Less nitrogen, but the same crop?

A Canadian field study suggests potato fertiliser rates can sometimes be reduced without sacrificing marketable yield — but the number that matters is local, not universal

By Lukie Pieterse, Editor/Publisher of Potato News Today

A potato grower cutting nitrogen by one quarter would reasonably want an answer to one question before considering anything else: what will happen to the marketable crop?

In field research conducted on Prince Edward Island, Canada, the answer was unexpectedly reassuring.

Scientists compared the locally recommended rate of 160 kilograms of nitrogen per hectare with a reduced rate of 120 kilograms. At two adjacent research sites, the 25% reduction did not lower marketable potato yield.

In one treatment, the reduced rate was supplied as an enhanced-efficiency urea product containing inhibitors intended to slow important pathways of nitrogen loss. Total nitrous oxide emissions were 5.7 times lower than those measured under conventional urea at the recommended rate.

The results, published in the Canadian Journal of Soil Science in 2026, deserve international attention. They also require careful interpretation.

This was not proof that every potato grower is applying 25% too much nitrogen. It was evidence that, under the studied soil, rotation, weather and management conditions, the crop did not benefit from the final 40 kilograms.

That is a more modest finding. Commercially, it may also be the more important one.

The trial was nearly a decade in the making

The work was led by Agriculture and Agri-Food Canada scientist Dr Judith Nyiraneza with Ikechukwu Agomoh, Tandra Fraser, Danielle Murnaghan and Morteza Mesbah.

Its foundation was laid in 2015, when three rotation systems were established at two neighbouring sites. Each rotation ended with potatoes:

barley underseeded with a forage legume, followed by another forage-legume year and then potatoes;
maize followed by sorghum-Sudan grass and potatoes; and
soybean followed by brown mustard and potatoes.

During the potato phase in 2024, the researchers divided the plots to compare nitrogen treatments.

At the first site, conventional urea at 160 kilograms of nitrogen per hectare was compared with 120 kilograms supplied through Super U, an enhanced-efficiency product. At the second, conventional urea at 160 kilograms was compared with conventional urea at 120 kilograms.

The researchers measured potato yield alongside emissions of nitrous oxide, carbon dioxide and methane.

Marketable yield was maintained at the reduced rate at both sites. The greenhouse-gas response, however, depended strongly on rotation, fertiliser form and location.

That variation is central to the story.

Where unused nitrogen goes

Nitrogen is essential to potato production. Too little restricts canopy development, reduces light interception and limits yield. Too much can prolong vegetative growth, delay maturity, reduce dry-matter concentration and complicate skin set and storage preparation.

The crop does not recover every unit applied.

Nitrogen that remains outside the harvested crop may be immobilised in soil organic matter, retained temporarily in microbial biomass, leached as nitrate, lost to the atmosphere as ammonia or converted by soil microorganisms into nitrous oxide.

Nitrous oxide is produced mainly through microbial processes associated with nitrification and denitrification. Its release is strongly influenced by soil nitrogen, moisture, temperature, aeration and carbon supply.

This explains why an apparently simple fertiliser comparison can produce different results between neighbouring fields. Depressions, compacted areas, wet soil and zones containing easily decomposed residues may emit differently from the rest of the field.

The fertiliser bag is only the beginning of the nitrogen story. Soil and weather decide much of what happens next.

How the enhanced product works

Super U is granular urea treated with two inhibitors: NBPT, which slows the conversion process associated with ammonia volatilisation, and DCD, which slows nitrification. The intention is to keep nitrogen in plant-available forms for longer while reducing losses to air and water.

At the first Prince Edward Island site, applying Super U at 120 kilograms of nitrogen per hectare produced total nitrous oxide emissions 5.7 times lower than conventional urea at 160 kilograms, without reducing marketable yield.

It is an impressive comparison, but three variables changed together: fertiliser rate, fertiliser formulation and the processes affected by the inhibitors. The trial does not allow every benefit to be attributed solely to cutting the rate.

At the second site, where both treatments used conventional urea, reducing the rate from 160 to 120 kilograms still maintained marketable yield. That strengthens the evidence that the additional nitrogen was unnecessary under those particular conditions.

Enhanced-efficiency fertilisers also carry a price premium. The relevant calculation is therefore not merely kilograms of nitrogen saved. It includes product cost, yield, grade, maturity, application logistics and the value—whether through regulation, incentives or supply-chain requirements—assigned to reduced emissions.

Rotation altered the emissions

The preceding crops left different quantities and qualities of carbon and nitrogen behind.

At the first site, the barley–forage legume–forage legume–potato rotation generated significantly more nitrous oxide during the potato phase than the other rotations. Legume residues can contribute biologically fixed nitrogen, but when residue nitrogen becomes available during wet or poorly aerated conditions, it can also provide material for nitrous oxide production.

At the second site, the soybean–brown mustard–potato rotation emitted 66% less nitrous oxide than the forage-legume rotation and 50% less than the maize–sorghum-Sudan grass rotation.

