Soil sampling after banded N fertilizer application. An impossible task?

Sometimes customers will ask, “What’s the best method to collect a representative soil sample after a banded nitrogen fertilizer application?” This question often comes up when customers are trying to figure out if fall applied nitrogen is still present in spring or if the correct rate of fertilizer was applied.  Sometimes they are just trying to figure out if nitrogen fertilizer was applied at all!  Of course, we all understand that if you take a soil core directly into a fertilizer band, the nutrient concentration will be extremely high compared to soil cores collected away from the fertilizer band. The discrepancy between in-band and out of-band soil cores makes collecting a representative soil sample incredibly difficult after N fertilizer has been band applied.

We recently conducted a demonstration project to assess in-band and out of -band soil test variation after a banded nitrogen fertilizer application. Urea fertilizer was banded 4 inches deep with 30-inch spacing at rates of 75 lb N/acre and 150 lb N/acre. After three weeks, we used square steel tubing to collect soil samples from across the entire 30-inch row (see picture).  We tested each 0-6” soil sample for ammonium and nitrate. We used a hammer and 2×4s to pound the square steel tubing into the ground to collect soil samples about every 2” (we went through a lot of 2×4s!).

This intensive sampling protocol easily located the N fertilizer band. The soil sample from the center of the fertilizer band had total inorganic N (NH4 + NO3) of over 600 lb/a for the 75 lb rate and over 800 lb/a for the 150 lb/a N rate respectively.  Once you moved a few inches away from the band, the total inorganic N in the soil samples was very low.  This extreme variation in inorganic N shows why soil sampling after banded N applications must be done in a very methodical way.   In our project we collected a sample every 2” across the row and averaged 78 and 119 lb N/acre for low (75) and high (150) N treatments in this one spot in the field.

We spent a lot of time sampling for this project and determined the total inorganic N for only “one” spot in this field.  If you wanted to know the average N for the entire field, you would have to replicate this process another 15 or 20 times! (this would be an impossible task!)

As you can see from this project, soil testing to try and audit the amount of inorganic N remaining after a banded N application is not practical. If you know where the N fertilizer band is located, you could collect a soil sample from the center of the band and a second sample away from the band to confirm that N fertilizer was applied, but determining the rate of N applied with any degree of certainty is impossible.

While soil testing after banded N is an impossible task, you can soil test after a broadcast N application to determine the amount of inorganic N (NH4 + NO3) left in the soil profile.  You would just collect 15-20 cores from the field (or zone) and have the topsoil tested for ammonium and nitrate.  This information can be very useful in seasons with excessive rainfall.  If you have any questions about this information please give our staff of soil scientists and agronomists a call.