The severity of nitrogen losses from extreme summer rainfalls, and how you react to them, depends on many factors.
Generally speaking, excess moisture in the soil profile is one of the main causes of nitrogen (N) losses in crop production. These types of losses are due to either leaching or denitrification of the nitrate form of N. Leaching losses occur when water percolating through the soil profile carries nitrate below the rooting zone. Denitrification losses occur when anaerobic soil conditions cause microbes to convert nitrate to nitrous oxide or di-nitrogen gases which escape from the soil into the atmosphere.
The extent to which these types of N losses occur after extreme precipitation events in mid-summer, such as those occurring in July and August 2018, when there was 10 to 20 inches of rainfall across Pennsylvania within a 3-week period, depend on many factors. These factors include the timing of precipitation relative to crop N uptake patterns, the quantity of N in the soil profile in the nitrate form during the precipitation events, and soil drainage patterns.
When extreme precipitation events occur later in the growing season, much of the N needed by a crop such as corn has already been taken up by the plant. For instance, corn in the silking stage (R1) has already taken up about 60% of the nitrogen it needs and corn in the blister stage (R2) has taken up 75% of the nitrogen that it needs. The remaining N requirement of the crop may come from fertilizer that has been applied earlier in the season, or from N mineralized from organic sources, such as soil organic matter, cover crop residues, or manure applications.
If a majority of the N fertility program for a corn crop is coming from inorganic fertilizer applications, it is likely that most of the fertilizer N applied has been converted to the nitrate form by the end of July. The exception to this is if a nitrification inhibitor was used with the fertilizer, which will provide about 6 weeks of protection against the conversion of N from ammonium to nitrate forms. The nitrate remaining in the soil profile is what will be susceptible to leaching and denitrification losses. If a significant portion of the N fertility program for a corn crop is coming from the residual manure history, a previous legume crop, and current year manure applications, these organic sources of N will continue to mineralize through the summer. Mineralization of these organic N sources creates the ammonium form of N, which later in the season may be taken up directly by the crop before it is nitrified and susceptible to loss. Moist soil conditions in late summer may create more favorable conditions for mineralization, possibly increasing the amount of N available to the crop. Therefore, the severity of N losses due to mid-summer extreme rainfall will be greater in systems where inorganic fertilizer is the primary N source for the crop as compared to systems where N mineralization from organic materials is a major source.
Constructing a simplified water budget based on the daily precipitation amounts that occurred in State College, PA (one of the drier areas of the state in summer 2018) showed that the 8.5” of total rainfall occurring between July 15 and July 30, 2018 would generate 4.8” of drainage water. Doubling the daily precipitation amounts to simulate the wetter parts of the state in 2018 showed that 17” of rainfall would generate 13.25” of drainage water. To translate these drainage volumes to the distance water would move within the soil profile, a simplified rule of thumb would be to multiply by two, since soil is about 50% pore space by volume. So 4.8” of drainage water would have moved existing water and nitrate in the soil downwards by 9.6”, and 13.25” of drainage water would move nitrate downward by 26.5”. The first scenario probably leaves nitrate still within the rooting zone while the second scenario represents a complete flush of water and nitrate from the root zone. These estimates represent worst case scenarios, because they assume all precipitation infiltrated the soil profile, whereas much of the precipitation that falls on an already saturated soil will likely run off the surface.
Because of the many interacting factors that affect N losses, computer simulation models can provide some useful information about potential losses during the season. For the 2018 growing season, we have been following soil N levels using the Adapt-N model, which takes into account rainfall patterns and soil nitrogen cycle dynamics to predict N losses and fertilizer N requirements in corn. We tested two scenarios in the tool, an N recommendation based on the Penn State Agronomy Guide and one based on the Adapt-N recommendation at sidedress time. The Agronomy Guide recommendation, based on the previous soybean crop N credit and the corn yield goal, called for 130 lbs N/ac. To achieve this, 80 lbs N/ac was applied at planting followed by 50 lbs N/ac sidedressed on June 21. The Adapt-N recommendation, accounting for 80 lbs N/ac applied at planting, called for an additional 100 lbs N/ac sidedressed, which we applied on June 21. As of July 30, Adapt-N was suggesting the Agronomy Guide recommendation was 25 lbs N/ac short in reaching the yield goal, while the sidedress rate based on the Adapt-N recommendation was 35 lbs/ac in excess of what the crop needed. For both scenarios, the amount of N loss between July 14 and July 28, when 7.2” of rainfall occurred, was only 2 to 3 lbs N/ac and was due exclusively to denitrification. Doubling the precipitation levels (by adding irrigation events to simulate increased rainfall) did not increase the level of N losses. In this case, one of the main factors that led to limited N losses was that much of the original N applied had already been taken up by the crop and there were fairly low levels of nitrate remaining in the topsoil. For the Adapt-N recommended sidedress scenario, the corn crop had already taken up 144 lbs N/ac by July 14, which is when the rain really started to fall consistently. By July 31, 161 lbs N/ac had been taken up of the total target of 184 lbs N/ac. The N uptake by the crop had scavenged soil nitrate levels down to only 35 lbs N/ac on July 14 and only 18 lbs N/ac on July 31, so relatively little of the total N applied was available for losses at this time of the season. While computer models certainly aren’t perfect, they do allow us to evaluate how different parts of the system work together to affect an outcome.
For those producers trying to decide about whether to add more nitrogen to their corn crop, the answer will certainly be site and scenario specific. Many indicators point towards N losses being rather limited during extreme precipitation events occurring in late July. Areas within a field that will suffer the worst will probably be low lying areas where water ponds, causing denitrification. Accessing fields with equipment that can apply additional N fertilizer into a 6’ tall corn crop also creates its own limitations. Since the areas within a field that need to be treated with additional N may be patchy, the economics of treating small areas with large equipment, or the economics of treating whole fields where much of the crop may not be responsive to additional N, may not be favorable. As always, understanding the processes that lead to N losses and integrating this understanding into adaptive management tactics will help you make a decision that best fits your individual scenario.