Nitrogen is one of the most yield-limiting nutrients for rice production, and proper nitrogen fertilizer management is critical for high yields. In rice research, more time and energy are devoted to managing nitrogen fertilizer than any other nutrient because of the high potential for a return on the fertilizer investment. Ironically, though, of the nutrients used for rice, nitrogen is probably still the least predictable.
Several environmental and cultural factors affect the uptake and use of nitrogen by rice. And, depending on the type of fertilizer used, the nitrogen could be lost before the rice plant even has a chance to begin absorbing it through the roots. Current rice varieties respond well to large amounts of nitrogen fertilizer, but these varieties are not totally immune to the problems associated with over-fertilization in older varieties. For instance, excessive vegetative growth, lodging, disease damage, delayed maturity and reduced grain yields of lower quality can occur if nitrogen fertilizer applications are made at unnecessary rates or at the wrong growth stage.
The development of Clearfield rice varieties has added a new dimension to rice production in Louisiana. This technology has prompted many rice farmers in the state to change at least a portion of their acreage from the traditional water-seeded system to a drill-seeded system. Both the water- and drill-seeded systems place unique restrictions on nitrogen fertilizer management, but the essential components of a successful nitrogen management plan are the same for either system. In developing a successful nitrogen fertilizer management plan, the source of the nitrogen fertilizer, the placement of the fertilizer in the field, the application rate and application timing should all be carefully considered.
Rice uses nitrogen in the form of the ammonium ion in flooded soil, so ammoniacal nitrogen fertilizer sources are recommended for rice. Urea (46:0:0) is the most common and best source of nitrogen for rice. Its relatively high nitrogen analysis compared with other nitrogen fertilizer sources also makes urea the most economical nitrogen source. Urea is prone to losses through ammonia volatilization if applied to a moist soil or if left on the soil surface for an extended period (more than three to five days) after application.
Ammonium sulfate (21:0:0) is another common nitrogen source for rice. It is not subject to ammonia volatilization and is a good choice if soil tests recommend sulfur; however, if used strictly as a nitrogen source, ammonium sulfate is less desirable than urea because price per pound of actual nitrogen is much higher than urea.
Another nitrogen source popular for rice in southwestern Louisiana is a 50% blend of urea and ammonium sulfate, which has a nitrogen analysis of approximately 33:0:0. This combination brings the best traits of both sources together. It is less prone to volatilization than urea and has a higher nitrogen analysis than ammonium sulfate. The urea portion of this mixture, however, is still subject to ammonia volatilization, and the mixture has 13% less total nitrogen than urea. So, for the fertilizer investment, urea is the nitrogen source of choice for most situations.
When nitrogen fertilizer is applied early in the growing season, the fertilizer needs to be placed where it is least prone to loss and most readily absorbed by the plant. Simply stated, the fertilizer must be incorporated into the soil. In a water-seeded system, this could be accomplished by applying most of the nitrogen fertilizer onto the soil and mechanically incorporating it before flooding and seeding. Also, the fertilizer could be applied during the drained period in a pinpoint flooding system and incorporated with the floodwater after pegging. In a drill-seeded system, most of the nitrogen fertilizer should be applied to the soil surface and incorporated with the floodwater as the permanent flood is established.
“What is the proper application rate for nitrogen fertilizer?” That’s been a burning question for years. And, the answer is, “It depends.” The application rate for nitrogen fertilizer depends on (in no particular order) rice variety, stand density, previous crop, straw management, fertilizer source, application method, water management, soil texture, soil pH and tillage system. Naturally, the odds on all these factors aligning themselves to allow for a “cut and dried” recommendation are pretty low.
