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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 Are We Doing?

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 potential for potassium chloride fertiliser applications to leach cadmium from a grazed pasture soil

R. McDowell
Geoderma, November 2019

Cadmium (Cd) is a biotoxic element that can enter the human food chain via plants grown in Cd-enriched soil. Chloride (Cl) can solubilise Cd in soil. Although fertilisers containing Cl are not recommended as they may increase plant uptake of Cd, potassium chloride (KCl) is regularly applied to replenish and maintain K in the soil of grazed pastures. A trial was conducted to see if an autumn application of low (0.02 mol L−1, equivalent to 30 kg K ha yr−1) or high (0.04 mol L−1) KCl and the same molar rates of CaCl2 (used as a check to other studies) would leach Cd over 4-months of normal (300-mm) or high (600-mm) autumn-winter rainfall without enriching Cd in plant shoots. Leaching losses of Cd were greater from the high KCl and CaCl2 treatments (1.80 and 1.70 g ha−1, respectively) than the low KCl treatment (1.27 g ha−1). All loads lost from the Cl treatments were greater than from the control treatment (0.53 g ha−1). Losses from Cl treatments represented 29–41% of the Cd applied in an annual application of 250 kg superphosphate ha−1. Pasture shoot Cd concentrations were not different to the control (mean = 0.088 mg kg−1). Applying Cl (e.g. as KCl) in autumn may be a simple strategy to help slow the enrichment of soil Cd and does not increase the uptake of Cd by pasture shoots.

Direct exports of phosphorus from fertilizers applied to grazed pastures in Australasia

D. Nash, R. McDowell, L. Condron, M. McLaughlin
Journal of Environmental Quality - Special Section: Celebrating The 350th Anniversary Of Discovering Phosphorus—For Better Or Worse, August 2019

Since its discovery in 1669, phosphorus (P) in the form of fertilizer has become an essential input for many agroecosystems. The 4R concept was developed to help mitigate P exports. This review investigates the factors affecting P exports soon after the application of mineral fertilizers and studies quantifying its potential impact in different systems. Initially, P fertilizers and reactions that might affect their short-term P export potential are reviewed, along with P transport pathways. We then review studies quantifying the short-term impact of fertilizer application in different regions. Under poor management, recently applied fertilizer can contribute a considerable proportion (30–80%) of total farm P exports in drainage, but when fertilizer is well-managed, that figure is expected to be <10%. We use three model systems of varying hydrology common to Australasia to demonstrate the principles for selecting fertilizers that are likely to minimize P exports soon after their application.

A Global Perspective on Phosphorus Management Decision Support in Agriculture: Lessons Learned and Future Directions

R. Drohan, N. Bechmann, F. Djodjic, D. Doody, J. Duncan, A. Iho, P. Jordan, P. Kleinman, R. McDowell, P Melander, I. Thomas, P. Withers
Journal of Environmental Quality - Special Section: Celebrating The 350th Anniversary Of Discovering Phosphorus—For Better Or Worse, August 2019

The evolution of phosphorus (P) management decision support tools and systems has been most strongly affected in developed regions by national strategies (i) to optimize levels of plant available P in agricultural soils, and (ii) to mitigate P runoff to water bodies. Recent developments focused on integrated digital mapping of hydrologically sensitive areas and critical source areas have rapidly advanced runoff modeling and education. Advances in technology will facilitate the development of decision support systems that predict heterogeneity over wider geographical areas. However, significant challenges remain in incorporating “big data” in an acceptable format that accounts for catchment variability, farming systems, and farmer behavior. Future efforts will undoubtedly focus on improving efficiency and conserving phosphate rock reserves in the face of future scarcity or prohibitive cost. The principles reviewed here are critical for sustainable agriculture.

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).

Transforming phosphorus use on the island of Ireland: A model for a sustainable system

K. Macintosh, J. Chin, B. Jacobs, D. Cordell, R. McDowell, P. Butler, P. Haygarth, P. Williams, J. Quinn, V. O’Flaherty, J. McGrath
Science of the Total Environment, March 2019

Phosphorus is an essential part of the world food web and a non-substitutable nutrient in all biological systems. Losses of phosphorus occur along the food-supply chain and cause environmental degradation and eutrophication. A key global challenge is to meet rising worldwide food demand while protecting water and environmental quality, and seeking to manage uncertainty around potential future phosphorus price or supply shocks. This paper presents a stakeholder-generated conceptual model of potential transformative change for implementing phosphorus sustainability on the island of Ireland via an ‘All-Island Phosphorus Sustainability’ workshop. Key transition pathways identified by stakeholders included: incentivising phosphorus recovery, developing collaborative networks to facilitate change, developing markets and value chains for recovered products; implementing data-informed practices on-farm to prevent losses and increase efficiencies, and harmonisation of technologies with end-user needs. A comparable model was previously produced for the North American region. We describe consensus and differences around key priorities between the two regions' conceptual models, and assess how the model produced for the island of Ireland can effect system-wide change and policy moving forward. Many of the transitional pathways and future aspirations presented in both models resonate globally and are highly pertinent to other jurisdictions.

