Phosphorus and Manure on a High-P Soil Field
A dairy operation in Wisconsin has a 40-acre corn field that has received manure for 15 years. Soil test P (Bray P1) is 95 ppm, well above the agronomic critical level (about 30 ppm for corn). The producer plans to apply 6,000 gal/ac of liquid dairy manure (about 25 lb N/1,000 gal, 10 lb P2O5/1,000 gal, 20 lb K2O/1,000 gal). The field drains to a trout stream 300 ft away. The nutrient management plan must comply with Wisconsin's NR 151 and the 590 standard.
1. Calculate the manure nutrient application per acre and determine whether the plan meets the P-based restriction.
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Per acre at 6,000 gal:
- N = 6 × 25 = 150 lb N/ac
- P2O5 = 6 × 10 = 60 lb P2O5/ac
- K2O = 6 × 20 = 120 lb K2O/ac
Because soil test P (95 ppm) is more than 3× the agronomic critical level (~30 ppm), the field is in the "high" or "very high" P category. Under NR 151/590, P application must be restricted to crop removal or a P-based rate, not N-based. Corn removal at 180 bu/ac is roughly 0.35 lb P2O5/bu = ~63 lb P2O5/ac - so 60 lb P2O5/ac is near crop removal and may be acceptable only if the plan demonstrates no P buildup and meets the P index. However, the field already has very high P, so the plan should target crop removal or less, and any application must follow the P index. The 150 lb N/ac also exceeds crop N need if no commercial N is added; N should be credited. The stream setback and P index must be addressed.
2. Identify the environmental risk and the management changes needed to protect the stream.
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Risk: High soil P plus a 300-ft stream proximity creates a high risk of P loss via runoff and erosion, which can cause eutrophication in the trout stream.
Management changes:
- Apply manure at a rate that does not exceed crop P removal (or lower) until soil P declines.
- Follow the Wisconsin P index to evaluate runoff, erosion, and distance to water; the index may require reduced rates or setbacks.
- Maintain a vegetated buffer (at least 35 ft under NR 151) along the stream and avoid application on frozen or saturated ground.
- Incorporate or inject manure to reduce surface runoff loss.
- Consider exporting manure to lower-P fields or treating it.
- Monitor soil P over time and rotate manure applications.