Gordon Johnson, Extension Vegetable & Fruit Specialist; gcjohn@udel.edu
I was recently asked about using the Presidedress Soil Nitrate Test for sweet corn. The Presidedress Soil Nitrate Test (PSNT) was developed to assess the nitrate levels in soils just prior to sidedressing in field corn and relate that to expected crop response to nitrogen fertilizer. As soils warm, the mineralization of organic matter increases along with nitrification. By measuring nitrate levels prior to sidedressing a “snapshot” of N available from organic sources is obtained. Therefore, the PSNT is used where manures have been applied or leguminous cover crops have been grown and limited fertilizer N has been applied preplant or at planting. This test has been adapted to several vegetable crops such as sweet corn, cabbage, peppers, and pumpkins. Soil samples are taken about a week prior to normal sidedressing at a depth of 12 inches. They are dried and then tested for nitrate at a laboratory or using a quick testing kit (available from several sources). An example for sweet corn from Rutgers University is shown below.
Sidedress Nitrogen Recommendations for Sweet Corn
Based on the PSNT Soil Test Level and Manure History
| PSNT Soil Test Level (ppm NO3-N) | Sidedress N Recommendation (lbs/acre)* |
| Manured Soils | |
| 0 to 10 | 160 |
| 11 to 15 | 120 |
| 16 to 20 | 80 |
| 21 to 25 | 40 |
| greater than 25 | 0 |
| Non-Manured Soils | |
| 0 to 15 | 160 |
| 16 to 20 | 120 |
| 21 to 25 | 80 |
| 26 to 30 | 40 |
| greater than 30 | 0 |
*When 100 lbs. or more of sidedress N are recommended on very light sandy soils, apply half of the sidedress when the corn is 12 inches tall and half when the corn is 18 to 24 inches tall.
One grower reported that they used a Pre-Sidedress Nitrogen Test (PSNT) in areas that had received manure and values indicated that no additional N was needed. However, nitrogen deficiencies were found in very sandy areas and low spots. These are areas where the most N loss would be expected. While the PSNT is a valuable tool to manage nitrogen in sweet corn, any recommendations should take into account weather at and after the time of sampling. Low PSNT values may result from heavy rains just prior to sampling (it is recommended to wait several days after heavy rains to take samples for PSNTs). High PSNT values (>30 ppm) would indicate no additional N is needed. However, extended heavy leaching rains could still render the crop N deficient even with these high values.