AAFC research finds blending recycled struvite with conventional phosphorus fertilizer can maintain crop performance while offering environmental benefits
Canadian researchers are exploring whether a fertilizer recovered from wastewater could become part of a more sustainable approach to phosphorus management on farms.
Research from Agriculture and Agri-Food Canada (AAFC) indicates that struvite, when used alongside conventional phosphorus fertilizer, could help farmers maintain crop yields while reducing phosphorus losses and improving nutrient-use efficiency.
Struvite captures phosphorus and nitrogen from municipal and industrial wastewater before those nutrients reach the environment. Following processing, the recovered material becomes a granular fertilizer that releases nutrients more slowly than conventional phosphorus products.
For agriculture, that creates an intriguing proposition: recover valuable nutrients from a waste stream and return them to crop production.
Why phosphorus management matters
As farmers know, phosphorus is an essential crop nutrient, but its primary source, phosphate rock, is non-renewable.
AAFC says nearly 82 per cent of the world's phosphate rock supply is used to produce phosphorus fertilizer, consuming more than one million tons annually.
There are environmental considerations as well.
Conventional phosphorus fertilizers dissolve readily in water. Where phosphorus applications exceed crop requirements, the nutrient can accumulate in soil as "legacy phosphorus." That stored phosphorus can subsequently contribute to runoff into nearby waterways.
The issue can be particularly important in areas of Canada that receive substantial precipitation.
Struvite's slower nutrient release gives researchers an opportunity to determine whether farmers can continue supplying crops with the phosphorus they need while reducing the potential for losses.
Potato trials put struvite to the test
AAFC researchers examined struvite in potato production systems in Eastern Canada, where higher rainfall and sloping landscapes can increase phosphorus runoff risks.
Field trials were conducted at AAFC research centres in Prince Edward Island, New Brunswick, Nova Scotia and Quebec. Researchers compared conventional phosphorus fertilizer with struvite and different blends of the two products.
One of the key questions was whether the environmental advantages could be achieved economically.
That is important because struvite is currently more expensive. AAFC reports its cost per unit of phosphorus is about "2.19 times the cost of conventional fertilizer", as recovery technology continues to be developed.
AAFC research scientist Dr. Mohammad Khakbazan evaluated the economics by considering production costs, crop yields and environmental impacts. The analysis included expenses such as seed, pesticides, machinery operation, hired labour, insurance, utilities, interest and land investment.
The analysis identified a blend containing "75 per cent conventional fertilizer and 25 per cent struvite*" as the most effective approach examined.
In Quebec and Prince Edward Island, that combination produced noticeable increases in potato yields while generating returns on investment that were comparable to, or better than, conventional fertilizer alone.
The results weren't uniform across the region, however. Outcomes differed in Nova Scotia and New Brunswick, which AAFC says was likely related to differences in soils and climate.
That variability reinforces the importance of considering local growing conditions when assessing alternative nutrient sources.
Environmental costs fell by up to 22 per cent
AAFC's environmental analysis found that adding struvite reduced environmental costs by as much as 22 per cent, largely through lower phosphorus runoff.
Researchers also found that fewer fertilizer applications during the growing season reduced operating costs.
For growers, those findings suggest the economics of alternative fertilizer sources cannot necessarily be judged on the purchase price of the nutrient alone.
"Despite higher upfront costs, the right mix of struvite fertilizer can offer a renewable alternative for potato farmers without sacrificing yields," Khakbazan said in the AAFC information, adding that incorporating even a relatively small amount could provide environmental benefits by reducing phosphorus runoff into waterways.
What about Prairie farms?
Researchers also examined struvite under Prairie conditions, where phosphorus losses present a different challenge.
AAFC says much of the phosphorus loss on the Prairies can occur during spring snowmelt. Frozen soils restrict infiltration, allowing dissolved nutrients to move with runoff.
Dr. Henry Wilson, an AAFC researcher in Brandon, Manitoba, studied struvite in organic forage systems and compared runoff losses in conventional soybean and wheat fields receiving struvite with fields receiving phosphorus from conventional fertilizer or manure.
The research found soil phosphorus levels continued to be an important influence on runoff risk. However, phosphorus losses during snowmelt were consistently lower than expected following struvite applications, including at similar soil-test phosphorus levels.
AAFC says those findings suggest struvite's low solubility may reduce the movement of phosphorus into runoff while continuing to support crop production and soil fertility.
A complement, rather than a replacement
For farmers, perhaps one of the most important findings is that researchers aren't positioning struvite as a wholesale substitute for existing fertilizer.
Instead, AAFC describes it as a product that could "complement conventional phosphorus fertilizer".
The potential benefits also differ by region. In Eastern Canada, research indicates struvite could help address legacy phosphorus and runoff while supporting crop yields. On the Prairies, research points toward an opportunity to reduce phosphorus losses associated with runoff and leaching.
At the national level, recovering phosphorus and nitrogen from wastewater and returning those nutrients to agricultural production could also contribute to a more circular nutrient economy.
Cost remains an important consideration, particularly while struvite phosphorus carries a significant premium over conventional fertilizer.
But the AAFC research suggests that farmers evaluating the technology may eventually have more to consider than the price per unit of phosphorus. Yield response, application costs, nutrient efficiency, soil conditions and the potential to reduce nutrient losses could all play a role in determining where struvite fits on Canadian farms.
Photo Caption: Measuring phosphorus run-off in Manitoba.
Photo Credit: AAFC