Residual Nitrogen After Harvest: A Free Resource for Next Year’s Crop?

As we discussed in an earlier blog post (“The First Pass Matters: Water and Nitrogen Strategy for Planting 2026”), parts of Nebraska and the Dakotas were and have experienced varying levels of drought through the 2026 growing season, while on the eastern side of the Missouri River, Iowa contended with in places too much precipitation and little in the way of drought.  

But it wasn’t long ago that Iowa had its share of drought in 2012.  While some may want to forget that year (I also would like to not recall both replanting corn due to wet conditions only for it to make 60 bushels/acre in the drought!), it was an amazing learning experience that provided useful guidance for future situations.

In the fall of 2012, John Sawyer, now-retired Nitrogen Soil Fertility Specialist and Professor at Iowa State University, worked with extension agronomists and research farm staff to pull samples across Iowa for nitrate-nitrogen down at 0 to 2 or 3-foot depths.  From previous year’s sampling they typically found 40 to 60 pounds per acre of nitrate-nitrogen, but after the drought of 2012, they found an average of 122 pounds of nitrate-nitrogen per acre, ranging from 27 to 348 pounds per acre.

These findings revealed a great deal of residual nitrogen left in the soil that was unused by that year’s corn crop.  While there was not a strong correlation between total N applied as fertilizer and measured nitrate-nitrogen, there did appear a weak correlation between precipitation from planting to sampling date and soil nitrate-nitrogen, with lower precipitation levels correlating to higher residual nitrate-nitrogen.

Admittedly, deep soil nitrate sampling is not a common practice in Iowa and other states with more timely and sufficient rainfalls; it is, however, a common practice in areas with drier conditions to measure these residual amounts for planning in next year’s crop.  Also, residual nitrate-nitrogen is of importance for crops like sugar beets; lowering the residual nitrate-nitrogen going into sugar beets can improve likelihood of higher quality and yield of sugar.

In my previous work coordinating dozens of field agents to pull nitrogen samples to two feet, I often found a total residual nitrogen level to be around 40 to 60 pounds per acre (total nitrate- and ammonium-nitrogen, but usually 75% nitrate, 25% ammonium).  These samples were often from Iowa, Illinois, Indiana and Minnesota, but some came from North Dakota to Texas and everywhere in between.  Most of the residual nitrogen laid in the top six inches to foot, with less in the lower 12-to-24-inch range.  Sometimes we saw spikes in nitrogen in certain high organic matter soils or where the field had a history of alfalfa and/or manure. 

Where drought is a concern, capturing these residual nitrogen levels is a key element for next year’s management.  Because of reduced likelihood of heavy precipitation over the winter, farmers and consultants can benefit from the guidance provided by these samples to be reasonably assured most, or all the measured N will still be there for next spring.

Taking these samples, especially at two- or three-feet depth, isn’t as easy as typical soil fertility samples at 6” depths.  Hand probes are possible in sandier conditions, but powered drills or hydraulic probes are the likely go-to for heavier and wetter soils.

Despite the effort, soil nitrate sampling post-harvest can be a great way to better understand the previous year’s residual nitrogen as you go into the next.  Give yourself the proper credit, especially in a drought year.  Consider returning that field back to the previous crop (corn on corn, sorghum on sorghum) to take advantage of both residual nitrogen and possibly crop-protection active ingredients.

Nature doesn’t always deal us the best cards, but with careful sampling and analysis for residual nitrogen, we can make the best of the situation and use the guidance to set up profitability in the next year.