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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.
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.
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;
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.
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).
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.
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.
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.
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.
Table 1: Suggested N fertilization rates based on PSNT values.1
1 Hart, J., Sullivan, D., Gamroth, M., Downing, T., and Peters A. 2009. Silage corn nutrient management guide. Oregon State University Ext. Pub. EM 8978-E. https://catalog.extension.oregonstate.edu.
2 Brown, B., Hart, J., Horneck, D., and Moore, A. 2010. Nutrient management for field corn silage and grain in the inland Pacific Northwest. A Pacific Northwest Extension Publication PNW615. University of Idaho. https://www.cals.uidaho.edu.
3 Marx, E., Christensen, N., Hart, J., Gangwer, M., Cogger, C., and Bary, A. 1997. The pre-sidedress soil nitrate test (PSNT). Oregon State University Ext. Pub. EM 8650. https://ir.library.oregonstate.edu.
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