Rice Farming

Nitrogen fertility in rice is as much art as it is science. Research in recent years has provided nitrogen guidelines specific to variety, soil texture and crop history. But the fact is, growers today base their levels of applied N during a particular crop year on experience and intuition as much as hard science.

That is because for nearly a century, scientists have been unable to develop a soil nitrogen test to accurately predict the precise amount of N available in the soil for a rice crop in a given year. Guidelines today are fairly accurate.

A grower can know that, for a given variety on a particular soil texture and based on crop rotation and management, he will need a range of nitrogen within about 30 pounds of applied N.

Still, researchers concede that even with those recommendations, many growers may be overfertilizing, a practice that not only wastes money and threatens to move excess nitrates out of the soil, but also can leave plants more prone to disease. A nitrogen soil test could change that.

A mix of science and art
Tim Walker, an assistant research professor at the Delta Research and Extension Center in Stoneville, Miss., says growers are doing the best they can without an accurate soil test to determine true N needs on a given field.

“Those recommendations are just guidelines, based on what we see in small plot research on different soil types,” Walker says. “Some guys get by with less, some more and it just takes experience in the field to know that.”

Chuck Wilson, Extension rice specialist at the Rice Research and Extension Center in Stuttgart, Ark., agrees, noting that what guidelines can’t do is address changes in nitrogen levels from season to season or within a given field.

“Without a soil N test, we just have to establish recommendations through these fertilizer trials to get an average across a number of years at various locations,” Wilson says. “Research trials take all those years and average them together. The guidelines are really the best we can do without a soil test.”

The elusive soil N test
Rick Norman, a University of Arkansas soil fertility scientist, says researchers for more than 80 years have attempted to develop a soil nitrogen test for rice that is accurate, consistent and relatively easy to perform, but that test has been elusive. While other macronutrients, such as potassium and phosphorous, lend themselves successfully to an accurate preplant soil test, it is much more difficult to predict how much actual N will be available to the crop the following year.

“For potassium and phosphorous, we have soil tests for those nutrients because they are not involved in microbial processes in the soil the way N is,” Norman says. “Also N doesn’t form minerals in the soil like P and K in order to stay there. In other words, when you apply nitrogen, something is going to happen to it—either the plant will get it or microbials will get it and put it into a gaseous form that is lost, so what we would measure in N now doesn’t mean that N is going to be around for awhile.”

And that for decades has been the sticking point for developing an accurate soil N test: Finding a form of nitrogen in the soil that will be a true indicator of what will remain in the soil and be available to the rice crop.

Learning from corn research
Researchers, however, in the last couple years have been determined to see that change. Emboldened by research in the 1990s that helped to develop a true soil nitrogen indicator in corn, Norman assembled a team of graduate students that have been exploring taking the successful premise in corn and transferring it to the various soil types in the rice belt.

The corn research, conducted by Richard Mulvaney at the University of Illinois, led to the creation of the Illinois Soil Test based on organic N—the first true preplant predictor for the release of N during the season for corn.

Mulvaney has said he expects the Illinois Soil Test to make intensive preplant soil sampling on very small grids within a field a reality because it provides a very accurate prediction of native release of N during the season.

That intensive sampling procedure tests for organic N in the form of nitrogen amines rather than the more volatile nitrates as a precursor to nitrogen needs within a given site.

The test itself involves distillation and diffusion—treating a soil sample with an alkaline solution, heating it and collecting amino sugar N in the form of gaseous ammonia. That ammonia level then indicates the soil’s N-supplying capacity.

Mulvaney’s test has provided a soil test that is 90 percent accurate, a far cry from previous yield-based methods of calculating nitrogen needs in corn.

With the success of the research in corn, Norman had hoped he could transfer the technology to a similar test in rice. His hope was to have an accurate soil test for rice within three years.

The search continues
Norman sent a team of graduate students to Illinois to be trained in Mulvaney’s anaerobic incubation method, and the two based their graduate work largely on making a similar organic N connection in rice.

However, as has been the case for more than a generation, the soil test for rice remains elusive.

“It is still very much in the preliminary stages, but we’re not seeing what we wanted to see,” Norman says.

While Norman says the corn research certainly pointed research on a rice nitrogen test in the right direction, it doesn’t appear that with the silt loam soils in Arkansas and Louisiana, amino sugar will be the best form of organic nitrogen to determine plant available N.

