Late fall rainfall: Where did the nitrate go?

Fall soil sampling in 2019 started normally after wheat. But then, excessive rainfall started in September and even snow in October, exceeding 12 inches of total precipitation in some places (Figure 1). After the rain and snow came, we received many questions about potential nitrate-nitrogen loss from the soil profile. In northern portions of the region, soils were quite dry when the first large rains arrived, and soils had capacity to absorb the first several inches of rain. As the rain continued to accumulate and soils became saturated (waterlogged), the fate of nitrate in the soil profile became uncertain. In poorly drained soils, nitrate can be lost to the atmosphere via denitrification. In well drained soils, nitrate is more likely to be lost via leaching.

Figure 1. Accumulated rainfall in Fall 2019 (September 1 – December 31). North Dakota Agricultural Weather Network (NDAWN), NDSU, Fargo, ND.

In the Red River Valley sugarbeet production region, we receive several thousand soil samples after wheat with 48-inch soil nitrate tests (0-24 and 24-48 inch). The deep 24-48 inch soil nitrate information helps sugarbeet producers manage nitrogen for highest recoverable sugar. As we received questions about soil nitrate loss last fall, the data from these fields with 48-inch soil nitrate tests helped illustrate the fate of soil nitrate (Figure 2).

Figure 2. Average soil nitrate after wheat and soil sampling date in the northern Red River Valley. While soil nitrate in the upper soil profile decreased over time, soil nitrate in the deeper profile increased.

In the northern Red River Valley (582 zip code area), the average nitrate-N (0-24 inch) in September was 45 lb/acre, but this declined to 30 lb/acre in late November, indicating some nitrate loss. During the same period, the average deep nitrate-N (24-48 inch) increased from 15 to 30 lb/acre, almost in parallel with the nitrate loss in the upper soil profile. The parallel decrease in upper soil profile nitrate (0-24 inch) and increase in deep soil profile nitrate (24-48 inch) suggest that nitrate in the upper soil profile (0-24 inch) was not entirely lost; it just moved deeper in the soil profile.

This is good news for farmers with long-season, deep-rooted crops like corn, sugarbeet, or sunflower. Deep-rooted crops can access nitrate well below 24 inches, and the nitrogen fertilizer rate for deep-rooted crops should not require much adjustment. Short-season crops, like wheat or barley, may not access deep nitrate (24-48 inch) in time, especially if soils are wet in spring. For small grains, an extra 10-15 lb/acre nitrogen may be good insurance if you collected soil samples before the large rain and snow storms in early October. This will help account some nitrate movement into the deeper soil profile.