The official LSU AgCenter recommendation for nitrogen (N) fertilizer application in rice is to apply approximately two-thirds of a variety’s seasonal need just before flooding on dry soil, followed by flooding as soon as possible. A second split application is recommended midseason.
The midseason N application window is between panicle initiation and panicle differentiation. Panicle initiation can be estimated by splitting a stem and looking for green ring development, while panicle differentiation can be estimated by visual observation of the panicle or approximately one-half internode elongation.
During this window, the remaining one-third of the seasonal N fertilizer need is applied. While this 2-way split N application method is the recommended method, it is not actually the most efficient way N fertilizer can be applied.
Applying N in a single preflood (SPF) application on a dry soil and flooding immediately is the most efficient method of applying N fertilizer in rice. When ammonium N fertilizer (ammonium sulfate) or ammonium N forming fertilizer (urea) is applied on a dry soil, the flood incorporates the N deeper into the soil.
The flooded field causes the soil to convert to an anaerobic (without oxygen) state which stabilizes the N in the ammoniumN form where it will remain available for plant uptake as the rice needs it. If the SPF N fertilizer application is the most efficient application method, you may wonder why it is not the official recommended method in Louisiana.
The answer is because the application method has more risks associated with it.
The first risk is that once the flood is established, it must remain on the field for a minimum of 3 weeks without ever losing the flood. Remember, floods from a rice field can be lost in many ways, including the inability to keep a flood on the field due to low pumping capacity and dry weather or blown out levees due to excessive rain, animals, or pests.
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If the flood is lost within the first 3 weeks and oxygen is reintroduced in the soil, the ammonium N will begin to convert to nitrate N. The nitrate N is stable while the flood is off of the field.
However, when the flood is reestablished and the soil goes anaerobic, the nitrate N will be lost as a gas (nitric oxide, nitrous oxide, or dinitrogen gas) very quickly through a process called denitrification. Therefore, more N fertilizer will need to be applied to compensate for the expected loss of N once it is reflooded.
The end result is that more total N fertilizer will be applied using the SPF as compared to the recommended split-N application method by season’s end. Currently, we do not have a mechanism for predicting the amount of N you will need to replace, but you should expect to have to apply more when the flood is lost during the first week as opposed to the third week.
The second risk associated with the SPF method is the inability to apply the large amount of N evenly across the field. If the N application is overlapped during the application, you will have applied twice the recommended N rate.
This overlapped area will be a prime candidate for lodging at the end of the season, especially for lodge-prone varieties and hybrids. In addition, the overlap areas will also have an increased potential for disease. Both issues will cause yield loss at the end of the season.
Skips in fertilizer can also happen during application. The skip areas can be cleaned up during midseason applications when using the 2-way split method. However, additional N applications when using the SPF method may cause excessive N application in the areas just outside of the initial skipped areas and should be avoided.
While fertilizer overlaps and skips are risky when applying SPF applications, it should be mentioned that advances in some fertilizer applicators have made these applications more precise over the last several years.