Rice preflood nitrogen management
Nitrogen is important for crop growth and production. Managing this valuable input for maximum plant availability is critical to ensure optimum economic returns while minimizing the potential for negative environmental effects. Urea is the standard N fertilizer product for rice production. Urea is a high analysis (44-46% N) granular N fertilizer that is ultimately converted into ammonium and nitrate, the plant available forms of N. The N in urea is subject to loss by ammonia volatilization if it begins to break down on the soil surface. Several factors including soil temperature, soil moisture, soil pH, wind-speed, cation exchange capacity, organic matter, and humidity influence the rate of ammonia volatilization.
Agrotain ([N-(n-butyl) thiophosphoric triamide] abbreviated NBPT) became commercially available around the year 2000. Since then, other products have entered the market that contain NBPT as the active ingredient. Arborite and N-FIXX are two that we have tested in our lab that have performed similarly to Agrotain. These products work by slowing the transformation of urea to ammonium by temporarily inhibiting the urease enzyme. This allows growers more time from urea application until the next rain or irrigation event needed to incorporate the fertilizer into the soil. At higher rates Agrotain and other NBPT containing products can minimize ammonia volatilization losses for up to 14 days. Note that NBPT containing products only stabilize N from loss against ammonia volatilization. Agrotain will not provide stabilization from the other N loss mechanisms once the urea fertilizer is incorporated into the soil and urea is converted into ammonium. The ammonium will convert to nitrate in the soil and may be lost via leaching, or lost through denitrification in saturated soil conditions. We have also tested Nutrisphere and NZone Maxx for their ability to stabilize N from ammonia volatilization loss. Both of these products have failed in our laboratory experiments.
The choice you make regarding the N-stabilizer product can be likened to car insurance. For many people, insurance premiums are never capitalized on because accidents have not occurred. However, if the accident does occur, you want the insurance to provide stability against repairs or replacement. In crop production, N losses cannot be predicted; however, when conditions promote loss, be sure you have a product that is proven to insure N will be available to the plant rather than in the atmosphere or in the closest stream.
After urea application, it is critical to establish a permanent flood as quickly as possible, but preferably within five days. This will ensure maximum availability of the applied N. Rates of nitrogen and application splits vary by soil type, pump capacity, the ability to apply urea uniformly, and grower preference. If fields can be flooded within a few days after urea application, my recommendation is to apply 50 to 70% of the total amount of N prior to flood and apply the remaining at midseason. For hybrids, RiceTec, Inc., recommends applying 30 lbs of the total N at 5% heading. Lodging susceptible varieties like CL151 and CL111 should not be fertilized for maximum yield potential. Our research shows that lodging can be greatly reduced when we fertilize to achieve 95% of the maximum yield potential. On clay soils, this equates to 135 to 150 pounds of N per acre. As a comparison, Rex and Cocodrie on clay soils will require typically require 165 to 180 pounds of N per acre.
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