Current recommendations suggest rate ranges based on varieties and/or soil types. And, as with many other inputs, there is no substitute for personal experience. Most varieties grown in the United States require 120 to 180 pounds of actual nitrogen per acre to produce acceptable grain yields with good milling quality, and, in most cases, 30 to 60 more pounds of actual nitrogen per acre will be required for a variety when grown on a clay soil than a silt loam soil. The LSU AgCenter’s recommendation for nitrogen on rice is 120 to 165 pounds of actual nitrogen per acre for rice varieties and 100 to 140 pounds of actual nitrogen per acre for rice hybrids.
Now that the water is thoroughly muddied with the “What?”, “Where?” and “How much?” of nitrogen fertilization, the last question is: “When does the application need to be made?” And, again, the a[Image: Water-seeded rice]nswer is, “It depends.” Regardless of whether rice is water-seeded or drill-seeded, the uptake of nitrogen early in the season is critical and affects uptake of nitrogen throughout the remainder of the season. So, for optimum growth and yield, the nitrogen supply should be adequate during the tillering stage of rice development.
There are three scenarios for nitrogen application timings in water-seeded rice. The first includes a preplant application followed by a postflood application. This is the ideal scenario for water-seeded rice although it may not be practical in every situation. The best grain yields of water-seeded rice are achieved when 50% to 100% of the nitrogen is broadcast and incorporated before flooding and seeding. Soil must remain saturated when the field is drained for pegging. A midseason nitrogen fertilizer application can be made as needed based on tillering and plant coloration.
The second includes an application during the pinpoint drainage followed by a postflood application. In this case, 33% to 66% of the nitrogen is applied immediately before flooding and after pegging. This application is followed with approximately 33% of the nitrogen when rice plants are actively tillering. A midseason nitrogen application can be made if needed. While not as efficient as a mechanically incorporated application made before flooding and seeding, this is the most readily applicable scenario to the current water-seeded system. Naturally, preplant application and mechanical incorporation is not always feasible, but the nitrogen must be incorporated to be absorbed by the roots of young rice plants. If mechanical incorporation is not an option, then incorporation with the floodwater is the best alternative.
The last scenario, which is probably the most common for water-seeded rice in Southwest Louisiana, is to apply all the nitrogen fertilizer into the flooded field following seeding. In this case, about 33% of the total nitrogen is applied at intervals after seeding and flooding. If this system is used, then at least one application should be made during active tillering, which is the critical growth period for rice. Applying all nitrogen into a flooded field is the least efficient nitrogen management scenario for water-seeded rice. Applications made to flooded fields early in the season are inefficient because the young rice plants do not have the volume of roots needed to absorb fertilizer effectively. In contrast, when nitrogen fertilizer is “dropped in the water” later in the season, the nitrogen is rapidly taken up because, during the reproductive growth stages, rice plants have developed extensive, shallow root systems with high nitrogen uptake capacity.
Not only has Clearfield rice opened a new world for southwest Louisiana rice farmers in the area of red rice management, drill-seeding Clearfield varieties has provided the opportunity to optimize use of n[Image: Rice panicles at maturity]itrogen fertilizer. When drill-seeded, newer rice varieties can absorb enough nitrogen for high yields from a single application of the total nitrogen requirement applied immediately before flooding; however, applying the entire amount of nitrogen in one application is not always feasible: aerial application. This is also a pretty big gamble because the field may not be flooded in a timely manner, the flood could be lost following the preflood application or the field may require drainage to combat rice water weevils. Splitting the application would be a safer means of delivering the balance of the nitrogen requirement. If split applications are used, 50% to 65% of the total nitrogen is applied immediately before flooding, and the balance is applied at midseason when rice is at the panicle initiation to panicle differentiation growth stages.
Proper management of nitrogen fertilizer is one of the biggest challenges to the rice farmer. But, no other nutrient can return greater benefits in the form of increased grain yields if managed properly. There are few black and white areas when discussing nitrogen fertilizer management on rice. The price of nitrogen fertilizer is at an all-time high; however, if acceptable yields are to be achieved, this is not an input to cut back on. With proper planning, nitrogen management systems that fit an individual operation can be developed that will, we hope, provide a return above and beyond the fertilizer investment.