Lime, Manure Nitrogen, and Phosphorus Decisions on a Corn Field
A 240-acre corn–soybean farm in the eastern Corn Belt has a 60-acre field, Field 7, that will be planted to corn this spring. The field is somewhat poorly drained and has a 2–3% slope. A recent composite soil test (0–8 in) reports pH 5.6, buffer pH 6.7, Bray-1 P 12 ppm, K 140 ppm, CEC 14 cmol/kg, and organic matter 2.8%. Last October the producer applied 20 tons/acre of bedded dairy manure and incorporated it within 48 hours. The previous crop was soybean. A spring preplant soil nitrate test (0–12 in) is 18 ppm. The producer plans to apply 180 lb N/acre as urea plus a starter fertilizer and wants to know whether to also apply lime and additional P. The farm has a no-till drill available, but the manure was incorporated; the producer is willing to change timing and source. Rainfall in the next 10 days is forecast to be 1–2 inches, and the field drains to a small stream.
1. How would you handle lime for Field 7? Include what additional information you need, a target, and a timing and incorporation plan.
Read the worked answer
The pH of 5.6 is below the 6.2–6.5 range usually targeted for corn–soybean, so lime is justified. Do not estimate the rate from pH alone: the buffer pH of 6.7 is only an index, and the lab's lime-requirement calibration, lime ECCE, and fineness must set the actual tons per acre. I would ask for the lab's recommended rate, the lime source analysis including ECCE and magnesium, and the soil magnesium level, then confirm whether the field is rented and has a different target. Apply and incorporate the lime before planting if soil conditions allow; incorporation speeds the pH change and reduces stratification. If the field must stay no-till, surface-applied lime is acceptable but will act more slowly, so I would not expect full correction in one season. Avoid overliming: pH above about 6.8–7.0 can reduce micronutrient availability and increase carryover risk for some herbicides. The manure application does not replace lime. Re-test pH in 12–24 months.
2. The producer asks whether the spring nitrate test and manure history justify reducing the 180 lb N/acre rate. What is your recommendation and what calculations or checks support it?
Read the worked answer
Start by separating reliable credits from unreliable ones. The previous soybean crop justifies a small N credit, commonly about 30–40 lb N/acre in this rotation. The 20 tons/acre of incorporated dairy manure is a real credit, but only the first-year available fraction should be subtracted-often roughly 30–40% of total manure N, depending on bedding and incorporation. That is a credit, not the full 200 lb of total N. The preplant nitrate value of 18 ppm is below the usual presidedress nitrate test critical range of about 21–25 ppm, so it does not justify an additional credit; a PSNT taken at V4–V6 is more useful. A 180 lb N/acre base rate is therefore likely above the economic optimum. I would reduce the base rate to roughly 100–120 lb N/acre, split it, and sidedress according to PSNT and rainfall. If early-season rain is excessive, recheck before sidedress because nitrate can move. The goal is to avoid both over-application and a yield-limiting rescue application.
3. What phosphorus management and environmental precautions would you recommend given the soil test and manure application?
Read the worked answer
Bray-1 P of 12 ppm is in the low-to-medium range, but the manure already applied is a significant P source. I would not add broadcast P. A small banded starter may be justified if the planter setup allows it and the starter is placed away from the seed, but the first step is to credit manure P using a manure analysis and a realistic availability factor. Applying manure on a P-based rate-not an N-based rate-is the key to limiting surplus soil P. Because the field slopes and drains to a stream, manure should have been incorporated (it was), and future applications should be injected or incorporated within 24–48 hours, kept back from the stream by the local setback, often 35 feet or more, and avoided before forecast heavy rain or on saturated or frozen ground. A cover crop after corn would take up residual N and reduce erosion. Soil test every 3 years and track P; do not let manure history turn a low-P field into a high-P field.