Soil Nitrate Sampling – When it Works and When Extra Help is Needed

If you’ve been following my previous posts, you’ll remember that I am based in south-central Iowa with some flat and rolling soils with primarily a clay pan not too far under the surface.  For our highly productive flat soils, this clay pan is both a blessing and a curse: In dry spring years like 2023 this allowed roots to grow deeper and tap into long-held water and nutrients.  Even under near drought conditions we had record yields, especially on the flat tablelands. 

But in this spring 2026, the more usual scenario occurred – excess rains (those rains that Nebraska wasn’t getting, unfortunately) in May continued to pound the fields.  The clay pan caused the precipitation to back up (water doesn’t flow through it very quickly, and tile drainage is not uniformly used here).  This caused large areas of saturated soils and fears of fall and spring anhydrous ammonia to be lost due to denitrification. 

Soil Nitrate Sampling in South Central Iowa 

Knowing that I have some experience working with and modeling nitrogen dynamics, a few of my neighbors asked me to help them out and determine if they need to apply more.  The corn was at V4-V6 growth stage, the optimal time for side-dressing. 

So, I went out to their fields, picked a couple of locations in the flattest terrain of each, and pulled 12” nitrogen samples to determine what nitrogen was still in the soil.  I pulled 11 cores, approximately 3” apart from each other, between two 30” corn rows.  This way, if any residual nitrogen from the fall or spring NH3 band still existed, I’d likely capture it and average it together with soil between the bands. 

I had the samples analyzed not only nitrate, but also ammonium and the full set of typical analytes (OM, pH, macros and micros).  The analysis for many samples showed how much of the NH3 had converted from ammonium to nitrate (for fall-applied it was all of it; spring-applied was 90%) and what the organic matter was to provide for potential mineralization. 

To determine if extra N was needed, I used Iowa State University’s recommendations from the CROP 3140 - Use of the Late-Spring Nitrate Test in Iowa Corn Production.  Full disclosure – I am an associate professor of practice at Iowa State University in the Agronomy department.   

In some fall-applied fields with 150 lbs/ac of N, extra N was warranted according to the nitrate tests.  In fall and spring NH3 applied fields with 190 lbs N/ac applied, all fields appeared to have sufficient N and didn’t require extra nitrogen, according to the ISU guidance. 

This approach worked; the farmers received guidance for nitrogen applications that they didn’t have before and followed them.  This information likely saved them money or increased their yield outcomes. 

Why Share This? 

So why do I share all of this in a post for Sentinel Ag, which utilizes remote sensing, crop modeling, and check blocks to determine nitrogen needs? 

Scalability:

With soil nitrate sampling, scalability becomes a big issue.  In one field, I took two samples for 205 acres.  Is this sufficient?  No – I focused on the flattest areas that were likely to denitrify nitrogen.  Could there have been N loss on the slopes?  Possibly, but this wasn’t measured. 

With remote sensing, as the crop canopy develops, it is possible to start identifying areas of nitrogen deficiency.  This addresses the scalability drawback of nitrogen sampling.  While not every farmer has the equipment to address late-season N deficiency, more equipment is becoming available. 

Point in Time:

These samples provided a snapshot of what was plant-available at that time, right before crop N uptake took off.  Shortly after sampling another 4” of rain hammered these fields.  Did the nitrate nitrogen stay there or continue to denitrify under saturated conditions? 

With crop modeling, a level of guidance of where nitrogen is moving and being lost can be determined.  Models are only as good as the data used to train them initially and fed to them in production, but they can help us understand if scenarios are likely that cause excessive N loss after high precipitation events. 

Cost: To scale nitrate soil sampling to a 5 acre per sample intensity, the cost for both labor and analysis would be around $8-10 per acre.  This provides a one-time snapshot of nitrate-nitrogen levels in the field.  The time window to perform this work is generally two weeks. 

By combining crop modeling (with proper inputs like planting info, nitrogen applications, and an estimated starting nitrogen value) with remote sensing as the crop develops, the cost to scale across area and over a season is less than what nitrate sampling can provide at a single point in time.  Granted, both modeling and remote sensing rely on training from boots-on-the-ground soil and plant sampling and data analysis performed, but crop and soil dynamics do not radically change from year to year.  Some variability will never be captured, but cannot the same be said for soil nitrate sampling? 

Summary 

I believe soil nitrate sampling is a useful tool to use in certain circumstances.  After three relatively drier springs, my neighbors may have become used to lower potential N losses from denitrification, only to be awakened by the deluges that were brought upon us.  By utilizing the nitrate soil sampling, they were able to make better broad choices (yes/no to supplemental N); I know of some area farmers who instinctively applied more N despite having applied 200 pounds of N per acre last fall without utilizing soil nitrate sampling. 

The science behind the Late-Spring Nitrate Test approach is sound, based on thousands of data points across Iowa over many years.  Much effort has gone into developing and maintaining this approach and the recommendations from it.  I highly recommend using CROP 3140 if soil nitrate sampling is to be performed. 

As noted, however, scalability, point in time, and scaled cost can be recognized as drawbacks to extensive soil nitrate sampling.  Spot sampling like I provided my neighbors can be performed, but it must be recognized that the entire fields were not sampled at one time, nor were multiple sampling campaigns performed.   

Tools like crop modeling and remote sensing, with proper inputs, can provide scalable nitrogen guidance over more area and across the growing season.  Soil nitrate samples may provide a calibration opportunity to augment these modeled values.  The two approaches are not necessarily opposed but have their places for different needs. 

With the overall goal of reducing excess nitrogen applications and providing the best ROI to the farmer, both soil nitrate sampling and approaches like Sentinel Ag’s are better than doing nothing and hoping things turn out well.  More knowledge, whether from soil samples or derived from models and imagery, is better than guessing.  Your crop, as an expensive biological factory, deserves better.