Anyone who produces rice knows the importance of nitrogen fertilizer for maximizing yields and optimizing profits. It is, however, a common misconception that “more is always better” or “if 150 units is good, then 300 units is even better.”
Pre-flood N fertilizer sets your rice crop’s yield potential, and it is very important that the proper measures be taken to maximize your pre-flood N fertilizer use efficiency. In recent years, with unusually hot summers and suboptimal growing conditions, we have started to realize just how important properly managed N fertilizer can be for a rice crop. Pre-flood N fertilizer can often represent the largest line-item expenditure for most producers, and with current urea prices bumping $800 per ton, a producer can’t leave anything to chance.
The following steps can help producers and consultants best manage pre-flood N.
(1) Always apply pre-flood N fertilizer to a dry soil.
This time of year, when things are going hot and heavy, it is not easy to be patient, but waiting an extra day or two for the soil to dry prior to applying N and flooding will pay huge dividends come harvest time. Urea applied to wet or muddy soil can easily lose 50-60 of the total applied N to ammonia volatilization. Even when a urease inhibitor is used, you can lose as much as 25 to 30 percent of the applied N.
So, how do you know when the soil is dry? This is a judgment call, but if the soil is moist enough for you to leave tracks when you walk through the field, it is too wet to apply pre-flood N fertilizer.
A second concern — “My field is wet, but my pilot says today is the only day he can get to me.” If you have to apply your pre-flood N to a wet or muddy soil — and I mean absolutely have to — make sure and use an recommended urease inhibitor, but let the soil dry before you flood.
(2) If you require more than two days to establish a permanent flood, use a recommended urease inhibitor on pre-flood urea.
A quality urease inhibitor that contains NBPT is worth its weight in gold when it comes to mitigating ammonia volatilization losses from urea applied pre-flood. If you are on clay soils or require two days or less to flood, then you probably will see no benefit from a urease inhibitor. If you are on a silt loam soil and conditions are right you can lose as much as 50 percent of your applied urea-N in as little as seven days.
The University of Arkansas Division of Agriculture currently recommends two products for combating ammonia volatilization losses from urea fertilizer — Agrotain and Arborite. These products have been extensively tested in both the lab and field and have shown to mitigate ammonia losses from pre-flood urea-N applications. Work is currently being conducted to evaluate several other products that have shown promise in other states but must be proven under Arkansas growing conditions.
With the price of urea skyrocketing, several producers have chosen to use a blend of ammonium sulfate and urea because the cost per unit of N is quite similar. Notes to remember when using ammonium sulfate — ammonia volatilization losses are seldom an issue when using ammonium sulfate and there is no need for a urease inhibitor (urease inhibitors are only for urea).
If you are blending urea and ammonium sulfate, have the co-op treat the urea with a urease inhibitor separately before blending. Do not let them blend the products and then charge you for the cost of treating both the ammonium sulfate and the urea.
(3) Maintain a permanent flood for at least three weeks following pre-flood N application and flood establishment.
An essential part of direct-seeded delayed-flood rice production that allows us to boast the highest N use efficiency of any cereal crop is the ability to apply N in an ammonium form and then lock that N in place using a permanent flood. There are several things that an established and well-maintained flood will do for your N fertilizer management.
By applying urea to a dry soil and establishing a flood, you are effectively moving the N into the rice root zone where it is readily available for crop uptake. By maintaining that flood for three weeks you are ensuring that the N remains as ammonium and does not undergo nitrification/denitrification losses. Research has shown that three weeks following flood is the critical time period where the flood has to be maintained in order to maximize N uptake by the rice plant and increase your yield potential.
It is best to maintain a quality flood until rice maturity, but if you have to drain, try to wait a minimum of three weeks to ensure that the rice crop has ample opportunity to utilize the pre-flood N fertilizer.
Following these three easy steps will help increase the N use efficiency of your 2012 rice crop and ensure that your hard-earned money is well-spent and well-used. For those of you utilizing the N-STaR program for N fertilizer recommendations, these guidelines are even more important for you because there is no room for error when managing your N fertilizer inputs. Please refer to your N-STaR report for guidelines on proper pre-flood N management using the N-STaR system.