Winter Rains Cause Concerns about Nutrient Loss

Many farmers are asking about the possibility of nutrient loss from the unprecedented rains we received in December. While there is definitely cause for concern, don’t assume all fertilizer that was fall-applied is lost. Also keep in mind that the amount of loss will depend on the specific nutrients that were applied, as well as whether it was in the dry, liquid or gaseous form.

Phosphorus (P) and Potassium (K):  Most experts agree that P & K applications are very stable no matter the time of year of application.  It’s not unusual to have these products applied on frozen ground and see little, if any, loss prior to planting.  One “watch out” here is if the December rains caused soil loss due to runoff. If this happened, then it is most likely that you also lost some P & K.

Nitrogen (N):  Two main reasons for N loss are leaching and nitrification. Leaching refers to the actual loss of nutrients from downward movement through the soil profile. Since the ground in most areas was not frozen prior to the rains, this process could have a significant effect. Because the rain came fairly fast, there was considerable ponding and runoff in some fields.  These will be the greatest areas of concern. Nitrogen loss here will not be uniform and will be extremely difficult to quantify.  If drainage tiles ran, that is another avenue for N loss that may need to be considered.

The most common form of fall-applied nitrogen is anhydrous ammonia (NH3).  When applied, it quickly converts to ammonium (NH4), which is a positively charged particle.  Nitrogen is fairly stable in this form as it is held onto the negatively-charged soil complex. Ammonium eventually converts to the nitrate form by the process of nitrification.  Since this is a biological process, the rate of conversion is determined by the soil temperature.

Nitrification does not stop, however, it slows down considerably when soil temps are below 50° Fahrenheit. This is the reason most agronomists recommend fall N applications after soils have fallen below this temperature.  The nitrate form of N is the most susceptible to leaching. There’s a very good chance of N loss from the nitrification process if you applied nitrogen before soil temps fell below 50°F.  Applications made later on (in colder soils) have a much less chance of significant loss.

It’s too early to determine nitrogen loss from December rains. We’ll wait until early spring to see how the weather develops and see the fields look after they thaw.  For additional information on the potential for nitrogen loss, click here for an article from Dr. John Sawyer, Extension Soil Fertility Specialist, Iowa State University.