Precision N application test costs 'outweigh returns'

The study found that potentially mineralisable nitrogen (PMN) - a measure of nitrogen release from the decomposition of soil organic matter - can vary widely, both within dairy farms and across farms on different soil types and with different management histories. This variability means that targeted nitrogen application, which adjusts the rates of nitrogen fertiliser based on PMN tests, can boost yields in low-PMN areas and reduce excess N application in high-PMN areas.

"Previous studies have shown PMN testing can improve the N use efficiency of annual crops. In this study we've shown that it can also predict variation in soil N supply and the response to N fertiliser on pastoral farms," says Dr Mike Beare, lead researcher from Plant and Food Research.

"Where PMN measured in soils was high, we found an increase in pasture dry matter production per unit of fertiliser N applied to be typically 32 to 48% less than where PMN was low.

"Where more than 40kg N/ha was applied (as N Protect, PMN had no effect."

However, while the concept works, the increase in yield isn't enough to offset the costs of testing, mapping, and variable rate application technology.

"We need practical solutions to improve N use efficiency and minimise N losses that farmers can afford and apply at scale. The data we've gathered are valuable, but for this approach to become viable, the costs of the increased testing and to a lesser extent variable rate application will need to come down and we need a better understanding of how PMN testing can help to reduce N losses and impacts on the environment,” Beare says.

The research was part of N-Vision, a collaborative programme with MPI, Ravensdown, Plant and Food Research, and Lincoln University aimed at improving nitrogen management on dairy farms, with the ultimate goal of improving nitrogen use efficiency and reducing environmental impacts all within the 190 kg/ha N cap. The programme is co-funded by the Government’s Sustainable Food and Fibre Futures (SFF Futures) fund.

The study took measurements from seven different farms from five different regions over an 18-month period. Researchers took measurements of PMN at 0-0.75cm depths, and also 0-15cm depths, and found that pasture dry matter production was greater in areas with higher PMN at both depths. However, the increase in pasture growth due to fertiliser was greater where PMN test values were lowest.

The study has now wrapped up, but the data gathered could still prove valuable for future research or if market conditions change.

“This research adds to our knowledge,” Beare notes. “If the cost of nitrogen rises or we find ways to lower testing costs, this could become a useful tool for farmers in the future.”

“We hope to use the data to identify the soil and environmental factors that could explain the wide variation in PMN observed on farms,” Beare says. Data could also be used to further improve the testing protocols for PMN, including options for reducing the costs.

For now, current best practice N fertiliser recommendations remain the most practical approach, but the findings offer insights that could lead to more efficient nitrogen management strategies in the future.