Since the end of the second world war, adding phosphorus to soils in the form of mineral fertiliser has played a crucial role in increasing and maintaining global food production in response to population growth.
Author
- Leo M Condron
Distinguished Professor of Soil Science, Lincoln University, New Zealand
However, plants only take up 20-50% of the phosphorus from fertiliser applications.
The unused phosphorus is mainly stabilised through chemical reactions which bind it to soil minerals. It accumulates as "legacy soil phosphorus", with an increased risk of contributing to water pollution.
In New Zealand, phosphorus for fertilisers is manufactured exclusively from imported phosphate rock. But there is an alternative to using rock-based phosphorus.
Struvite is a phosphorus-rich compound that can be recovered from wastewater treatment processes.
As our research shows, struvite raises phosphorus uptake by plants and may reduce transfer to waterways. It could at least partially replace conventional phosphorus fertilisers.
Recovering phosphorus from waste
A combination of factors has resulted in significant cost increases for mineral phosphorus fertilisers , while economic and geopolitical instability disrupted supply chains and caused further price volatility.
For example, after decades of relative stability, the cost of single superphosphate has doubled in price from NZ$200 to $400 per tonne since 2000, with notable spikes in 2008 ($480 during the Global Financial Crisis), 2022 ($500 at the start of the Russian invasion of Ukraine) and 2026 ($550 because of Middle East conflicts).
It is widely acknowledged that we need to improve the efficiency of phosphorus use in primary production systems . This can be achieved by using different plants in crop and grassland systems to enhance uptake from fertiliser and legacy soil phosphorus, and by recovery and reuse of phosphorus from waste.
Struvite can be recovered at scale from nutrient-rich municipal wastewater and industrial waste from sources such as milk processing. The technology for isolating and purifying struvite from wastewater is well established.
For example, Watercare Services recovers struvite at the Mangere Sewage Treatment Plant and sells it mainly as garden fertiliser .
Struvite removal from wastewater also benefits the environment by reducing direct discharge of phosphorus and nitrogen into waterways.
Better uptake by plants, lower loss from soil
Research from North America and Europe shows that struvite can be as effective as conventional mineral fertiliser as a source of phosphorus for plants.
However, most of this research has been carried out on annual crops grown in neutral to alkaline soils (pH higher than 6.5). New Zealand primary production is mainly based on perennial mixed-species grasslands grown in acidic soils (pH lower than 6.5).
We recently completed a comprehensive investigation of struvite as a source of phosphorus for New Zealand grasslands. This involved a combination of field, controlled-environment and laboratory experiments, and included soils with different phosphorus retention capacities.
In a two-year field trial, we found that applying phosphorus as struvite increased perennial ryegrass uptake by more than 30% , compared with single superphosphate.
This means that using struvite instead of single superphosphate, a similar plant yield could be achieved with a third less phosphorus added to the soil.
Our findings are consistent with data from a three-month glasshouse experiment carried out on two acidic grassland soils with contrasting relative phosphorus retention capacities (12% and 96%).
On average, plant phosphorus uptake was 49% higher from struvite compared with single superphosphate, especially for soils with a lower phosphorus retention capacity.
Soil analyses conducted at the conclusion of both the field and glasshouse experiments revealed that concentrations of potentially plant-available phosphorus were higher when struvite was applied.
Further confirmation came from a laboratory incubation experiment carried out on three acidic grassland soils with different phosphorus retention capacities (12%, 45% and 94%). Results show, on average, increases in plant-available phosphorus 63 days after application were markedly higher for struvite compared with single superphosphate.
Finally, results from a controlled-environment study reveal that struvite significantly reduces immediate phosphorus loss from soil through drainage compared with single superphosphate. Total phosphorus measured in leachate from a soil with a low phosphorus retention capacity was 80% higher for applications of single superphosphate compared to struvite.
Overall our findings clearly demonstrate that struvite recovered from wastewater could be a viable and sustainable alternative source of phosphorus fertiliser for primary industries in New Zealand.
The next step is to quantify how different soil types, plants and environmental variables influence the effectiveness of struvite phosphorus and to assess its commercial viability.
Leo M Condron does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.