Transforming soil phosphorus fertility management strategies to support the delivery of multiple ecosystem services from agricultural systems

K. Macintosh, D. Doody, P. Withers, R. McDowell, D. Smith, L. Johnson, T. Bruulsema, V. O’Flaherty, J. McGrath
Science of the Total Environment, February 2019

Despite greater emphasis on holistic phosphorus (P) management, current nutrient advice delivered at farm-scale still focuses almost exclusively on agricultural production. This limits our ability to address national and international strategies for the delivery of multiple ecosystem services (ES). Currently there is no operational framework in place to manage P fertility for multiple ES delivery and to identify the costs of potentially sacrificing crop yield and/or quality. As soil P fertility plays a central role in ES delivery, we argue that soil test phosphorus (STP) concentration provides a suitable common unit of measure by which delivering multiple ES can be economically valued relative to maximum potential yield, in $ ha−1 yr−1 units. This value can then be traded, or payments made against one another, at spatio-temporal scales relevant for farmer and national policy objectives. Implementation of this framework into current P fertility management strategies would allow for the integration and interaction of different stakeholder interests in ES delivery on-farm and in the wider landscape. Further progress in biophysical modeling of soil P dynamics is needed to inform its adoption across diverse landscapes.

Managing diffuse phosphorus at the source versus at the sink

K. Macintosh, B. Mayer, R. McDowell, S.Powers, L. Baker, T. Boyer, B. Rittmann
Environmental Science and Technology, September 2018

Judicious phosphorus (P) management is a global grand challenge and critical to achieving and maintaining water quality objectives while maintaining food production. The management of point sources has been successful in lowering P inputs to aquatic environments, but more difficult is reducing P discharges associated with diffuse sources, such as nonpoint runoff from agriculture and urban landscapes, as well as P accumulated in soils and sediments. Strategies for effective diffuse-P management are imperative. Many options are currently available, and the most cost-effective and practical choice depends on the local situation. This critical review describes how the metrics of P quantity in kg ha–1 yr–1 and P form can influence decision-making and implementation of diffuse-P management strategies. Quantifying the total available pool of P, and its form, in a system is necessary to inform effective decision-making. The review draws upon a number of “current practice” case studies that span agriculture, cities, and aquatic sectors. These diverse examples from around the world highlight different diffuse-P management approaches, delivered at the source in the catchment watershed or at the aquatic sink. They underscore workable options for achieving water quality improvement and wider P sustainability. The diffuse-P management options discussed in this critical review are transferable to other jurisdictions at the global scale. We demonstrate that P quantity is typically highest and most concentrated at the source, particularly at farm scale. The most cost-effective and practically implementable diffuse-P management options are, therefore, to reduce P use, conserve P, and mitigate P loss at the source. Sequestering and removing P from aquatic sinks involves increasing cost, but is sometimes the most effective choice. Recovery of diffuse-P, while expensive, offers opportunity for the circular economy.

Impacts of long-term biomass management on soil phosphorus under temperate grassland

Boitt G, Black A, Wakelin SA, McDowell RW, Condron LM
Plant and Soil - Plant Science, June 2018

We assessed and quantified the cumulative impact of 20 years of biomass management on the nature and bioavailability of soil phosphorus (P) accumulated from antecedent fertiliser inputs. Soil (0–2.5, 2.5–5, 5–10 cm) and plant samples were taken from replicate plots in a grassland field experiment maintained for 20 years under contrasting plant biomass regimen- biomass retained or removed after mowing. Analyses included dry matter production and P uptake, root biomass, total soil carbon (C), total nitrogen (N), total P, soil P fractionation, and 31P NMR spectroscopy. Contemporary plant production and P uptake were over 2-fold higher for the biomass retained compared with the biomass removed regimes. Soil C, total P, soluble and labile forms of inorganic and organic soil P were significantly higher under biomass retention than removal. Reserves of soluble and labile inorganic P in soil were significantly depleted in response to continued long-term removal of P in plant biomass compared to retention. However, this was only sufficient to sustain plant production at half the level observed for the biomass retention after 20 years, which was partly attributed to limited mobilisation of organic P in response to P removal.

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.

