Crops · Corn

Corn

Corn has a big nutrient appetite and a short window to feed it. Problems usually come from something running short, not something obvious going wrong.

Corn field

Corn pulls nutrients hard from knee-high through tasselling. It needs nitrogen moving steadily through the plant and phosphate in the root zone from the day it comes up. The rest of the program (sulfur, zinc, potassium) is there to back up the N and P. If yields are off, the cause is usually a nutrient the crop ran short on weeks before you noticed.

Spotting deficiency in the field

The nutrients most likely to limit corn yield, what each one looks like before you sample, and how to confirm.

Nitrogen diagnostic
photo coming soon

Nitrogen

Keeps the plant green, drives leaf area and grain protein.

What it looks like
Older lower leaves turn yellow from the tip back toward the stalk, making a pale V pattern down the centre of the leaf. Plants look pale and thin. If it gets bad, the bottom leaves die and fall off.
Where to look
Start at the bottom of the plant. Nitrogen moves inside the plant, so it gets pulled out of old leaves to feed new growth.
When it shows up
Knee-high through tasselling. Can show again later if N runs short during grain fill.
Confirm with
A soil nitrate test taken when corn is knee-high (often called a PSNT). A tissue test from the ear leaf at tasselling will confirm if it shows up later.

Phosphate

Roots, energy, early vigor.

What it looks like
Young plants look stunted and dark green with a purple or reddish tint on leaf edges and the underside. Growth stalls. Cold, wet springs make it more visible.
Where to look
Early-season plants, up to about the six-leaf stage. Worst on cool clay ground.
When it shows up
Emergence through early vegetative growth. Late phosphate can't fix an early miss.
Confirm with
Fall or early-spring soil test. If purple shows up after the ground warms, a tissue test helps — cold alone can cause similar colour.
Maize leaf showing phosphorus deficiency symptoms
Phosphate-short maize: purpling and darkening across the leaf.
Maize plants with severe potassium deficiency on weathered soil
K-deficient maize: yellowing and browning along leaf margins.

Potassium

Stalk strength, water handling, finishing the ear.

What it looks like
Leaves brown along the edges and work inward. Stalks get brittle and lean. Lodging risk goes up.
Where to look
Bottom leaves first. Like nitrogen, the plant pulls potassium from old growth to feed new growth.
When it shows up
From knee-high through tasselling. Most dramatic if reserves run out late in the season.
Confirm with
Soil test is the starting point. A tissue test from the ear leaf at tasselling confirms.

Zinc

Even stand, kernel fill, enzyme activity.

What it looks like
White or yellow stripes between the leaf veins on new growth. Plants look short and bunched up with tight spacing between leaves.
Where to look
Upper, newer leaves. Zinc doesn't move inside the plant, so symptoms show in the newest growth.
When it shows up
Early to mid vegetative. Most common on sandy fields and fields with high soil pH.
Confirm with
Tissue test from the upper leaves. Soil tests showing low zinc back it up.
Maize leaf showing zinc deficiency symptoms
Zinc-short maize: pale striping between the veins on new growth.
Sulfur diagnostic
photo coming soon

Sulfur

Protein, helps the crop use nitrogen efficiently.

What it looks like
The whole plant looks pale yellow-green, top and bottom roughly the same shade. Easy to mistake for nitrogen deficiency, but sulfur is more uniform.
Where to look
Whole plant, but new growth is usually the clearest signal. Sulfur doesn't move around inside the plant.
When it shows up
Early vegetative through tasselling. More common on sandy fields and after cool, wet springs.
Confirm with
Tissue test. Soil tests for sulfur aren't very reliable. A high ratio of nitrogen to sulfur on the tissue report is the usual clue.

When it isn’t a nutrient problem

Other things to rule out before you tissue-test or change the program.

Compaction from working wet

What you see — Short, yellow plants in tire tracks. Roots growing sideways instead of down.

What to do — Dig a pit and check for a hard layer at 4 to 6 inches. Deep tillage or a taprooted cover crop can help. The real fix is staying off wet ground next year.

Uneven emergence

What you see — Plants at different leaf stages right next to each other. Gaps and doubles in the row.

What to do — Usually a planter or seedbed problem, not a fertility problem. Check seed depth, downforce, and whether the soil was uniform when you planted.

Sidewall compaction

What you see — Seedlings with roots growing along the seed slot instead of down into the soil.

What to do — From planting when the ground was too wet. You can't fix it once it's there. Wait for crumbly soil next time.

Herbicide carryover

What you see — Stunted or twisted plants in patches, often in strips across the field.

What to do — Check last season's residual herbicide label for re-cropping intervals. Dry weather and high pH slow the breakdown. Rotate chemistries if you see this more than once.

Drought that looks like a deficiency

What you see — Rolled leaves and dull colour during dry stretches. Plants look stressed but recover after rain.

What to do — Wait for the next rain before running a tissue test. Drought can mimic several deficiencies. If symptoms stick around after moisture returns, then sample.

The program, at a glance

Rates and timing for corn. We’ll match the specific product to what’s available for your season.

Pre-plant
Broadcast about 3 tonnes per acre

Sets up the season. Work it in shallow so the nutrients sit in the active root zone.

At planting
Starter MicroBoost broadcast at about 0.5 tonnes per acre

Gets corn out of the ground quickly with a phosphate boost — especially important on cold clay where mineralization is slow.

Side-dress (optional)
Extra nitrogen before canopy close

Useful on sandy ground that leaches nitrogen. Usually not needed on clay or loam.

What changes by soil

Same crop, different ground — emphasis, placement, and timing shift.

Sandy

Sandy soil leaches nitrogen and dries out fast. Lean on products with organic matter, consider splitting your nitrogen, and broadcast a starter pass to get the crop ahead of the first dry spell.

Clay

Cold clay is slow to release nitrogen in spring, so the crop leans on what you applied up front. Broadcast phosphate at a higher rate to overcome clay's tie-up — concentration matters when you're spreading wide.

Loam

Loam does most of the work. The job is to keep soil capital from drifting downward. A balanced broadcast plus a light starter covers most fields.

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Brent Veens
Brent Veens
Product Advisor
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From the Knowledge Hub

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Image credits

Photos on this page come from Wikimedia Commons under public domain or Creative Commons licenses. Each is attributed to its author and linked to the source.

Images are for reference and may not match your exact field conditions. Always confirm a suspected deficiency with a soil or tissue test before treating. We do not imply endorsement by the photographers.