Mid-Season Corn Fertilization

​​Nitrogen (N) is the most yield-limiting nutrient for corn production. The Pre-sidedress Soil Nitrate Test (PSNT) can be used to determine the need for mid-season N applications.

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Corn nutrient uptake was discussed in the previous newsletter. Early-season N uptake is minimal as corn grows slowly for the first 30 days. Corn growth becomes rapid after 10 leaves are produced and new leaves are appearing every couple of days. N uptake is rapid between the 10-leaf stage and silk emergence. About two-thirds of the total seasonal accumulation of N occurs by the time corn silks are appearing.

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Figure 1. Perform the Pre-sidedress Soil Nitrate Test (PSNT) when corn is in the V5 growth stage with a plant height of about 12 inches at the center of the whorl. Sample soil to a depth of 12 inches between rows and away from fertilizer bands. Collect 15 to 20 soil cores taken randomly across a corn field, and mix thoroughly to produce a representative sample. Immediately send sample to a soil testing laboratory to be analyzed for nitrate-N. 13;10;

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Supplying N to corn when it needs it is important. Split applications of N between pre-plant and side-dressing is generally the best approach. If needed, N should be applied shortly after corn has five or six leaves to meet the high demand of uptake through silk emergence.

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Pre-plant soil tests cannot be used to make mid-season fertilizer decisions because they do not tell how much N will be available during the season.1,2 Pre-plant soil testing can be effective for determining the N requirements of non-manured fields, but tend to over-estimate the fertilizer N required in manured fields. The Pre-sidedress Soil Nitrate Test (PSNT) can be a better predictor of whether or not a mid-season N application is needed (Figure 1).

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The PSNT measures soil nitrate-N early in the season after some N has become plant available, but before the crop’s time of greatest need. The test predicts how much N will be mineralized or released for crop use. Growers who apply manure to fields are especially likely to benefit from the test.3 The organic matter in manure contributes N that is not well accounted for by other soil test methods. Mid-season applications of N to manured fields without using PSNT usually provides more N than needed by a silage corn crop.1 If PSNT values are greater than 25 ppm, there should be sufficient N available for the corn silage crop. Additional N should be applied if it is below 25 ppm. Estimated amounts of N required to meet crop N need at PSNT values are shown in Table 1.

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Fields regularly receiving manure should have ample phosphorus (P) and potassium (K) for silage corn production.1 Manure can also supply N, and manured fields may not require any additional N fertilization. The PSNT will let you know if additional N is needed mid-season in manured as well as non-manured fields.

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Applying N using sprinkler irrigation of corn under center pivots or linear move systems is an efficient and flexible way to provide N during the season.2 Supplying N with lagoon water usually requires multiple applications, making the last application at or before corn silk emergence. A well-fertilized corn silage crop will not produce additional yield if fertilized with N after silk emergence.

13;10;N application options during the season can be more limited for furrow-irrigated fields.

Once corn growth prevents trips through the field with equipment, applying N becomes difficult. N applications on furrow-irrigated fields are typically accomplished with pre-plant and side-dress applications.

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PSNT Value (ppm nitrate-N)Estimated N to apply (lb N/acre)
0-10100-175
10-20 50-100
20-250-50
Over 250
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Table 1: Suggested N fertilization rates based on PSNT values.1

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