PSNT, Yellow Corn, and Sidedressing

Gordon Johnson, Extension Ag Agent, Kent Co.;

Corn sidedressing has begun throughout Delaware. One important tool for nitrogen management in corn is the pre-sidedress nitrogen test (PSNT). The PSNT is based on the concept that amounts of plant-available N for the growing season can be predicted by taking samples from the surface foot of soil when corn plants are 6-12 inches tall. The timing of sampling is important because it allows you to detect any unexpected losses early in the growing season. It is also important because you are sampling just before the corn begins rapid uptake of N.

Fields that have had applications of animal manures or organic wastes and received less than 50 lbs of N as a starter and/or preplant are good candidates for the PSNT. On irrigated fields where farmers have the option of adding N later in the growing season, other methods, such as use of the leaf chlorophyll meter (LCM) may be more appropriate. Fields where more than 50 lbs of N has been applied preplant and/or as starter or fields receiving no manure are not appropriate for the PSNT.

Sample collection and handling are extremely important. Multiple cores, 15-30 per sample, should be collected to a depth of 12 inches to represent a uniform area of a field (in terms of soil type, management, etc.) no larger than about 20 acres. Be careful not to sample bands of previously applied fertilizer or injected manures! The cores should be mixed and quickly air-dried by spreading them on paper in a warm area. Samples may also be refrigerated until it is possible to dry them.

The PSNT level will provide an estimate of the likelihood of seeing a response to additional nitrogen fertilizer. Fields with PSNT values 21 ppm or higher are unlikely to benefit from additional nitrogen fertilizer, and the higher the value the less likely the need for supplemental nitrogen. The problem arises when PSNT values are less than 21 ppm. PSNT values below this level may or may not respond to additional nitrogen fertilizer, but the stock recommendation would be that they do require more nitrogen. There can be sites that have a low PSNT value but show no response to nitrogen fertilization. Bottomline: if the PSNT values are above 21 ppm, adequate nitrogen should be available for this year’s corn crop. If it is less than 15 ppm, the normal nitrogen rate should be applied. Between 15 and 21 ppm, other factors should be considered before reduction of the normal nitrogen rate.

In Delaware, Conservation Districts in Kent and Sussex Counties offer PSNT testing free of charge and will take the samples. A number of different laboratories, including many private laboratories and the University of Delaware Soil Testing Laboratory, can analyze these samples and provide guidelines for interpreting the results.

Many corn fields are showing patches of yellow plants. This is most likely due to a temporary nitrogen deficiency. On sandy soils, heavy rains may have leached nitrate deeper than the depth of the corn roots. On wet soil areas, nitrogen losses due to denitrification may have occurred. In compacted soils with broadcast N but no starter N, roots may not have expanded sufficiently to pick up enough nitrogen.

The key in all cases is to sidedress as soon as possible with additional N. Nitrogen rates may need to be increased from what was originally planned in areas with heavy leaching or denitrification. One common question is whether or not knifing nitrogen is better than dribbling nitrogen. When using nitrogen solutions (such as 30% UAN), there are slight increases in efficiency by knifing the N into the root zone and limiting volatilization losses of ammonia from surface applied urea. This loss of ammonia from urea in dribbled on UAN nitrogen is generally minimal because there is limited contact with the urease enzyme on the soil surface. One additional advantage of knifing in nitrogen is that it can open up compacted soils and improve aeration to a limited degree. Knifing is required when using anhydrous ammonia and the slot needs to be closed to avoid ammonia loss to the air.

Information used in this article came in part from Dr. Dave Hansen, Extension Nutrient Management Specialist, UD; the University of Maryland; and the Ohio State University.

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