This does not mean brown mustard will always produce the lowest emissions, or that forage legumes should be removed from potato rotations.

Legumes can reduce the need for manufactured fertiliser, support soil biology and provide valuable forage. High-residue grasses can improve soil structure and carbon inputs. Brown mustard may offer other rotational benefits but presents its own management considerations.

The lesson is that previous crops contribute to the potato nitrogen budget. A fertiliser recommendation based only on the crop being planted ignores nutrients already moving through the system.

The danger of a universal percentage

The headline temptation is obvious: potatoes can be grown with 25% less nitrogen.

That statement would travel quickly and be agronomically irresponsible.

The study used one potato phase, two adjacent sites and a defined group of rotations under Prince Edward Island conditions. It did not test every cultivar, soil texture, irrigation regime, yield target or market specification.

A crop following a vigorous legume stand may receive substantial nitrogen from decomposing residues. A crop planted in low-organic-matter sand after a nitrogen-demanding cereal may not. Heavy rain can move nitrate below the active roots, while drought can prevent both uptake and mineralisation.

Seed crop, fresh-market and processing potatoes also have different priorities. A nitrogen rate that produces acceptable total yield may alter tuber size, specific gravity, fry colour, skin maturity or storage behaviour.

The proper conclusion is not that every rate should fall by 25%. It is that standard recommendations may contain room for site-specific improvement.

A global review supports moderation—not austerity

The Canadian findings fit a wider body of evidence.

A 2026 global meta-analysis drawing on 3,397 datasets examined the combined effects of irrigation and fertilisation on potato yield, quality, water productivity and nitrogen-use efficiency. Its overall assessment favoured sufficient irrigation paired with a mild reduction in nitrogen, rather than aggressive restriction of both inputs.

An earlier potato-specific meta-analysis found that enhanced-efficiency fertilisers can improve nitrogen-use efficiency and reduce losses, but their performance varies with soil, weather and management. Nitrification inhibitors frequently lowered nitrous oxide emissions without significantly changing yield, yet no product performed identically in every situation.

These results point towards optimisation, not deprivation.

A well-fed crop that converts a larger share of applied nitrogen into saleable tubers is the objective. Reducing the rate below crop demand merely exchanges an environmental concern for a yield and income loss.

The commercial question is the marginal kilogram

Nitrogen recommendations are often discussed as if there were a sharp boundary between the correct and incorrect rate. Crop response is usually less tidy.

Yield rises as nitrogen is added, then approaches a plateau. Near that plateau, another unit of nitrogen may create little additional saleable crop. The economic optimum can therefore be lower than the rate producing the absolute maximum biological yield.

The final kilograms may also carry hidden costs. Excessive canopy delays maturity, increases haulm-management difficulty and can leave the crop less prepared for harvest or storage. Unused nitrogen represents money that never became product.

Yet growers understandably protect themselves against under-application. Weather uncertainty, uneven soil supply and high contract values make a small nitrogen reserve feel safer than a visible deficiency.

Moving beyond that insurance mentality requires credible field evidence. It also requires buyers and sustainability programmes to avoid transferring all experimentation risk to the grower.

If processors want lower-emission potatoes, they may need to support replicated trials, soil and tissue testing, measurement technology and contractual protection when a responsibly designed treatment underperforms.

Better measurement before lower rates

A first step is to construct a field-specific nitrogen budget.

Soil nitrate testing, organic-matter history, manure records, previous crops and anticipated residue mineralisation all contribute useful information. Petiole or tissue analysis can help assess crop status during the season, although sampling method, growth stage and cultivar-specific interpretation matter.

Canopy sensing and satellite imagery can identify spatial differences, but a green canopy is not proof of efficient nitrogen conversion. Yield, grade, dry matter and maturity must close the loop at harvest.

Strip trials offer a practical route. A grower can compare the standard programme with one or more modest reductions across representative parts of a field, preserving treatment identity through grading and quality assessment.

The experiment should be repeated. One wet or dry year can reverse an apparent advantage, while soil nitrogen supply changes across a rotation.

Variable-rate application may eventually allow higher rates where the crop is likely to respond and lower rates where mineralisation or manure history already supplies more. That potential depends on reliable management zones and equipment capable of executing the prescription.

Technology is useful only when it improves the decision.

A better question for the fertiliser plan

The Prince Edward Island trial does not make nitrogen less important. It makes knowledge of nitrogen supply more important.

The researchers demonstrated that 120 kilograms per hectare could preserve marketable yield where 160 kilograms had been the recommended comparison. They also showed that fertiliser formulation and rotation altered the emissions outcome substantially.

The opportunity lies in finding the nitrogen the crop does not need—field by field, season by season—without withholding the nitrogen it does.

That requires more effort than following a familiar rate. It also offers something increasingly valuable: lower input exposure, reduced loss to the environment and evidence that sustainability need not be purchased with fewer saleable potatoes.

The question at the start of the season should therefore not be, “How much nitrogen did we apply last year?”

It should be, “How much nitrogen can this field already supply, and what additional amount will the next kilogram genuinely return?”