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Phosphorus Best Practice

Investigating whether current effluent and fertiliser guidelines are strong enough to prevent phosphorus loss in shallow-stony Canterbury soils

Rich McDowell photo by Dairy Farmer


Challenge funding: $50,000

Research duration: June 2018 – February 2019

What Did We Do?

Phosphorus can be leached from intensive land uses, including grazed dairy farming, especially where fertiliser or farm dairy effluent (FDE) is applied. When phosphorus leaches through the soil and into freshwater it can stimulate algal growth, leading to impairment of water for swimming, fishing, drinking, and reduced biodiversity.

This research investigated whether current practice for fertiliser and FDE application are sufficient to minimise phosphorus loss in stony free-draining soils with low capacity to absorb phosphorus, such as those common in the Canterbury region, the second-largest dairy-producing region in New Zealand.

This research found that current practice and regional rules for the application of FDE to stony free-draining soil under irrigation were not sufficient to prevent phosphorus losses.

To decrease losses, farmers should avoid applying FDE to these soils, manage depths (and rates) carefully or use a technology that reduces phosphorus to very low concentrations.

How Can The Research Be Used?

  • Our research indicates that despite adhering to regional regulatory rules, significant phosphorus losses still occur when farm dairy effluent (FDE) is applied to well-drained shallow soil.
  • It is unclear if applying less FDE would decrease phosphorus losses, because depths of application were already low (<2mm).
  • In freely draining shallow stony soil (or similar soils) under irrigation, phosphorus should be made less available for loss, for example with effective solids removal, or FDE should not be applied.
  • Industry bodies and regional and central government could use these results to strengthen guidelines and regulations beyond current recommendations, particularly regarding the use of FDE on soils of low sorption capacity, to meet community and government expectations.

In the Media

Challenge looks at how to build trust and capture more of our agricultural earnings

Irrigation NZ News, Winter 2019 (page 19)

“More research needs to be done to determine whether applying less FDE at lower rate would decrease phosphorus losses,” says lead researcher Rich McDowell, chief scientist of the Our Land and Water National Science Challenge. “Until we know this, the research suggests that farmers should try to avoid applying FDE to freely draining shallow stony soils under irrigation.”


Team Snapshot

Research Outputs


The efficacy of good practice to prevent long-term leaching losses of phosphorus from an irrigated dairy farm

R.W.McDowell, C.W.Gray, K.C.Cameron, H.J.Di, R.Pellow
Agriculture, Ecosystems & Environment – March 2019

We measured P fractions in leachate at 70cm depth from two soils of low-P sorption capacity in an intensively grazed dairy farm. In response to best-practice annual applications of P as either fertiliser or a lower rate of fertiliser plus FDE, dissolved and particulate P concentrations increased annually 4% to 7%. Mean total P load over 14 years (2001–2015) from the FDE-treated, free-draining shallow soil was 1.46 kg ha−1 yr−1, much greater than the same soil without FDE (0.25 kg ha−1 yr−1) or the moderately well-drained soil with or without FDE applied (0.12 kg ha−1 yr−1, for both treatments).

A review of regulations and guidelines related to winter manure application

Jian Liu , Peter J. A. Kleinman, Helena Aronsson, Don Flaten, Richard W. McDowell, Marianne Bechmann, Douglas B. Beegle, Timothy P. Robinson, Ray B. Bryant, Hongbin Liu, Andrew N. Sharpley, Tamie L. Veith Ambio
A Journal of the Human Environment – February 2018

Winter manure application elevates nutrient losses and impairment of water quality compared to applications in other seasons. We reviewed worldwide mandatory regulations and voluntary guidelines on efforts to reduce off-site nutrient losses associated with winter manure applications. Most of the developed countries implement regulations or guidelines. Developing countries lack such official directives, despite increasing concern over water quality. An analysis of five case studies reveals that directives are affected by local socio-economic and biophysical considerations. Successful programs combine site-specific management strategies along with expansion of manure storage to offer farmers greater flexibility in winter manure management.

A national assessment of the potential linkage between soil, and surface and groundwater concentrations of phosphorus

R.W. McDowell, N. Cox, C.J. Daughney, D. Wheeler and M. Moreau
Journal of the American Water Resources Association – August 2015

A meta-analysis of three national databases determined the potential linkage between soil and surface and groundwater enrichment with phosphorus (P). A subset of sites dominated by aquifers with gravel and sand lithology showed increasing P concentrations with as little as 10 years data. These data raise the possibility that groundwater could contribute much P to surface water if: there is good connectivity between surface and groundwater, intensive landuse occurs on soils prone to leaching, and leached-P is not attenuated through aquifers. To avoid poor surface water quality, management and planning may need to consider the connectivity and characteristics of P in soil-groundwater-surface water systems.


Science to policy and back again: compliant land use practices still cause P leaching

Rich McDowell
NZ Freshwater Sciences Conference – December 2018

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