Where did all the soil N come from?

The drought of 2012 left its mark in many areas of the Midwest. In the southern areas of the Midwest, the heat and lack of rain greatly reduced corn and soybean yields. In many northern areas like ND, MB, parts of SD and most of Minnesota, crop yields were good, inspite of the extreme heat and the lack of rainfall. Most areas that came through with good yields had good subsoil moisture down 8-10 feet at planting time due to excessive rainfall in previous years.

As soil testing got started after small grain harvest, it became obvious that there was more nitrate left in the soil profile than most agronomists expected following good crop yields. Instead of averaging 20-40 lb/a nitrate-N in the 0-24″ soil profile, many fields had soil nitrate levels testing over 100 lb/a. Our staff received many questions about the higher than normal soil nitrate levels and how this was possible after such a good crop. Many fields were retested, just to confirm the high N levels in the soil. Each field situation is different, but there are several reasons why we should not be surprised by the higher than normal soil nitrate-N levels this fall, even after very good crop yields.

Seeding started early and expectations were high for crops like wheat and corn. With early seeding and high yield expectations, growers did not skimp on nitrogen fertilizer and applied high rates of N. The growing season started off great with warm temperatures early and plenty of soil moisture. Crops progressed well ahead of schedule and everything looked great. Then hot temperatures settled in and the rain stopped in most areas. The topsoil dried out quickly and by mid-June, crops were forced to root deeper to find moisture and mobile nutrients like nitrogen. By about July 1, the topsoil in many areas was so dry that there was little root activity in the top 6″. Plants responded by continuing to root deeper into the soil profile. Since most subsoils were full of water when the season started, crops were able to keep growing and produce very good yields. Rooting depths for crops like wheat were probably 4-5 feet and corn probably rooted as deep as 6-7 feet. Both crops were able to get the water and nitrogen they needed from these deep depths to produce good yields and quality.

Many soil samples this fall tested higher than normal for nitrogen and much of the nitrogen was located in the topsoil. The average amount of N in the topsoil this fall is much higher when compared to a normal moisture year like 2009 (see table). This is probably due to N that got stranded when the topsoil got so dry plants could not take up nutrients from that part of the soil profile. Some of the N remaining in the topsoil was probably from fertilizer N the crop had not taken up before the topsoil got very dry. Some of the N remaining in the topsoil may also be from nitrogen mineralization by microbes breaking down soil organic matter during a very hot summer. With higher than normal temperatures this summer, the nitrogen mineralization rates were probably higher than normal as well. Another reason we have more N left in the topsoil may be because we did not lose any N from the soil due to excessive rainfall. In past wet years, we always had some losses of N to leaching or denitrification, so we kind of got used to losing some N each year.

All of these things contributed to the higher amount of nitrate-N left in the 0- 6″ profile as well as the total 0-24″ soil profile this fall. The regional maps show the average nitrogen left in the 0-24″ soil profile in different areas following wheat or corn production in 2012 compared to 2009. As you can see, many areas had higher soil N levels in 2012 compared to 2009, which was a year the region had good yields and received ample moisture.