By: Brad Carlson

Recent heavy rains have many farmers questioning the fate of nitrogen (N) fertilizer that they applied for this year’s crop. The primary N loss processes are water based and occur when the soil is saturated, a condition that is widespread across southern Minnesota right now.

The two primary loss pathways are leaching and denitrification, both of which happen when N is in the nitrate form. The conversion of N to nitrate is biological in nature, and is therefore time and temperature dependent. The extent to which applied fertilizer has undergone this transformation varies based on application practice and local conditions.

Fall applications of anhydrous ammonia that did not use a nitrification inhibitor will be largely converted to nitrate by now. Anhydrous applications will have less conversion, but still a significant amount due to the amount of time that has passed since application, allowing for the inhibitor to degrade. Spring anhydrous applications are probably largely still in the ammonium form. The extent to which spring urea has converted is largely a factor of how much time has passed since application and this past weekend’s rains.

Nitrate Leaching

Rates of nitrate leaching vary based on soil conditions, but a general rule of thumb is that nitrate moves about 6 inches down in the soil profile for every inch of drainage in fine textured soils. A good question to ask yourself is: how much rainfall did I receive beyond the point of soil saturation? This will give you a rough idea of how far your nitrogen moved down in the soil profile. However, a significant portion of excess rainfall will run off (runoff is not a significant loss pathway for N).

Another general rule of thumb is that drainage systems across southern Minnesota are typically designed with a half inch drainage coefficient. This means that a system flowing at capacity will require two days to move an excess inch of water. While primarily qualitative in nature, observing tile outlets can give you an idea of how much water has been drained. The bottom line is that, with corn rooting to about three feet deep, about five to six inches of drainage are required before nitrate-N is lost “out the bottom,” and this is not likely to have occurred yet. It is likely that significant amounts of nitrate have moved deeper in the profile and will cause the crop to have a yellowish color until the roots grow down to it.

Denitrification

Denitrification happens when oxygen is lacking in the soil due to saturation. It takes about two days after a heavy rainfall before the process of denitrification really gets going. Research has shown that soil temperatures of 70 to 80 degrees Fahrenheit are ideal for denitrification loss. Soil temperature at 6 inches is currently in the vicinity of 60 degrees in south-central Minnesota. Fundamental research conducted in the 1950s shows that N loss at 60 degrees when the soil is saturated for four days is only 6% of the total applied, and climbs to 12% when saturation lasts for ten days. The open question right now is whether we continue to receive precipitation with enough frequency to keep the soil saturated for a significant amount of time. If this happens, and soil temperatures continue to warm, losses could add up. As of right now, however, it does not seem likely that we have lost a significant amount of N via denitrification.

Carryover Nitrogen

It is worth noting that there likely was a significant amount of N that moved into surface water over the weekend. However, this was probably carryover N from either last year’s fertilizer application or mineralized N from the decomposition of organic matter and crop residues since last year’s harvest. If you soil sampled for carryover N, and took credit for this N, it is possible that you may need to compensate for this loss. Also, the areas with the most concern are sandy soils, where an excess inch of drainage can move nitrate a whole foot. In addition, farmers who were conservative with N rates may experience some deficiency symptoms.

As with most things in nature, it is difficult to be precise with what is happening to applied N fertilizer this spring. The U of M Supplemental Nitrogen Worksheet for Corn (PDF) can help you determine whether supplementing N will be necessary.