Every year we receive questions from growers and agronomists about soil testing for N in the spring after nitrogen fertilizer has been band applied. There are several reasons why growers want to know the soil N level after N fertilizer was band applied. They may want to know if they lost any N if they applied fertilizer early in the fall when soil temperature was a little too warm. They may have had problems with application equipment or they cannot remember if they applied fertilizer at all (yes, we actually get this question)!
The issues we encounter are where to collect soil cores and how many cores are needed to get an accurate test value that represents the banded N along with the background N in the soil profile. The N concentration in the middle of a fertilizer band right after application may be as high as 800 lb/acre and a few inches away the N concentration may be much lower, such as 30 lb/acre. Whether present as ammonium-N or nitrate-N, the higher N concentration in the fertilizer band makes sampling very difficult if not impossible to determine the average amount of N across a field.
Research from SDSU looked at different ways to do this (Clay et al., 1997). If you have a situation where you do not know the location and direction of the fertilizer bands, their research showed 60+ random soil cores were required to get within 20% of the actual N value for the field. You could use this sampling technique to tell if a field had been fertilized the fall before (testing nitrate and ammonium). There are not any sampling techniques that would be practical for measuring N losses from a fall band application. Theoretically, it would take hundreds of soil cores inside and outside of the band if you were to try this. This is not practical.
The best answer we can give when asked how you can get a representative sample after N fertilizer has been banded is that it is very difficult and is not practical. This summer our staff will be doing a demonstration project on a corn field where N will be sidedressed between the rows with a coulter. After this sidedress N application, we will collect soil cores about every two inches from one corn row to the next in several locations. These results will give us data to reinforce why it is difficult/impossible to get a representative sample after N fertilizer has been band applied. We will be presenting the data from this project at our fall customer update in August and at our educational seminars this coming January 9, 10, 11. If you have any questions about sampling situations where N fertilizer has been banded, please give one of our technical support staff a call.
Clay, D.E., C.G. Carlson, K. Brix-Davis, J. Oolman, and B. Berg. 1997. Soil sampling strategies for estimating residual nitrogen. J. Prod. Agric. 10(3):446-452. https://dl.sciencesocieties.org/publications/jpa/abstracts/10/3/446