Nitrogen for Corn

Corn Nitrogen Requirements

Corn takes up nitrogen throughout the growing season, with peak uptake between V8 and tasseling (June-July in the Corn Belt):

• Total N requirement: 1.0-1.2 lb N per bushel of yield
• 200 bu/ac crop: 200-240 lb N/acre total
• 180 bu/ac crop: 180-216 lb N/acre total
• 150 bu/ac crop: 150-180 lb N/acre total

Not all of this N needs to come from fertilizer. Soil organic matter releases N through mineralization — about 30 lb N per 1% organic matter above 1%. Previous crops also provide N credits: soybeans leave about 50 lb N/ac, alfalfa 100-150 lb N/ac.

Nitrogen Credits from Soil and Previous Crops

Reduce fertilizer N rate by these credits:

Soil organic matter credit: 30 lb N per 1% OM above 1%. Example: 3% OM = 60 lb N credit.

Soybean previous crop: 50 lb N credit (soybean residue decomposes quickly, releasing N)

Alfalfa previous crop: 100-150 lb N credit (depends on stand density; up to 4 years of credit)

Wheat previous crop: 20 lb N credit (small but measurable)

Manure application: Variable — test manure for nutrient content. Typical: 4-8 lb N/ton for dairy, 8-12 lb N/ton for poultry.

Legume cover crops: 30-100 lb N credit depending on species and biomass. Crimson clover = 50-80 lb; hairy vetch = 80-120 lb.

Nitrogen Sources for Corn

Several nitrogen fertilizer sources are available, each with different properties:

Anhydrous Ammonia (82-0-0): Cheapest per lb N. Apply fall or spring pre-plant. Most volatile — needs N-serve or waiting for soil <50°F. Pressurized equipment required.

Urea (46-0-0): Dry, easy to handle. Surface-applied urea loses N to volatilization — incorporate within 3 days or use Agrotain inhibitor. Good for sidedress.

UAN Solution (32-0-0 or 28-0-0): Liquid. Easy to apply with herbicide or split applications. Sidedress UAN is very efficient. Most flexible N source.

Ammonium Sulfate (21-0-0-24S): N + sulfur. Good for no-till and high-residue situations. Acidic reaction helps in high-pH soils.

DAP (18-46-0) and MAP (11-52-0): Primarily P sources, but provide some N. Account for this N when calculating total N rate.

Manure: Variable analysis. Test before application. Organic N releases over 3-4 years. Great for soil health and microbial activity.

Nitrogen Application Timing

Corn takes up most of its N between V8 and tasseling. Split applications match N availability with crop demand:

Fall anhydrous: Apply after soil <50°F (late October-November). Use N-serve inhibitor. Best for poorly drained soils that dry out slowly in spring.

Spring pre-plant: Apply 30-50% of total N before planting. Allows early weed-and-feed program with UAN.

Sidedress (V3-V8): Apply remaining N as UAN with coulters or Y-drops. Most efficient timing — matches crop demand.

Foliar/late N: Small amounts (10-20 lb N/ac) at tassel can boost protein, but minimal yield response. Generally not cost-effective.

Best practice: 30% pre-plant + 70% sidedress around V5-V6. Reduces N loss risk and improves efficiency.

Nitrogen Loss Pathways

Nitrogen can be lost through several pathways:

• Leaching: Nitrate moves with water through soil profile. Worst on sandy soils and with excessive rainfall.
• Denitrification: Bacteria convert nitrate to N gases under saturated conditions. Worst on heavy clay soils in wet springs.
• Volatilization: Urea converts to ammonia gas and escapes from soil surface. Worst with surface-applied urea in warm, windy conditions.
• Runoff: Dissolved N moves off-field in surface water. Worst on sloped fields with bare soil.

Each pound of N lost is one pound of yield potential lost. Using the right N source, timing, and inhibitor can reduce losses significantly.

Nitrogen Efficiency Strategies

Improve N efficiency with these strategies:

• Split applications: Apply 30% pre-plant + 70% sidedress. Reduces loss risk.
• Use inhibitors: N-serve for fall anhydrous, Agrotain for surface urea. Cost-effective insurance.
• Sidedress at V5-V6: Matches N availability with crop demand. Improves efficiency 10-20%.
• Variable rate application: Apply more N on high-productivity areas, less on low-productivity. Use yield maps and soil tests.
• Cover crops: Rye captures residual N after corn. Cuts N loss and improves soil health.
• Nitrification inhibitors: Keep N in ammonium form longer. Especially valuable in wet springs.

Nitrogen Cost Management

Nitrogen is typically 25-30% of corn production cost. Optimize N spending:

• Soil test: Don't apply N blindly. Use soil tests and yield goals to determine rate.
• Source wisely: Anhydrous is cheapest per lb N; UAN is most flexible. Price varies by year.
• Time it right: Sidedress applications are most efficient. Don't apply all N upfront.
• Capture credits: Account for soybean/alfalfa credits and manure N.
• Use inhibitors strategically: Pay $5-10/ac for inhibitor to protect $80-120/ac of N.
• Variable rate: Don't apply high N rates to low-productivity areas where yield is limited.