At-Harvest Stalk Nitrate Testing for Sweet Corn

Over 60 field experiments were conducted to examine the relationship between sweet corn yield and the nitrogen status of corn stalk tissue sampled at the time of harvest. Immediately following sweet corn harvest, stalk samples were collected by cutting 8-inch segments of corn stalk from 10 randomly selected plants. The tissue samples were dried and analyzed for both NO3-N and total Kjeldahl-N (% N).

The relationship between stalk N status and sweet corn yield are presented in Figures 1 and 2. The optimal stalk NO3-N concentration range appears to be between 13,000 and 15,000 ppm. When the stalk NO3-N concentration was less than 8,000 ppm, sweet corn yield was generally less than 90% of maximum yield. When the stalk NO3-N concentration was above 13,000 ppm, yield was generally greater than 90% of maximum yield.

The optimal % N concentration range in sweet corn stalk tissue appears to be between 1.2 and 2.2% N (Figure 2). Corn plants with stalk N concentrations less than 1.2% generally produced less than 95% of maximum yield. Stalk N concentrations greater than 1.2% were generally associated with yields better than 90% of maximum.

The same experiments used to study stalk N status also measured the soil NO3-N concentration (PSNT) when the plants were 8 to 12 inches tall. A critical PSNT value of 25 ppm NO3-N has been established in previously conducted research (Heckman et al, 1995). PSNT soil test values less than 25 ppm indicate that soil N supply during the growth of the sweet corn is probably not adequate and that sidedress N fertilizer should be applied. PSNT values of 25 ppm or greater indicate that the soil N supply is adequate.

The relationship between the presidedress soil nitrate test and the at-harvest stalk N test were also examined (Figures 3 and 4) and found to be complementary. Although the PSNT samples were taken early in the season when the plants are 6 to 12 inches tall, and the at-harvest stalk N test on the day of harvest, the different tests have agreeable results. When the soil NO3-N concentration was above 25 ppm as measured by the PSNT, the stalk NO3-N and % N values were in or above the optimal range. Thus, when the PSNT predicts that that sidedress N is not needed, the sweet corn crop clearly has enough N supply from soil to complete the growing season. In other words, following a no-sidedress N recommendation, when indicated by the PSNT, has a low risk of later causing the crop to become N deficient.

The ASNT is also useful to sweet corn growers not using the PSNT. A survey of 37 sweet corn fields in New Jersey revealed that about half of the commercial grower fields examined with the ASNT had optimal concentrations of N. In 30% of the fields the ASNT results suggested that the grower applied too little N fertilizer and in 20% of the fields too much N fertilizer was applied. Growers will be able to use these results to adjust their N fertility practice as needed. This is expected to benefit growers with better crop yields or reduced cost of production.

New Jersey sweet corn acreage is approximately equally split between sandy textured coastal plain soil and finer textured or loamy soils in the piedmont. We conducted our field trials on a wide range of soils to include soil textures that are representative of where sweet corn is commonly grown. Field sites were located in Warren, Hunterdon, Somerset, Monmouth, Burlington, Middlesex, and Cumberland counties. Approximately a third of the sweet corn PSNT trials were conducted on manured land. Most sweet corn study sites were irrigated and produced useful data.