Presidedress Soil Nitrogen Test
Where corn is grown on land that has had manure applications, nitrogen fertilizer recommendations can be made using the presidedress soil nitrogen test (PSNT) for corn. This new nitrogen soil-testing approach involves taking soil samples just before sidedressing—after the spring wet period but before the period of major nitrogen demand by corn—and determining the nitrate nitrogen available in the soil sample. The results of the test then are used to make sidedress nitrogen recommendations.
This test is recommended primarily for use on fields with a history of manure application. This test is of limited value on most fields without organic nitrogen contributions, because these fields generally have low soil nitrate levels. For these situations, standard recommendations are preferable.
- Apply only a minimum of fertilizer nitrogen in the spring (starter fertilizer and/or nitrogen used as a herbicide carrier).
- Apply manure based on the history of the field, a manure analysis, manure handling, and on crop requirements for nitrogen estimated from the yield goal and crop history.
- Take soil samples when the corn is 12 inches tall, or at least a week before planned sidedressing.
- Sample the fields by taking 10 to 20 cores to a 12-inch depth, if possible. If not, sample as deep as you can. Avoid starter bands and other atypical areas. Because of sampling problems, this test cannot be used on fields that have received injected fertilizer or manure.
- Combine and crumble the cores and dry them as quickly as possible (within one day of sampling). Spread the samples out in the sun or under a heat lamp in a well-ventilated area to dry.
- Send the sample to a reputable soil testing lab for soil nitrate-N analysis. A reliable field test kit also can be used to determine the nitrate-N level in the sample.
- From Table 1.4-4, determine the sidedress nitrogen recommendation. See the PSNT recommendation guidelines below for more details.
PSNT Recommendation Guidelines
These N rates should be considered guidelines for making recommendations. It is important that the actual recommendation be made in the context of the field information. Check for consistency between the test results and the field history. Double-check inconsistencies by rerunning the sample and/or taking a new sample and having it analyzed. A very low test on a field where you would expect a high level of residual N, such as on a field with a history of heavy manure applications or a recent legume in the rotation, or a very high test on a field with little or no history of manure or legumes in the rotation, should warn you of a potential problem.
If a rerun of the analysis confirms that the test is very low, but the field’s history indicates that a high level of residual N should be present, apply no more than 50 to 75 pounds of N per acre. On the other hand, if the test is very high but the history indicates a low level of residual N, the recommendation should be to apply nearly a full rate. As with any soil test recommendation, use all available information to arrive at the best final recommendation for the situation. For more information, see Agronomy Facts 17: Presidedress Soil Nitrate Test for Corn.