The trial:

Portsmouth Water commissioned S. Woodley Crop Services to conduct an in-depth trial set to understand and measure the effect cutting back N applications has on a crop of winter wheat in terms of yield (income for the grower) and the reduction in nitrogen leaching into the groundwater. A 5ha field drilled with Skyfall on a chalk loam soil was chosen. Five different fertiliser regimes where planned looking to cut back applications by:

1. Control (farmers standard practice)

2. 5% reduction total N

3. 10% reduction total N

4. 15% reduction total N

5. 20% reduction total N

The conclusion:

Nitrogen use efficiency was significantly higher in 2020 compared to 2021. The main contributor to this is the weather patterns of the growing season in 2021. The lack of sunshine and levels of rain was the perfect storm for high disease resulting on average a 20% yield reduction in West Sussex. 

Nitrate leaching potential is difference between spring and autumn SMN results. The trends seen in year two follow a practical understanding. The less N you apply, the lower the residual N after harvest. This was the logic across the trial with the exception of plot 4 (20% reduction) that saw a higher-than-expected figure. 

The porous pot figures show a correlation between nitrate leaching into the porous pots and the amount of N applied on the surface. In both years (20% reduce) saw the lowest levels of nitrate recorded in the porous pots. The control plots where also consistent in producing the highest nitrate levels in the porous pot samples. Note the step up in nitrate levels between 2020 and 2021, on average twice as high!

Yield differences where negligible across all 5 plots in the 2021 trial. Weather had a big role in this, but it does suggest that adding more nitrogen to the crop does not necessarily add yield, even on a dull year.