Wisconsin’s Pre-sidedress Soil Nitrate Test

Scott Sturgul, Nutrient & Pest Management (NPM) Program

The large amount of rain some portions of the state have received in the spring of 2009 may have some corn growers wondering about the nitrogen (N) status of their fields. Is the N from earlier fertilizer applications still there? Are the manure-N or legume-N credits really available to this year’s corn crop?  

A diagnostic tool that can answer these questions is the pre-sidedress soil nitrate test (PSNT). The PSNT is a soil test available to Wisconsin corn growers for improving the efficiency of their N fertilizer applications. Not to be confused with routine soil tests, soil N testing allows N fertilizer application rates to be adjusted for field-specific conditions that influence corn N need.  

The PSNT is most useful for confirming legume and manure N credits and providing site-specific estimates of soil N availability. The test is particularly important when information is not available to assess these credits using standard techniques, such as when previous manure application rate and/or nutrient content are unknown or when the stand density of a previous alfalfa (or other legume forage) crop is unknown. Another situation where the PSNT is of particular value is when abnormal weather conditions occur that may impact N mineralization rates (cool weather) or loss of N (excessive rainfall).

Soil samples for the PSNT are taken after planting when corn is 6 to 12 inches tall. At this stage of the growing season most of the conversion of organic N sources to plant-available forms of N has usually occurred. PSNT soil samples are collected to a depth of 1 foot at a sampling density of 15 soil cores per 20 acres. Cores are mixed to obtain a 1-cup composite subsample for submitting to the soil testing laboratory. PSNT samples need to be kept cool until taken to the lab. If samples are to be stored for more than two days, they need to be either air-dried or frozen to prevent changes in nitrate content during storage. The PSNT is not recommended on sands and loamy sands.

Because of the rapid growth of corn at this point in the growing season, it is imperative that PSNT soil sampling, lab analysis, and supplemental sidedress N applications (if needed) be completed within one to two weeks.

Results of the PSNT are interpreted using a critical value of 21 ppm nitrate-N. Fields testing above 21 ppm N are not likely to respond to additional N.  Fields with PSNT values below 21 ppm N probably will benefit from additional N. The amount of N to apply to these fields is determined by subtracting the PSNT N credit (Table 1) from your target N application rate. 

1. Corn following alfalfa - Abnormally cool spring weather can delay the release of N from legume residues. If a PSNT result for corn following alfalfa is less than 21 ppm nitrate-N, apply no more than 40 lb/a of supplemental N.  
2. Corn following soybean - The PSNT does not work well in assessing the N contribution from a previous soybean crop and should not be used for this purpose.