Corn Nutrient Guide

Corn is a heavy feeder. A 200-bushel crop removes roughly 150 pounds of nitrogen, 75 pounds of P2O5 (phosphorus), and 55 pounds of K2O (potassium) per acre in the grain alone, with another 80-100 lb of N, 25 lb of P2O5, and 150 lb of K2O tied up in the stover that may or may not return to the soil. Replacing what the crop removes is the foundation of a fertilizer program; maintaining soil test levels above critical thresholds is how you know the program is working.

The three primary macronutrients play distinct roles. Nitrogen drives leaf area, chlorophyll, and protein — it's the biggest yield driver among nutrients and the most prone to loss. Phosphorus drives root development, early vigor, and energy transfer; a deficiency shows up as stunted, purple seedlings. Potassium drives stalk strength, disease resistance, and water regulation; a deficiency shows up as yellow leaf margins and lodging. Getting all three right is the foundation of high-yield corn.

Corn takes up about 1.0 to 1.4 pounds of nitrogen per bushel of grain produced, with 1.2 lb N/bu being the standard university recommendation. A 200 bu/ac crop therefore needs about 240 lb of N from all sources — soil mineralization, manure, legume credits, and fertilizer. After subtracting credits, the remaining N comes from fertilizer, typically applied as anhydrous ammonia, urea, or UAN solution.

Timing matters as much as rate. Corn takes up most of its nitrogen between V6 and tasseling (June to mid-July in the Corn Belt). Applying all N pre-plant in April exposes it to 8-10 weeks of leaching and denitrification risk before the crop can use it. Split applications — 30-50% pre-plant, 50-70% sidedressed at V5-V6 — keep N in the root zone when the crop needs it. On sandy or tiled ground, split applications are essential; on heavy untiled ground with low leaching risk, all pre-plant can work.

Deficiency symptom: yellowing (chlorosis) starting at the tip of the lower leaves and progressing up the midrib. N is mobile in the plant, so the plant pulls it from older leaves to feed newer growth — deficiency shows on the oldest leaves first. By the time you see deficiency, yield has already been lost.

Nitrogen stabilizers (nitrapyrin, DCD, NBPT) slow the conversion of ammonia to nitrate, keeping N in the root zone longer. They're worth the cost on tiled or sandy ground, wet springs, and any field with leaching history. On well-drained heavy ground in a dry year, the payback is less clear.

Corn needs about 0.35-0.45 lb P2O5 per bushel — roughly 75-90 lb/ac for a 200 bu crop. Unlike N, P is relatively immobile in the soil and doesn't leach. The challenge with P is availability: it's most available at soil pH 6.5-7.0 and gets tied up at lower pH (acidic) or higher pH (calcareous). Cool, wet soils also reduce P uptake, which is why deficiency often shows up as purple seedlings in early planting.

Soil test P is the right way to set P rates. Build soil test P to the critical level (Bray P1 of 20-25 ppm, or Olsen P of 15-20 ppm on calcareous soils), then maintain it by replacing crop removal. Above the critical level, additional P fertilizer doesn't increase yield. Below it, yield is constrained regardless of how much N you apply.

Deficiency symptom: purple or reddish coloration on the older leaves, stunted growth, and delayed maturity. The purple color comes from anthocyanin buildup when phosphorus is limiting. Cool spring weather can cause temporary P deficiency even on adequate soils — the crop grows out of it as soils warm.

Placement: starter fertilizer (banded 2x2 or in-furrow) puts P where the seedling can reach it before the roots expand. Starter is especially valuable in no-till, high-residue, and cool-soil situations. Broadcast P works but is less efficient on low-testing soils.

Corn removes more potassium than any other nutrient in the stover — about 0.3 lb K2O per bushel in the grain (60 lb/ac for 200 bu) plus 150+ lb/ac in the stover. If stover is removed (baling, silage), K replacement needs to be much higher than if stover is returned. K drives stalk strength, water regulation, and disease resistance; a K-deficient crop lodges more and yields less.

Soil test K is the right way to set K rates. Build to the critical level (around 150-200 ppm depending on soil CEC), then maintain. Like P, K is relatively immobile and doesn't leach significantly. But K can be tied up in clay lattices (especially in 2:1 clays) and released slowly over time.

Deficiency symptom: yellowing (chlorosis) along the leaf margins of older leaves, starting at the tip. The interior of the leaf stays green. Severe deficiency causes lodging and stalk rot. Like N, K is mobile in the plant and deficiency shows on older leaves first.

Beyond N-P-K, corn needs smaller amounts of sulfur, zinc, and occasionally boron and magnesium:

• Sulfur (S): Corn needs about 0.1 lb S per bushel — roughly 20 lb/ac for a 200 bu crop. S deficiency has become more common as atmospheric S deposition has declined (Clean Air Act). Symptoms resemble N deficiency (yellowing) but show on younger leaves first, because S is relatively immobile in the plant. Apply 10-25 lb S/ac as ammonium sulfate, gypsum, or elemental S.
• Zinc (Zn): Deficiency shows as interveinal chlorosis on young leaves, often in bands or streaks. Most common on high-pH, calcareous, or sandy soils, and after heavy P application. Apply 1-5 lb Zn/ac as zinc sulfate if soil test Zn is below 1 ppm.
• Boron, magnesium, manganese: Deficiencies are rare in the Corn Belt but can occur on sandy, low-OM, or high-pH soils. Soil and tissue testing identify problems.

You can't manage what you don't measure. A soil test every 3-4 years (every 2 years on high-removal ground) is the foundation of a fertilizer program. Sample by management zone, not by whole field, to capture variability. A good soil test tells you pH, organic matter, P, K, and (with the right test) micronutrients. Set rates to build low-testing nutrients to the critical level, then maintain by replacing crop removal.

Tissue testing (collecting leaf samples at R1) confirms what the soil test predicts and catches in-season deficiencies. Pair a soil test (every 3 years) with a tissue test (annually) for the most complete picture of nutrient status. Both are cheap relative to the cost of a wrong fertilizer decision.