Twelve Years of Manure on a Tile-Drained Sandy Loam: Does the Nutrient Budget Protect the Stream?
A 48-ha grain farm on a sandy loam (7% clay, 1.4% organic carbon) has received liquid swine manure at agronomic rates for 12 years. Grid soil sampling (one sample per 0.4 ha, 0–15 cm) returns Mehlich-3 phosphorus averaging 210 mg/kg, with a maximum of 340 mg/kg; subsurface samples at 15–45 cm average 55 mg/kg. The field drains through 12 tile laterals that discharge to a roadside ditch feeding a Class 2 trout stream 900 m downstream. Depth to the water table is 1.8 m in spring. A 25-mm rainfall in June produced visible turbidity and green filamentous growth in the ditch within 48 h. The owner proposes to continue manure at the same rate because 'the crop removes it', and points to yield maps showing no decline. The county has asked for a phosphorus loss risk assessment before renewing the nutrient management plan; the state threshold for elevated soil test P is 150 mg/kg Mehlich-3 (agronomic), and the tile drains are classified as a direct conduit.
1. The owner argues the nutrient budget balances because crop removal equals application. Evaluate that argument and identify the dominant phosphorus loss pathway implied by the data.
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The mass balance does not answer the question. At field scale, P loss to water is governed by soil P concentration and by transport pathways, not by whether the annual budget nets to zero.
Three lines of evidence:
- Buildup and stratification. Mehlich-3 P of 210 mg/kg (max 340) at 0–15 cm is about 1.4 times the state's 150 mg/kg elevated-P threshold, while 15–45 cm averages 55 mg/kg. Subsoil values that high in a sandy loam do not come from parent material; they indicate decades of downward P movement. The field is already leaking.
- Saturation rather than concentration. The controlling metric is the degree of phosphorus saturation, DPS = [P_ox]/([Al_ox] + [Fe_ox]) on a molar basis. In coarse-textured soils, DPS above roughly 20–25% is associated with a steep increase in soluble P in drainage. A 7% clay, 1.4% organic carbon soil carrying 210 mg/kg Mehlich-3 has largely consumed its sorption capacity, so further P stays in solution.
- Pathway. Twelve tile laterals are a direct conduit: they intercept soil water, bypass any riparian buffer, and deliver dissolved reactive P to the ditch. The June storm explains the turbidity (particulate P from erosion), but the filamentous growth within 48 h is a dissolved-P signal - attached algae respond to soluble reactive P, not to sediment-bound P.
Dominant pathway: subsurface dissolved P through tile flow, with erosion a secondary contributor. Crop removal of roughly 25–35 kg P2O5/ha/yr cannot offset manure applied on a nitrogen basis, which typically over-applies P by a factor of two to four. The yield maps are irrelevant: P is not limiting yield, which is precisely why it accumulates.
2. Write the management and monitoring response you would put into the renewed nutrient management plan, including numeric action triggers.
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Source control. Stop P application on any grid cell above 150 mg/kg Mehlich-3 and apply only replacement-rate P elsewhere; because the field average is 210, that means no P this rotation and export of manure off the farm. Recalculate the plan on a P-based rate and credit all manure N and P.
Transport control. Treat the tile outlets: a tile-line phosphorus filter (slag or iron-based media) or a small constructed wetland/settling basin on the outlet; controlled drainage where grades permit, which raises the water table outside the growing season and cuts tile discharge; and a 10–15 m grass buffer plus a two-stage ditch section to reduce bank erosion and ditch scour.
Monitoring. Quarterly plus first-flush storm grabs of tile discharge for dissolved reactive P, total P, and TSS; soil sampling every 2 years at 0–15 and 15–45 cm; oxalate-extractable Al, Fe, and P on a composite to compute DPS.
Action triggers I would write in:
- Tile DRP > 0.05 mg/L in two consecutive events → zero P application for 3 years and reassess land application.
- Tile total P > 0.10 mg/L or TSS > 50 mg/L → inspect and upgrade outlet treatment and buffers.
- 0–15 cm Mehlich-3 > 150 mg/kg → no P.
- 15–45 cm Mehlich-3 > 30 mg/kg → treat as a legacy-leaching field; cap manure at crop removal and prioritise engineered outlet treatment.
Run the state P-index if one exists. Its score here will be driven by the direct conduit, high soil test P, and short distance to the stream - slope alone would understate the risk.
3. What would you state in writing to the county, and what residual uncertainty would you flag?
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Conclusion to state plainly: the plan cannot be renewed at the current application rate. A field-scale mass balance is necessary but not sufficient; the soil test P, the subsoil P, the tile conduit, and the ditch response together establish an ongoing loss.
Commitments: P-based application, no P on high-testing cells, outlet treatment, buffer establishment, and the monitoring programme with the triggers above, reported annually to the county.
Residual uncertainty to flag:
- No oxalate Al/Fe data yet, so DPS is inferred rather than measured; the first year of monitoring should include it.
- Tile flow is ungauged, so the annual P load cannot be quantified - only concentrations. Install a v-notch weir and continuous stage, or at minimum event-based flow estimates, before committing to a load-reduction target.
- The relative split between dissolved and particulate P is unknown; it determines whether outlet treatment or erosion control is the higher-value investment.
- A one-off tillage to mix the stratified surface P would dilute the 0–15 cm value but risks a short-term erosion and loss spike, and would mobilise the accumulated pool. I would not recommend it without two seasons of tile monitoring first.