Sweet corn growers evaluating levels of N application or new N management practices, such as the PSNT, may be particularly interested in whether their N fertility program was appropriate. This fits the function of the stalk N test, which in the case of field corn, can validate the current N fertility program or can determine if the N application was insufficient or excessive. Collecting stalk samples at the time of sweet corn harvest should allow observations about current crop performance to be related to the sufficiency of the N fertility program. Furthermore, this should allow growers to learn from that experience, and to direct efforts to improve N management in future growing seasons.

Corn stalk N testing is based on the assumption that nitrate (NO₃M) tends to accumulate in the lower portion of mature corn stalks when abundant amounts of N are available in soils. Sweet corn, which is harvested at a physiologically immature growth stage, may behave differently than field corn with respect to NO₃-N accumulations of stalk tissue. A recent study, however, showed that stalk NO₃-N testing was an effective method of defining excess N availability to silage corn, which is harvested about 3 weeks before physiological maturity. In very young corn plants (4 weeks after emergence) stalk NO₃-N was, however, found to be a poor predictor of soil N availability, as a result of the sensitivity of stalk NO₃-N concentrations at this early-growth stage to the influence of solar radiation and soil moisture availability. Because sweet corn is generally irrigated, the effect of soil moisture on stalk NO₃-N concentration may be less of a factor than for field corn. Stalk total Kjeldahl N (TKN) may be a better indicator of sweet corn N status because it measures both NO₃-N and reduced-N in the stalk tissue, and it may minimize the importance of solar radiation on NO₃ reduction.

This study was conducted to evaluate stalk N testing as a means to determine if the N management program used for sweet corn provided insufficient, optimum, or excessive plant-available N. Stalk N testing was evaluated both by measuring TKN and NO₃-N concentrations. The relationship between the PSNT and stalk N test results was examined.

For the purpose of this study, the concentration of nitrogen (N) in the lower portion of sweet corn stalks was examined on the day of harvest as a basis for evaluating the crop N status. Sweet corn stalk tissue was collected from N-rate experiments by cutting a stalk section at 15 and 35 cm above-ground and removing leaf material from the resulting 20 cm segment. Samples were dried and analyzed for total Kjeldahl N.

Relationships between crop yield and stalk N concentration indicated that concentrations <11 g.kg^-1 are N deficient and under-fertilized; N concentrations between 11 and 16.5 g.kg^-1 are marginally deficient; and between 16.5 and 21 g.kg^-1 the N status is optimum. Concentrations of N <21 g.kg^-1 are above optimum and indicate that sweet corn was overfertilized with N. When soil nitrate concentrations (PSNT >25 mg NO₃-N per kilogram) indicated sufficient N at time of side-dressing, stalk N concentrations generally indicated N sufficiency at harvest.