Nutrient stratification in no-till

Nutrient stratification can occur when farmers repeatedly apply fertilizer to the soil surface in a no-till system without doing any tillage.

This happens for two reasons, says Dr. John Lauzon, an associate professor at the University of Guelph. Lauzon spoke as part of a panel at the 2025 Southwest Agricultural Conference.

The first reason is the more obvious one: surface application of fertilizers leads to a build-up of nutrients in the uppermost layer of the soil.

However, Lauzon says that nutrient stratification also occurs naturally due to plants’ uptake of nutrients from the entire root zone. Some of these nutrients are removed from the harvested grain, but whatever is left in the crop residue on the soil surface is eventually released. For phosphorus, about 20 per cent of what the crop takes up remains in the residue, but for potassium, that number is 65 per cent.

Nutrient stratification primarily affects phosphorus and potassium. While nitrate and sulphate may be temporarily stratified, Lauzon says these ions are mobile in the soil water and will not remain stratified long-term. On the other hand, he says potassium occurs as an exchangeable cation in the soil and moves only slowly in the soil water. As a result, it will stay stratified in the soil. Phosphorus forms a precipitate with iron, aluminum, and calcium in the soil and moves even less than potassium, so it also becomes stratified.

WHAT ARE THE IMPACTS OF NUTRIENT STRATIFICATION?

When the soil dries out at the surface due to a lack of rainfall, nutrients stratified near the soil surface may be positionally unavailable to the roots which cannot access them in dry soil, says Lauzon.

Higher levels of phosphorus in the topmost layer of soil can also lead to downstream water quality problems. Sheet erosion is the loss of a thin layer of soil from the surface of a field. While not as obvious as rill or gully erosion, Lauzon says it can result in a significant amount of soil erosion. Erosion of soils with high potassium levels is a concern because it causes algae growth in lakes. While water-soluble potassium causes an immediate burst of algae growth, the potassium attached to soil particles, known as ‘legacy potassium,’ can continue to contribute to algae growth for a long time.

WHAT DOES NUTRIENT STRATIFICATION LOOK LIKE IN THE FIELD?

Dale Cowan, an agronomist with AGRIS Co-op in Chatham, compared soil samples taken at two-inch increments to a depth of six inches with six-inch cores. The samples were taken on both a clay loam and a sand in a no-till system. He found there was stratification in both soils, but that stratification of phosphorus and potassium was more significant in the clay loam soil compared to the sandy soil.

WHAT IS THE IMPACT ON SOIL SAMPLING?

Cowan concluded that it was important to maintain a consistent six-inch depth when taking soil samples. Taking samples at a shallower depth could overestimate phosphorus and potassium soil test levels when nutrient stratification is present.

In a field where fertilizer was applied in a band during strip tillage, Cowan compared random sampling across the rows with sampling between the rows, sampling in the band, and using a standardized template to guide sampling. Using a limited data set showed that a good randomized sampling regime but with attention to avoiding sampling right in the band, gave satisfactory results. He noted that “after several years of strip-till and moving the location of the strip, it may remove any concerns with soil sampling patterns.”

The traditional tried-and-true method for applying fertilizer with the planter is to band it two inches to the side and two inches below the corn seed. “The closer you get to the seed, the greater the chance of injury, so the application rate would need to decrease to avoid fertilizer injury,” says Lauzon. He emphasizes the importance of ensuring that the planter is set up properly so the band placement is actually where you want it. He says you shouldn’t assume that the manufacturer has set it up properly.

Applying fertilizer with a strip-till unit opens up new possibilities for fertilizer placement.

In an ongoing study, with funding from the Grain Farmers of Ontario, Lauzon, along with Ontario Ministry of Agriculture, Food and Agribusiness corn specialist Ben Rosser, aims to determine the safe rates of fertilizer application in various placements with a strip tillage unit.

“The optimal rate meets the needs of the plant but does not cause injury,” says Lauzon, who is comparing fertilizer toxicity to the seed when fertilizer is placed in one of four locations: in a band four inches below the soil surface, a double band (which had the advantage of having only half the concentration in each band), a deeper six- inch band, and dribbling it in front of a wavy coulter.

Early results showed that the placement four inches below the soil surface sometimes placed the fertilizer too close to the seed. However, the deeper six-inch band may be too far from the seed to see the benefit of early potassium in corn. In the treatment where fertilizer was dribbled in front of the coulter, the fertilizer did not get mixed in very well and resulted in toxicity to the corn seed.

IS IT A GOOD IDEA TO PERFORM TILLAGE TO REDUCE STRATIFICATION IN NO-TILL?

While anecdotally, there are reports of farmers experiencing a yield boost after performing a tillage operation in a previously undisturbed system, Cowan says it is not known if this yield increase is due to remixing phosphorus and potassium that’s near the surface, the release of nitrogen from decomposing organic matter, or getting more air into the soil.

“I’m a big fan of multiple methods of fertilizer placement,” says Cowan. “I think the whole top six inches needs a good distribution of phosphorus and potassium. So broadcast, strip-till, some incorporation…I see nothing wrong with managing nutrients in that manner.” •