg N/ha equates to £471/ha, more than double the cost in 2021.

Mined off the coast of north-east England, Polyhalite, sold worldwide by ICL as Polysulphate, is a mineral fertiliser containing 48% SO3­ as sulphate, 14% K2O as from sulphate of potash, 6% MgO as from magnesium sulphate and 17% CaO as from calcium sulphate. ICL claims that Polysulphate can increase crop yields by up to 40%.

As a relatively new form of fertiliser its contribution to performance has been the subject of trials spanning several crop types. Agrii has performed replicated trials to consider its contribution in a programme with other nutrients.

“Across winter wheat, hybrid barley and spring oats (both naked and covered), the plots that received Polysulphate consistently produced the highest yields,” says David Felce who co-ordinated the Agrii trials at Stow Longa.

It is the potassium component specifically that captured Agrii’s attention. At 14% K2O its contribution is modest and yet crops that received Polysulphate repeatedly outperformed those that received far higher quantities of K2O.

“Potassium uptake in winter wheat peaks during flowering at about 300 kg/ha, more in a high-yielding crop. Most growers would apply significantly less than this, yet where its availability is constrained, we see that nitrogen utilisation is poor and yields are reduced,” Mr Felce says.

“That prompts the question, ‘how much of what is in the soil can we expect to be plant available?’. The answer is, ‘it depends’,” Mr Felce says.

“Availability of all nutrients is a function of many interrelated factors, most importantly that of water. However, potassium availability is heavily influenced by the calcareous soils typical of much of lowland Britain While important in many soil functions, calcium binds strongly to exchange sites, reducing availability of less strongly bound nutrients, such as potassium, magnesium and ammonium nitrate,” Mr Felce says.

The soils at Agrii’s Stow Longa Technology Centre near Huntingdon are a classic example of the contradiction that is nutrient status and nutrient availability. The Hanslope series clay is not nutrient poor with an index of 2+ for potassium and magnesium. It can be expected to release about 30 kg K2O/ha annually. Despite the long-term use of the plough, soil organic matter is typically upwards of 5%.

“The straw is not removed, so the only K to be replaced is that taken away with the grain. This is roughly 5.6 kg/t of grain so not extreme. The high soil calcium level, typically above 5000ppm, however, greatly influences availability. We know from historical work that above 3500ppm the amount of potassium available is reduced.”

Nitrogen utilisation

Nitrogen is by far the most important nutrient in crop production, but there is an economic and a regulatory incentive to reduce its use.

Even before the war in Ukraine sent prices rocketing, proposals intent on cutting nitrogen use to protect the environment were under consideration. The Clear Air (Human Rights) Bill, if enacted, will introduce limits on ammonia emissions from agriculture while the DEFRA consultation on urea fertiliser proposes that all urea ammonium nitrate (UAN) is treated with an inhibitor from 2024. Both will influence application rates, performance and production costs.

The manufacturing, transportation and application of ammonium nitrate is a significant source of Co₂. Using less is the easiest and fastest way to address this reality.

Lifecycle analysis suggests that if growers use 50 kg AN/ha less, it could reduce emissions of Co₂ equivalent by 300 kg/ha. Doing so without compromising output means taking a different approach to crop nutrition.

For Mr Felce and his colleagues, this meant revisiting the accepted principles surrounding the relationship between macro- and micro-nutrients and to look afresh at how to improve fertiliser performance.

“This very quickly took our thinking beyond nitrogen use and into nutrient use efficiency in its widest sense. We wanted to understand if Polysulphate was more than the sum of its parts, we wanted to know if it would support milling wheat production at lower nitrogen rates,” Mr Felce says.

To answer the question, Agrii took a nitrogen rate that most growers would apply to a milling wheat in the second cereal situation – typically 264 kg N/ha – and reduced it by 50 kg/ha. This was then reduced further to a lower limit of 150 kg/ha.

To avoid suggestions that it was really a sulphur trial, all plots received a normalised application. Three treatments received their potassium, along with magnesium and calcium from Polysulphate while one received a heavy dose of potash. The final two treatments in trial received only nitrogen and sulphur.

Tissue tests were taken through the season to give an indication of nutrient levels in the plant.

“The crops which received only nitrogen and sulphur were always in need of more nitrogen than the other crops. This supports our understanding that potassium and calcium are important in supporting nitrogen uptake within the plant,” Mr Felce says.

Winter wheat’s need for calcium is significant and while the soils at Stow Longa are rich in it, there is not always a relationship between what’s in the soil and what’s in the crop.

“The plant’s need for calcium is significant as it supports many functions and, unlike some other nutrients, that requirement keeps climbing through to grain fill. Applying Polysulphate in this situation can only be beneficial,” Mr Felce says.

The findings make for compelling reading. Using the Group 1 variety Skyfall sown in late October across a range of rotational positions spanning second, third, fourth, fifth and sixth wheat, it was possible to meet yield and grain quality expectations from just 164 kg N/ha (see table).

Treatments 3 and 4 tell the story in its clearest form: grain yields and protein are broadly the same, but the use of Polysulphate in treatment 3 enabled nitrogen rates to be cut by 50 kg/ha. This contributed to an extra £129/ha in gross margin. Furthermore, where the same amount of N was used (164kg/ha) in Treatment 5, but with only sulphur in support, the yield was nearly 1t/ha lower and protein reduced from 13.7 to 10.9, with gross margin consequently down from £1030 in treatment 3 to £643/ha.

“The real hero here is 14 kg K₂O in the Polysulphate. It has outperformed the 80 kg K₂O in the standard programme [treatment 4]. This could be due to the additional magnesium and calcium, the balance of them, their sulphate form, or likely a combination of all these factors,” he says.

The results have prompted further discussion which future trials will consider.

“It’s worth noting that this is one year’s result and the NUE scores this season are not so dramatic. However, what the early results from this harvest do show is a dramatic yield response to the use of Polysulphate, with variations linked to product timing and cultivation practice.

“Our ambition is to develop an approach and understanding of plant nutrition that works at lower nitrogen rates and given a changing climate, is both resilient and cost effective,” Mr Felce says.

Table: Stow Longa wheat nutrition trial 2020/21