Guiding phosphorus stewardship for multiple ecosystems services

MacDonald G, Jarvie H, Withers P, Doody D, Keeler B, Haygarth P, Johnson L, McDowell R, Myyittah M, Powers S, Sharpley A, Shen J, Smith D, Weintraub M, Zhang T.
Ecosystem Health and Sustainability , December 2016

We propose a conceptual framework—the “phosphorus‐ecosystem services cascade” (PESC)—to integrate the key ecosystem processes and functions that moderate the relationship between P released to the environment from human actions and ecosystem services at distinct spatial and temporal scales. Phosphorus stewardship can have emergent ecosystem service co‐benefits. Applying the PESC framework, we identify key research priorities to align P stewardship with the management of multiple ecosystem services, such as incorporating additional services into agri‐environmental P indices, assessing how widespread recycling of organic P sources could differentially impact agricultural yields and water quality, and accounting for shifting baselines in P stewardship due to climate change. Ultimately, greater precision in targeting stewardship strategies to specific locations would help optimize for ecosystem services and more effectively internalize the downstream costs of farm nutrient management.

The effect of irrigation and urine application on phosphorus losses to subsurface flow from a stony soil

Gray CW, McDowell RW, Carrick, S, Thomas, S
Agriculture, Ecosystem & Environment, October 2016

Factors such as irrigation and soil pH can increase the solubility of P and its loss to water. Urine patches are known to alter pH but it is unknown if this increases P loss. This study investigated subsurface P loss from a stony soil under irrigation with or without the application of urine. Results showed that despite receiving urine average concentrations were significantly (P < 0.05) lower than non-urine treatments. Increasing the rate (not amount) of irrigation did not affect the load of P lost, thought to be due to attenuation. Lower P losses following urine application were due to the buffering effect of the soil and enhanced P uptake as dry matter production increased in response to N in urine. However, concentrations still exceeded guidelines. Strategies should be considered to decrease losses, especially where drainage occurs via preferential flow pathways that negate attenuation of P losses, or are well connected to surface waters.

The effect of soil moisture extremes on the pathways and forms of phosphorus lost in runoff from two contrasting soil types

Simmonds B, McDowell RW, Condron LM
Soil Research, September 2016

Soil moisture and Olsen P concentrations play an important role in phosphorus (P) losses in runoff. Our hypotheses were as follows: (1) rainfall applied to a dry soil would cause greater particulate P losses in surface runoff due to hydrophobicity; (2) P losses from a wet soil would be dominated by drainage and filtered P; and (3) both runoff processes would result in environmentally unacceptable P losses at agronomically productive Olsen P concentrations depending on the sorption capacity (anion storage capacity; ASC) of the soil. We found total P (TP) concentrations in surface flow from both soils increased linearly with Olsen P concentration. Compared with dry Organic soil, the wet Brown soil lost a greater proportion of TP as particulate via surface runoff. However, due to the high hydraulic conductivity and low ASC, the most important pathway for the Organic soil, wet or dry, was filtered P loss in drainage. These data can be used to more effectively target strategies to mitigate P losses.

Municipal composts reduce the transfer of Cd from soil to vegetables

Al Mamun S, Chanson G, Muliadi, Benyas E, Aktar M, Lehto N, McDowell R, Cavanagh J, Kellermann L, Clucas L, Robinson B
Environmental Pollution, June 2016

Cadmium (Cd) is a non-essential trace element that accumulates in agricultural soils through the application of Cd-rich phosphate fertiliser. Vegetables can accumulate Cd to concentrations that sometimes exceed food safety standards. We investigated the potential of low-cost soil amendments to reduce Cd uptake by spinach, lettuce and onion. Lignite and compost had the greatest ability to sorb Cd and were subsequently selected for pot trials. The addition of 2.5% (dry w/w) municipal compost reduced the Cd concentration in onions, spinach and lettuce by up to 60% in both soils. The addition of lignite gave variable results, which depended on the soil type and rate of addition. This Cd immobilisation was offset by soil acidification caused by the lignite. The results indicate that municipal compost is a low-cost soil conditioner that is effective in reducing plant Cd uptake.

A review of the policies and implementation of practices to decrease water quality impairment by phosphorus in New Zealand, the UK, and the US

McDowell RW, Dils RM, Collins AL, Flahive K, Sharpley AN
Nutrient Cycling in Agroecosystems, September 2015

The improper use of phosphorus (P) on agricultural land is related to P losses that impair surface water quality. We outline policy in New Zealand, the UK, and the US who have imposed limits for P. We contrast the strategies used and discuss their likelihood of being able to decrease P losses and improve water quality. A poor understanding of catchment processes and critical source areas of P loss has resulted in some areas where regulation has had minimal effect on P discharges. Furthermore, we give several examples where social and economic challenges are of equal if not greater relevance to P discharges (e.g. subsidies). Other policy instruments (e.g. trading schemes) may be needed at larger scales, but should be flexible and encourage innovation. There is increasing recognition among all 3 countries that while targeting good management practices can substantially decrease P losses from existing land use, to achieve ‘good’ water quality in catchment, policy may have to consider land use change.

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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