3 ways of getting the most out of your phosphorus

As environmental concerns around nutrient runoff persist, Canadian researchers both in and outside the Great Lakes region continue to investigate how nutrients – namely phosphoros (P) – can be better managed. Here are 3 examples.

1. Less P, more efficient bioavailability

A 20-year study from Canada’s national agriculture ministry (Agriculture and Agri-Food Canada) indicates that crops growing in soils consistently treated with applied P absorb more than they need.

At test sites in the prairie province of Manitoba, researchers looked at an organic system with no P inputs, an organic system with a single manure application made in 2007, a conventional system with steady nutrient application, as well as restored prairie grassland. Further research with these systems was also conducted in a greenhouse setting (using ryegrass as the plant measure) to better control other influencing factors, such as the presence of weeds.

Overall, results indicate that soil under organic management had lower concentrations of plant-available P compared with the conventional and grassland soils. Ryegrass-centred greenhouse trials showed the biomass uptake from soils receiving no P were significantly lower for the organic management group.

However, it was also observed that P use efficiency increased in the groups with a lower rate of P applications.

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Ben Rosser, a corn specialist with Ontario’s provincial agriculture ministry, discussing tillage style and nutrient management in corn with grain farmers during a University of Guelph management seminar in July 2019. – Photo: Matt McIntosh

Overfeeding makes for lazy plants

Scientist Tandra Fraser says the results indicate that a steady abundance of the nutrient means the soil microbes – which otherwise work to make P available to the crops – have less work to do. Over time this makes them ‘lazy’ and less effective.

In the conventional system, Fraser says the rate of plant P absorption was higher – compared to the level of absorption in other systems – despite the same application rates being used. Biomass also didn’t increase with rate of fertiliser in the conventional system.

This indicates what Fraser calls ‘luxury uptake’, where the plants absorbed more nutrients than they could actually use. “The plants were basically just being greedy. It wasn’t translating to yield at all,” she says.

Basically the bacteria become lazy when they are in an environment with a lot of bioavailable phosphorus

It was also observed that bacteria in the P-starved systems were doing a lot more work to make what P was present available to the crop in question. According to Fraser, bacteria and fungi evolved to find P when they need it, and make up for deficiencies. Enzymic activity, she says, was therefore a lot higher in the soils where P is not abundantly applied.

“It’s fairly widespread in bacteria to turn on a gene that produces enzymes that alter phosphorus more effectively when needed,” she says. “Basically the bacteria become lazy when they are in an environment with a lot of bioavailable phosphorus.”

Same yield possible with less P

Fraser says these revelations indicate less can sometimes be more when it comes to applying P. Limiting inputs can promote natural nutrient processes within the field, while saving farmers money and reducing the risk of P loading in waterways – something which is of particular concern for farmers in the Great Lakes region.

Those interested in reducing the amount of P they apply, says Fraser, can try test strips to determine any yield differences. Following 4R nutrient stewardship and precision nutrient application strategies also makes a significant difference. “You don’t always need a blanket application,” she says, adding that alternative nutrient amendments in organic and conventional systems – such as recycled waste products – should continue to be investigated.

2. Accounting for differences between sites

In the 2016-2017 crop year, Aaron Breimer, general manager for Veritas Farm Management – a drone tech and data-service company based in Ontario – headed a project analysing strip-till strategies which producers could use to mitigate off-target phosphorus movement, while maintaining or increasing productivity and profitability. This involved looking for both yield increases and the potential to reduce fertiliser applications through more accurate placement.

We don’t believe that a field is uniform from one end to the other. There’s considerable variability

Breimer says smart-grid soil analysis techniques were used to divide tillage test plots into zones of expected yield, with fertiliser applied variably based on the yield potential of each zone. Plant tissue samples were taken to see how much phosphorus was being absorbed by the crops, while samples of crop residue and the soil itself revealed how much of the nutrient was water extractable (a runoff risk).

Yield and yield related data were also recorded. “We don’t believe that a field is uniform from one end to the other. There’s considerable variability. We also believe there’s variability in soil texture levels as you go across the field,” says Breimer. “What we were doing is going back and sampling the same area time and time again.”

