Corn School: Which leaf identifies nitrogen needs?

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Growers and agronomists typically focus on the ear leaf at flowering when tissue testing corn to determine nutrient needs.

But is that still the best choice for determining the optimal nutrient prescription for the crop as it moves into the reproductive stages? Purdue University professor emeritus Dr. Tony Vyn believes the evolution of modern hybrids dictates that corn growers need to change how they read their corn leaves and reconsider tissue testing practices.

At the 2024 Ontario Agricultural Conference at the University of Guelph’s Ridgetown campus, Vyn shared a presentation on the changing characteristics of corn leaves and how this evolution impacts nutrient testing.

“First of all, the leaves are more erect, especially at the top of the plant and at the bottom of the plant,” says Vyn. The leaves are also appearing faster and maintaining their greenness longer in the reproductive period. As well, new hybrids are closing the canopy faster and silking earlier, which results in a quicker start to grain fill, he notes.

How does this leaf evolution impact nutrient testing? On this episode of the RealAgriculture Corn School, Vyn tells host Bernard Tobin that “the timing and the leaf that should be selected should be changing because of the change in hybrids, as well as the change in plant density.”

Vyn recommends that growers always start their nutrient plans with a robust soil testing plan. There are also opportunities to test for specific nutrient deficiencies that have historically been done up to the sixth leaf by sampling whole plants. And when the crop gets to the R1 stage, the tissue testing focus typically moves from the top leaf to the ear leaf. At this stage, however, mounting research evidence indicates that growers should adjust their sights and focus on different leaves.


When it comes to tissue testing for nitrogen, Vyn says growers should go lower. “In the preliminary work that we’ve done with modern hybrids, especially those growing at higher plant densities, if we want to get the best indicator or status of the nitrogen concentration in the whole plant, we should be at least two leaves below the ear leaf,” he says.

The story is similar for potassium. “We can probably go two or three leaves below the ear leaf and have a better indicator of an approaching potassium deficiency,” says Vyn who also shares leaf recommendations for phosphorus and sulphur.

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