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 Helen Lammers-Help Nutrient stratification in no-till What’s the reality? Nutrient stratification can occur when farmers repeatedly apply fertilizer to the soil surface in a no-till system without doing any tillage. ONTARIO GRAIN FARMER AGRONOMY Photo: OMAFA
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