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Aaron Breimer of Veritas Farm Management: “We need to do a better job of soil sampling.” – Photo: Veritas

Opportunities in higher fertility soils

Breimer says no positive or negative differences in crop productivity were observed between strip tillage and other approaches (minimum-till and no-till). Regarding the potential for applying less P per acre, he says their data indicated that farmers working with very fertile conditions might be able to successfully reduce input amounts without corresponding reductions in yields.

However, he and his colleagues did not find an economic or agronomic justification for doing so. If some areas are fertile, he says, the fertiliser savings there could be transferred to lower-fertility areas. “Traditionally the thinking is if you have phosphorus levels above 35 parts per million, you probably don’t need to apply a lot. But, if you talk to a farmer or a certified agronomist, they might say you need a little,” he says.

Nutrient absorption (temporarily) better in strip-till

The data also showed that crops planted in strip-till rows absorbed phosphorus at a higher rate early in the season – but that gain didn’t last. “By the time we got to maturity, the levels had basically levelled out. Strip-till seemed to give it a little extra kick in the spring,” says Breimer. Additionally, increases in water-soluble phosphorus in soil and crop residues were only significant in plots with very high phosphorus levels. Even in plots with very low levels of available phosphorus – the only places where sulphur deficiencies also appeared to be an issue – no system indicated statistically different crop residue levels.

“Each site was so unique […] The biggest difference was not between different treatments but between different sites,” says Breimer. He adds that useful insights into each test area were found, but those insights became lost once all test data was compiled in aggregate. While he says more research sites and seasons are needed to draw concrete conclusions, Breimer believes strip-tillage is a good compromise between conventional and no-till systems.

He reiterates, however, that basic agronomy practices need to be followed in all systems. “We need to do a better job of soil sampling in Ontario,” he says. “It’s hard to make recommendations without good soil test data.”

3. Practical information for strip-till users, and potential adopters

Other research in Ontario has shown greater opportunities that are specific to fertiliser timing and placement. An ongoing three-year project (from the province’s agriculture ministry) featuring four to five different annual corn trials, was designed to determine the efficiency of fertiliser placement in both autumn and spring applications.

In order to better highlight fertiliser response potential, the test plots were also placed on locations with low P and potassium fertility. The impact of different tillage and fertiliser placements methods are also being analysed. This includes measuring phosphorus loss in strip-till systems where nutrients are incorporated and comparing it to broadcast application in conventional tillage.

Ben Rosser, corn specialist for the provincial ministry – and the head researcher behind the project – says the results are intended to translate to practical fertiliser application strategies for farmers getting into or already using strip-tillage.

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Whether through granular or liquid application, some farmers are incorporating new fertility insights to increase their fertiliser efficiency, while reducing costs. – Photo: Matt McIntosh

Considering soil quality

Data from the first full project year suggests that there was a positive yield response for spring strip-till fertiliser application compared to spring full-width tillage with broadcast fertiliser. Rosser says this was true ‘across most locations’. “We can’t truly say whether this was fertility placement response or difference in tillage systems, but we can speculate that it was more fertility related,” he says.

He elaborates by saying this is because the research began using low-fertility test soils, moderate amounts of applied fertiliser, and other application factors. “I’m surprised there was not much response for moving a portion of broadcast fertiliser to starter placement for full-width tillage. We would have expected a strong response,” says Rosser. Other results showed consistently higher yields for spring strip-till fertility compared to autumn strip-till fertility (149 compared to 141 bushels per acre).

Spring fertility or spring tillage

“We can’t say whether the response is due to the spring fertility or spring tillage, or some combination of the two. Given spring conventional till yields, I would speculate that a large part of this response is from fertility,” says Rosser.

The yield performance in strip-till systems was, on average, very similar to yields in full-width tillage. “Because of fertility applications, we don’t have a true apple-to-apple tillage comparison. The spring strip-till fertility treatment was much higher yielding, but I suspect this was strongly influenced by high-rate fertiliser placement, not just tillage.”

Like Breimer and the Veritas research team – and from a grower recommendation perspective – Rosser adds that it might be important to consider whether the same yield response would be seen on better, more fertile soils. “The results are only from one growing season at this point, so we should interpret this with caution.”