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Fertilizing wet rice soils

STUTTGART, Ark. — With the extensive rainfall recently obtained in several Mid-South areas, some rice farmers have escaped the early-seedling stage without the need to flush. However, as the rice approaches the tillering stage, these fields need to be dry for nitrogen fertilizer application. Only time will tell if Mother Nature cooperates.

Every year I get dozens of phone calls on how to manage nitrogen fertilizer when the fields are wet.

Because pre-flood nitrogen sets the yield potential of the varieties currently produced, it is critical to manage the pre-flood nitrogen as efficiently as possible.

Urea has been the standard nitrogen fertilizer source for rice for many years. It is relatively inexpensive, has a high nitrogen analysis and is an effective fertilizer for flooded soils when applied and managed properly.

However, it has limitations that can make it inefficient when not applied and managed appropriately.

Nitrogen derived from urea is prone to ammonia volatilization losses when it is applied to the soil surface. When urea is applied to dry soils, establishing the flood incorporates the urea into the soil to minimize this loss.

However, when the soil is muddy at application or if several days are required to establish the flood, the amount of nitrogen lost by ammonia volatilization increases.

Ammonia volatilization tends to be amplified when urea is applied to wet soils. In addition, when urea is applied to wet soils and the field is flooded while still muddy, the urea is apparently not incorporated in to the soil deep enough where it is protected against nitrification-denitrification losses.

Application of urea into the water after flood establishment, during the rice vegetative stage, is the least efficient method, with as much as 75 percent loss possible.

In recent studies, Rick Norman, Nathan Slaton, and I have found as much as 20 to 25 percent of the nitrogen fertilizer is lost when the flood is delayed only five days after application.

This is significant since many of the rice fields in Arkansas take as much as 10 to 20 days to establish a flood.

Yields were reduced in that study by 12 to 23 percent when the flood was delayed for five or 10 days after fertilizer application.

Because of the potential for loss and the reduced yield, we encourage producers to establish the flood as quickly as possible.

If extended time is required, evaluate opportunities to improve water management through techniques such as multiple inlet rice irrigation with the use of the plastic tubing. Keep fields small to improve management capabilities. Larger fields are easier during field preparation and harvest but are more difficult to irrigate.

From the study mentioned above, ammonium sulfate was the most efficient nitrogen source. When ammonium sulfate was used instead of urea, nitrogen loss was only about 5 percent and the yield loss was negligible when the flood was delayed.

Ammonium sulfate is an excellent source of nitrogen for rice. This is particularly true for fields that require an extended amount of time to flood. Many producers routinely include ammonium sulfate as part of their nitrogen fertilizer requirements, particularly if early nitrogen applications are made prior to a flush.

The biggest limitation to widespread use of ammonium sulfate is the cost per unit of nitrogen and the associated application cost in comparison to urea.

Urea contains 46 percent nitrogen whereas ammonium sulfate contains 21 percent nitrogen.

Because of the difference in analysis, 219 pounds of ammonium sulfate is required to supply the same amount of nitrogen as 100 pounds of urea. The costs of the materials are similar, which means that ammonium sulfate may cost as much as two times that of urea.

Also, application costs are generally assessed on the basis of pounds of material applied. Since roughly twice as much material is needed with ammonium sulfate, the application costs are generally going to be twice as much compared to urea.

Because of the potential for nitrogen loss, the best approach is to apply urea onto dry soil and establish the flood as quickly as possible.

If fields are wet, allow seven to 10 days for the field to dry before applying the nitrogen.

If the fields do not get dry, apply the nitrogen onto the wet soil and flood as quickly as possible.

If you know it will likely take a long time to get flooded although the field is already muddy, consider adding an additional 20 to 30 pounds of nitrogen to your preflood application to help compensate for loss that will occur.

Charles E. Wilson Jr. is the Extension rice agronomist at the University of Arkansas Rice Research and Extension Center in Stuttgart. e-mail

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