Dairy Manure Injection on Cornfield in Wisconsin

Scenario

Project Type: Precision nutrient management for irrigated corn production Location Context: Central Wisconsin, loam soil, humid continental climate (cold winters, warm summers), 2023 spring application Constraints: Must comply with Wisconsin NR 243 manure application rules (no surface application within 10 days of forecasted >0.5" rain); field has 5% slope; farmer seeks to replace 60% of sidedress N with pre-plant manure while minimizing leaching and volatilization risk.

Given Data

• Total Nitrogen in Manure (wet basis): 125 kg/ha
• Organic Nitrogen in Manure (wet basis): 92 kg/ha
• Decay Constant for Ammonia Volatilization: 0.035 day⁻¹ (lower due to immediate injection <2 hrs after spreading)
• Mineralization Rate Constant: 0.0065 day⁻¹ (warmer-than-average April soil temps, 12°C average)
• Time Since Application: 22 days (applied April 18; assessed May 10, pre-V6 corn stage)

Calculation

The tool computes Plant-Available Nitrogen (PAN) as:

PAN = (Total_N − Organic_N) × e^(−k_decay × t) + Organic_N × (1 − e^(−k_min × t))

Where:

• (Total_N − Organic_N) = 125 − 92 = 33 kg/ha (inorganic N, primarily NH₄⁺)
• e^(−0.035 × 22) = e^(−0.77) ≈ 0.463 → Inorganic N remaining = 33 × 0.463 ≈ 15.28 kg/ha
• Organic N mineralized = 92 × (1 − e^(−0.0065 × 22)) = 92 × (1 − e^(−0.143)) ≈ 92 × (1 − 0.867) ≈ 92 × 0.133 ≈ 12.24 kg/ha
• PAN = 15.28 + 12.24 = 27.52 kg/ha (rounded to 27.52 per tool precision)

Result and Decision

The tool returned 27.52 kg/ha of plant-available N at 22 days — far below the target 80–100 kg/ha needed for high-yield corn. Based on this, the agronomist recommended retaining the full planned sidedress urea application (90 kg/ha) and not reducing it. Soil nitrate testing confirmed low residual NO₃⁻ (4 ppm), validating the model’s conservative estimate.

Lesson

Injection reduces volatilization but does not accelerate mineralization — early-season PAN from dairy manure remains low in cool soils; relying on pre-plant manure alone for corn N demand is risky without supplemental mineral N or warmer conditions.