Recent Rain Patterns: Implications on Nitrogen Management for Wheat Production

Since the past week, many wheat fields in Ohio are battling with excessive soil water or even ponding water conditions (Fig 1). Ohio has received rainfall ranging from about 1.5 inches in the north to more than 4.0 inches in some south-western counties (Fig. 2a). Total precipitation amount in the past 30-days has been substantial with certain portions recording near 15 inches (Fig. 2b). Such rainfall patterns can have implications on wheat production, especially due to nitrogen (N) management concerns.

Recommended N application timing: Ohio State University recommends applying nitrogen between spring  green-up and Feekes Growth Stage 6 (first node visible): https://www.youtube.com/watch?v=D_f3VrqzV5c&list=PLYlh_BdeqniJKls4wRr4V_JNRA6ufv3Ne&index=11&t=4s). Ohio research has shown no yield benefit of N application prior to green-up, with major N loss and yield reductions  observed with such approach in certain years. On the flip side, waiting until Feekes Growth Stage 9 can also result in yield declines (Lentz and Lindsey, 2016).

Current wet soil conditions are posing two major concerns for wheat N management:

1. Waiting to apply N: Is it too late to apply N now? There is still time to apply N to meet wheat N demand without seeing a yield penalty. In most areas of Ohio, wheat enters Feekes Growth Stage 6 around mid- to late-April. If weather clears up and soil dries up enough to allow machinery into the field, growers should apply N.
2. Already applied N: What is the fate of applied N? Excessively wet soil conditions trigger N loss via two pathways: leaching and denitrification. As water coming from rain moves down the soil profile, it carries negatively charged nitrate ions with it via leaching process. It is hard to predict the exact amount of N leached, but one can expect more N losses in coarse textured soils compared to fine textured soils. Additionally, the N loss via leaching depends upon the type of N fertilizer applied. Ammonium-N fertilizers, such as urea and ammonium sulfate should still be in the ammonium-N form and not vulnerable to loss (positive ionic charge of the N form held by the negative charged soil particles). However, N fertilizers with a nitrate component (negative ionic charge; repelled by the soil) such as urea-ammonium nitrate (UAN, 28-0-0) has the potential to lose the nitrate portion after the intense and heavy rainfall events. Roughly speaking, UAN is 50% urea and 50% ammonium nitrate resulting in about 75% ammonium-N and 25% nitrate-N for the crop soon after application.  During the early season’s excessive rain events, 25% nitrate-N from UAN is generally vulnerable to be lost as moving water could carry it down the tile system or as run-off since the wheat crop utilizes very little N until Feekes Growth Stage 6. Thus, if a producer applied 100 lbs of N from UAN, 25 lbs would be vulnerable to loss from a heavy rain event that occurs before Feekes Growth Stage 6. After Feekes Growth Stage 6, much of the nitrate-N will be used by the wheat crop reducing the loss potential. The longer the time between fertilizer application and Feekes Growth Stage 6, the greater potential for this nitrate component to be lost from excessive rainfall. The Ohio State University research has not shown yield increases from very early nitrogen applications on wheat, thus, to minimize the N loss potential, OSU recommends applying N after full green-up.  
   Soil texture and temperature will also affect the potential of N loss. If soils are fine textured (high in clay content), water ponding may be more prominent which can trigger N losses via denitrification. Denitrification mainly occurs when soil temperatures are above 55 ^oF and water is standing for extended time in the fields. Fortunately, soil temperatures have been relatively low in the past rainy week, averaging <50 degrees in the northern portion of the state and mid to low 50s in the southern part of the state. Therefore, denitrification loss is expected to be minimal.

In summary, recent rainfall events have raised concerns about N management for wheat production. Growers are still in the window to apply N to meet the crop needs; it is not too late yet. However, if a grower has already applied N fertilizer, some fraction of applied N may have been lost after excessive rain events, especially if fertilizer contained N in the nitrate form and soils were relatively coarse textured.