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Residual nitrogen in the soil: Can you take an N credit this year?

residual nitrogen soil minnesota

By: Fabian Fernandez, Extension nitrogen management specialist; Jeff Strock, Professor, U of M Southwest Research and Outreach Center (SWROC); Jeff Vetsch, Researcher, Southern Research and Outreach Center (SROC); and Karina Fabrizzi, U of M researcher

Despite applying typical nitrogen (N) fertilizer rates for last year’s corn crop, many farmers across Minnesota had lower-than-usual crop yields due to the dry 2021 growing season. The dry conditions limited N uptake by the crop, but also minimized N from being lost through leaching or denitrification. All of these factors resulted in higher-than-normal residual N levels in the soil last fall. However, we know that substantial spring precipitation can lead to less residual N for the following crop. Higher-than-normal potential for residual N, soaring N fertilizer prices, and concerns over N loss to the environment are all factors leading farmers to ask: How much N is left in the soil from last year and how much should I credit for this growing season?

Fall 2021 vs spring 2022 soil nitrogen levels

The best way to know the answer is to measure soil N in the field of interest because local soil and weather conditions have a huge impact on what happens to residual nitrogen. However, to provide some general guidance, we collected soil samples from several study sites around Minnesota and analyzed them for N last fall and again this spring. What we present here is from selected treatments from an ongoing long-term N study funded by Minnesota’s fertilizer tonnage fee through the Agricultural Fertilizer Research and Education Council (AFREC). These kinds of studies are invaluable to be able to answer questions such as the ones we explore in this article. 
We sampled sites across Minnesota that were planted with corn in 2021 and selected three nitrogen rates representing A) the control plot with no nitrogen, B) an intermediate rate near what is typical for the economic optimum nitrogen rate (EONR) for the specific soil and cropping rotation, and C) an excessive rate that is well above the EONR. 

Normally, farmers measure nitrate-N to determine residual nitrogen. In this study, we looked at total inorganic nitrogen (TIN) by measuring both ammonium-N and nitrate-N in the first and second foot of soil depth in fall 2021 (Figure 1) and again this spring in early May (Figure 2). In Lamberton (southwest, Minnesota) (Figure 3), we measured soil nitrogen above the tile lines (the top four feet of soil) because N within the entire soil profile will be available once the massive corn root system fully develops. 

What is typical?

As a point of reference, in Table 1 we show residual soil nitrogen from a three-year study in a fine-textured soil in south-central Minnesota during more typical growing seasons. Based on this information, and several other studies across Minnesota, in the fall, the amount of residual N in the top two feet of the soil typically increases in relation to the amount of N applied, and it is around 60 pounds TIN/acre at the EONR.  In the spring, one can expect around 45 lbs TIN/ ac in the top two feet of soil, regardless of the amount of N fertilizer applied the previous season. That total amount in the spring is often about half nitrate and half ammonium. 

What is happening after the 2021 growing season?

Fall soil nitrogen in the last growing season (Figure 1) followed a similar pattern to what is typical, where residual N increased with N rates, but, in general, there was more residual N than normal last fall due to the dry conditions. This was especially true in western Minnesota, where grain yields were severely limited due to the drought and lower potential for N loss. The grain yield at the EONR for Lamberton (southwest Minnesota) was only 114 bushels/acre and, in Morris, (west-central Minnesota), it was 118 bu/ac. Whereas in Rochester (southeast Minnesota), grain yield at the EONR was 255 bu/ac and at Waseca (south-central Minnesota) it was 208 bu/ac because these locations received more timely precipitation. 

The site at Becker (central Minnesota), which is an irrigated sandy soil, also showed more residual nitrogen as N fertilizer rates increased. This is a highly unusual for this location because the sandy soils are prone to nitrate leaching. That said, by spring 2022 (Figure 2), residual N was very low at this site (similar to the control plot regardless of the N rate) indicating residual N was lost due to leaching in the spring on this coarse textured soil.

The fine-textured soils this spring (Figure 2) had higher amounts of residual N with higher N fertilizer rates, which highlights the fact that we have more residual N than normal this growing season. Except for Rochester, which has what could be considered typical residual N levels this spring, the other three sites (Waseca, Lamberton, and Morris) have around 25% to 30% more residual N than what is typical. This means that if weather conditions are favorable this season, it may be possible to reduce your N fertilizer rate obtained from the MRTN calculator by about a quarter.   

While this spring we are getting precipitation and there is potential for N loss with excess rainfall, it is important to recognize that the cool temperatures so far this spring reduce the potential for loss through denitrification. Also, nitrate must travel through the soil profile several feet before it is lost below the root zone or leaches into a tile-drainage line. Based on the total amount of drainage we have measured since the start of the spring in drainage research plots in Lamberton, we estimate that little nitrate (between 5 and 20 lbs/ac) has leached out.  

Comparing nitrogen levels by depth in the fall vs. the spring (Figure 3), it is clear that some nitrogen has moved deeper in the soil profile, but it is still present where crop roots can use it later this season. Ammonium levels decreased from fall to spring as the nitrification process occurred, but the nitrate that was produced and moved deeper in the profile resulted in greater nitrate levels this spring than last fall at the deeper layers. Overall, from fall to spring in the top three feet of the soil in Lamberton, we measured a decline of only 10 lbs TIN/ac. In the top four feet of the soil in Lamberton this spring, we measured 49 lbs TIN/ac for the control plot, 72 lbs TIN/ac for the 140 lbs. N/ac rate, and 107 lbs. TIN/ac for the 280 lbs. N/ac rate. 

We encourage farmers to use this information in the context of their individual weather, current and last season crop conditions, and soil/location information to decide how much N to credit.

residual N fall
Figure 1. Residual nitrogen in the top 24 inches of the soil in fall (November 2021) for different locations, cropping systems, and nitrogen rates applied for the 2021 growing season.

residual N spring

Figure 2. Residual nitrogen in the top 24 inches of the soil in spring (early May 2022) for different locations, cropping systems, and nitrogen rates applied for the 2021 growing season.

residual soil nitrate, residual ammonium in fall and spring
Figure 3. Residual soil nitrate (left) and ammonium (right) in the fall 2021 (top 36 inches) and the spring (48 inches in early May 2022) in Lamberton, MN for the 140 lbs N/ac rate.

Table 1. Residual nitrogen in the top two feet of soil in the fall and the following spring averaged across three growing seasons (2014-2016) in a fine-textured soil in south-central Minnesota as impacted by different nitrogen fertilizer rates (0 N control plot, near EONR, and high) for corn grown in rotation with soybean.

Fall Fall Fall Spring Spring Spring
Fertilizer rate (lbs/acre) 0 120 200 0 120 200
Nitrate-N 17 28 52 20 26 24
Ammonium-N 27 30 32 21 23 24
Total Inorganic Nitrogen (TIN) 44 57 85 41 49 48

The authors acknowledge the funding from the Agricultural Fertilizer Research and Education Council (AFREC) and the following co-principle investigators in this project: Daniel Kaiser, Yuxin Miao, Paulo Pagliari, Lindsay Pease, Carl Rosen, Albert Sims, and Melissa Wilson.


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