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How farming and changing irrigation affect nitrogen and phosphorus runoff

Irrigation to increase pasture and crop yields is often associated with an increase in nutrient inputs through fertiliser and changing management practices. Use of these practices has raised concerns about nutrient losses and their potential environmental impacts, especially for water quality.

While the losses are known to be happening, the exact extent of them is still uncertain. Manaaki Whenua researchers have been able to draw on more than 5 years of research to start to close the knowledge gap relating to variable-rate irrigation.

In a recent study in Soil Research, researchers were able to show that water and nutrient losses from farms can be reduced through precision agriculture practices, such as the use of management zones for improving the management of irrigation on spatially variable soils or crops – called variable rate irrigation (VRI).

The study, conducted on a mixed cropping site at Massey University’s No.1 Farm, near Palmerston North, measured nitrogen and phosphorus leaching losses under two contrasting management zones, Zone 1, a Manawatū fine sandy loam, a deep, free-draining soil, and Zone 2, a poorly drained Manawatū silt loam.

The site had a centre-pivot irrigator with variable rate control, and drainage flux meters were constructed and installed for each of the zones.

Senior researcher Dr John Drewry says the study sought to contribute to sustainable agricultural practices and water quality management. “The practical significance of our results is that, where spatially variable soils (or crops) exist under irrigators, irrigation can be managed in specific zones to reduce irrigation-induced drainage and nutrient leaching.”

“With some exceptions, and allowing for an unexpected rain event, there was generally more nitrate and nitrite leaching from the free draining Zone 1 soil,” says John. However, the nitrogen concentrations and loads generally had greater uncertainty in Zone 2. “We found the drainage flux meters worked well for the free-draining soil, but were less reliable in the poorly drained soil.”

John adds these findings highlight the importance of understanding nutrient leaching losses under zone-specific irrigation, as well as the need for improved monitoring and modelling techniques to accurately assess nutrient losses in different soil types and management zones. “It also highlights the need to map soil types and understand their water-holding characteristics under the irrigator footprints at a scale that land managers can make practical use of.”


Funding was provided by the Ministry of Business, Innovation and Employment (MBIE) Strategic Science Investment Fund. Funding was also provided in part by the Manaaki Whenua and Plant & Food Research-led Endeavour research programme, Maximising the value of irrigation.

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