Phosphorus-Limited Land Application for a Swine Lagoon
A 2,400-head swine finishing site produces about 1.8 million gal/yr of anaerobic lagoon liquid. The farm has 100 acres of rented corn ground with a 180 bu/ac yield goal and a soil test showing Olsen P of 48 ppm (Very High). Lagoon analysis: total N 1,000 mg/L, NH₄-N 700 mg/L, total P 250 mg/L, K 450 mg/L. The state requires a nutrient management plan that holds application to crop removal for P whenever soil test P exceeds the agronomic threshold. A drainage ditch runs along the west boundary, and the county requires a 35-ft setback from it.
1. Compute the phosphorus-based application rate and determine whether 100 acres is enough land for the annual volume.
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Convert the lagoon P to fertilizer units. Using lb = mg/L × 8.34 × MG (million gallons), for 1,000 gal = 0.001 MG:
Total P per 1,000 gal = 250 × 8.34 × 0.001 = 2.09 lb P P₂O₅ per 1,000 gal = 2.09 × 2.29 = 4.77 lb P₂O₅
Crop removal. Corn grain removes about 0.37 lb P₂O₅/bu, so at 180 bu/ac: 180 × 0.37 = 66.6 ≈ 67 lb P₂O₅/ac.
Because soil test P is Very High, the state rule sets the allowable rate at removal, i.e. zero buildup.
Application rate = 67 ÷ 4.77 = ≈14,000 gal/ac
Land check. 1,800,000 gal ÷ 14,000 gal/ac = ≈129 acres required against 100 available. The farm is roughly 29 acres short, about 22%.
Sanity check on N. 14,000 gal/ac delivers 14 × 8.34 = 117 lb total N/ac; first-year available N (70% of NH₄-N plus 25% of the 300 mg/L organic fraction) is about 565 mg/L → 4.71 lb/1,000 gal → 66 lb/ac available. Against a 160 lb/ac recommendation, that leaves ~94 lb/ac to be supplied as commercial N - so P-limited application under-feeds N, which is the normal trade-off and must be written into the plan.
2. The producer proposes applying all 1.8 million gallons on the 100 acres to avoid renting more ground. Quantify the consequence and characterize the compliance risk.
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What the "spread it all" option actually delivers:
1,800,000 ÷ 100 = 18,000 gal/ac
- P₂O₅: 18 × 4.77 = 86 lb/ac against 67 lb/ac removal → +19 lb P₂O₅/ac, about 28% over removal
- Total N: 18 × 8.34 = 150 lb/ac (available N ≈ 85 lb/ac)
- K₂O: 18 × (450 × 8.34 × 0.001 × 1.205) ≈ 18 × 4.5 = 81 lb/ac against ~49 lb/ac removal - also over.
Assessment. Agronomically it looks tidier for N, which is exactly why producers push for it. But the rule in force is P-based, and this rate exceeds removal by 19 lb P₂O₅/ac/yr - roughly 1,900 lb P₂O₅/yr across the farm. Olsen P is already 48 ppm; continued over-application drives it higher and increases dissolved and sediment-bound P loss to the ditch on the west boundary.
Compliance risk is not just a paperwork issue. It is a violation of the state-required plan, it creates a documented record that can trigger an enforcement action or loss of the operating permit, and it makes the farm the obvious source if the ditch is listed for impairment. The correct answer is to refuse to sign the plan at 18,000 gal/ac and to close the land gap first.
3. List the practical options to close the land gap, and flag the siting and timing constraints that reduce the effective acreage further.
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Close the gap (ranked by cost and speed):
- Rent ~30–35 more acres with low soil test P - cheapest, fastest, and also solves next year's growth. Do this first.
- Reduce volume at the source: cover the lagoon to exclude direct rainfall and roof the lot runoff, which cuts the 1.8 MG/yr materially without touching the herd.
- Export: sell or give liquid to a neighboring crop farm with a low-P soil test, under a written agreement with its own nutrient budget.
- Treat: solids separation plus chemical P precipitation (alum/ferric) moves P into a stackable sludge that can be hauled further economically than liquid.
- Reduce herd - the last resort, and the only one that changes the long-run balance permanently.
Constraints that shrink the 100 acres:
- The 35-ft setback to the west ditch is non-application area; on a rectangular field this removes several acres depending on geometry, so the plan should be built on net wetted acres, not gross.
- Timing: no application on frozen or snow-covered ground and none on saturated soil; the 1.8 MG must fit into the window between harvest and freeze-up. At typical application rates that window is the real binding constraint, so storage capacity and planned drawdown must be scheduled, not assumed.
- Incorporation/injection within the timeframe the plan specifies, to protect the available-N credit claimed in the budget.
Bottom line: secure the additional acres before the plan is finalized, and write the plan on net acres and a realistic calendar, not on an optimistic annual average.