Innovative Polysulphate Trials: Enhancing Nitrogen Efficiency

These trials were initially designed to improve nitrogen use efficiency in response to tightening environmental regulations. However, the onset of the war in Ukraine and the subsequent spike in gas prices brought economic considerations to the forefront, given that natural gas is a primary source for the production of ammonium nitrate.

Trial Setup and Key Findings

The trials incorporated Polysulphate, a relatively new form of fertilizer, into nutrition programs alongside other nutrients. Agrii was particularly interested in Polysulphate’s potassium content. Despite its modest contribution of 14% K₂O, crops treated with Polysulphate consistently outperformed those that received higher quantities of K₂O.

David Felce, who coordinated the Agrii trials, explained: “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 utilization is poor and yields are reduced.”

Potassium availability is heavily influenced by calcareous soils, where calcium binds strongly to exchange sites, reducing the availability of less strongly bound nutrients such as potassium, magnesium, and ammonium nitrate. High soil calcium levels, typically above 5000 ppm, significantly impact the availability of other nutrients. Felce added: “We know from historical work that above 3500 ppm the amount of potassium available is reduced.”

Economic and Environmental Incentives

While nitrogen is crucial for crop production, there are both economic and regulatory incentives to reduce its use. Excessive nitrogen use has led to environmental problems, prompting regulatory limits and a push for higher-efficiency fertilizers. Additionally, the CO₂ emissions associated with the production, transportation, and application of ammonium nitrate are significant. Reducing nitrogen use is a straightforward way to address this issue. Lifecycle analysis suggests that reducing ammonium nitrate use by 50 kg/ha could lower CO₂ emissions by 300 kg/ha.

To achieve this goal, the Agrii team shifted their focus from nitrogen use to nutrient use efficiency in its broadest sense. Felce noted: “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.”

Trial Methodology and Results

The team started the trial with a nitrogen rate of 264 kg N/ha, which is typical for milling wheat in a second cereal situation, and reduced it by 50 kg/ha increments down to 150 kg/ha. All plots received a normalized application. Three treatments included potassium, magnesium, and calcium from Polysulphate, while one received a heavy dose of potash. The final two treatments received only nitrogen and sulfur, with no potassium.

Tissue tests during the season showed that plants receiving only nitrogen and sulfur consistently needed more nitrogen. Felce explained: “This supports our understanding that potassium and calcium are important in promoting nitrogen uptake within the plant. The need for calcium is significant. Calcium supports many functions and, unlike some other nutrients, calcium demand is rising with the grain filling. In this situation, Polysulphate applications can only be beneficial.”

The findings were impressive. It was possible to meet yield and grain quality expectations with just 164 kg N/ha. The grain yields and protein contents were broadly the same in treatments 3 and 4, but the use of Polysulphate in treatment 3 enabled nitrogen rates to be cut by 50 kg/ha. Treatment 5, which used the same amount of nitrogen (164 kg/ha) but only sulfur in support, resulted in nearly 1 t/ha lower yield and reduced protein from 13.7 to 10.9.

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

Future Directions

The results of these trials have sparked discussions about further research to develop a reliable and cost-effective approach that delivers stable crop yields even with lower nitrogen inputs. This innovative approach could revolutionize agronomy practices, making them more sustainable and economically viable.