“Maybe we need to look more to amino acids, maybe we need to look at amides, we’re not sure yet,” he says.

Norman says that more advance analysis procedures, such as chromatography, rather than traditional “wet chemistry” methods, might better isolate the form of nitrogen that could be accurately used in a soil N test in rice fields.

“We need something that we can identify the fraction in the soil that is breaking down to make N available to the plant so we can analyze the level in the soil preplant to know how much is available.

“I just don’t think this wet chemistry is going to find it,” he says. “I think we are going to have to look at chromatography. We’re going to keep pursuing it.”

It’s a concept Norman and other researchers hope to continue looking at, because it’s one they think the industry demands.

“Growers would really like to have a method to test for nitrogen requirements,” he says. “They know by experience that they don’t have to put as much on one field as they do another, but they don’t know why. They want to be able to apply a certain amount of N based on that crop and based on that specific field.”

For questions or comments, contact Rice Farming editor Vicky Boyd at (209) 571-0414.

Fertility’s Holy Grail Researchers continue 80-year search for accurate predictor of crop nitrogen needs
By Doreen Muzzi
Nitrogen fertility in rice is as much art as it is science. Research in recent years has provided nitrogen guidelines specific to variety, soil texture and crop history. But the fact is, growers today base their levels of applied N during a particular crop year on experience and intuition as much as hard science. That is because for nearly a century, scientists have been unable to develop a soil nitrogen test to accurately predict the precise amount of N available in the soil for a rice crop in a given year. Guidelines today are fairly accurate. A grower can know that, for a given variety on a particular soil texture and based on crop rotation and management, he will need a range of nitrogen within about 30 pounds of applied N. Still, researchers concede that even with those recommendations, many growers may be overfertilizing, a practice that not only wastes money and threatens to move excess nitrates out of the soil, but also can leave plants more prone to disease. A nitrogen soil test could change that. A mix of science and art Tim Walker, an assistant research professor at the Delta Research and Extension Center in Stoneville, Miss., says growers are doing the best they can without an accurate soil test to determine true N needs on a given field. “Those recommendations are just guidelines, based on what we see in small plot research on different soil types,” Walker says. “Some guys get by with less, some more and it just takes experience in the field to know that.” Chuck Wilson, Extension rice specialist at the Rice Research and Extension Center in Stuttgart, Ark., agrees, noting that what guidelines can’t do is address changes in nitrogen levels from season to season or within a given field. “Without a soil N test, we just have to establish recommendations through these fertilizer trials to get an average across a number of years at various locations,” Wilson says. “Research trials take all those years and average them together. The guidelines are really the best we can do without a soil test.” The elusive soil N test Rick Norman, a University of Arkansas soil fertility scientist, says researchers for more than 80 years have attempted to develop a soil nitrogen test for rice that is accurate, consistent and relatively easy to perform, but that test has been elusive. While other macronutrients, such as potassium and phosphorous, lend themselves successfully to an accurate preplant soil test, it is much more difficult to predict how much actual N will be available to the crop the following year. “For potassium and phosphorous, we have soil tests for those nutrients because they are not involved in microbial processes in the soil the way N is,” Norman says. “Also N doesn’t form minerals in the soil like P and K in order to stay there. In other words, when you apply nitrogen, something is going to happen to it—either the plant will get it or microbials will get it and put it into a gaseous form that is lost, so what we would measure in N now doesn’t mean that N is going to be around for awhile.” And that for decades has been the sticking point for developing an accurate soil N test: Finding a form of nitrogen in the soil that will be a true indicator of what will remain in the soil and be available to the rice crop. Learning from corn research Researchers, however, in the last couple years have been determined to see that change. Emboldened by research in the 1990s that helped to develop a true soil nitrogen indicator in corn, Norman assembled a team of graduate students that have been exploring taking the successful premise in corn and transferring it to the various soil types in the rice belt. The corn research, conducted by Richard Mulvaney at the University of Illinois, led to the creation of the Illinois Soil Test based on organic N—the first true preplant predictor for the release of N during the season for corn. Mulvaney has said he expects the Illinois Soil Test to make intensive preplant soil sampling on very small grids within a field a reality because it provides a very accurate prediction of native release of N during the season. That intensive sampling procedure tests for organic N in the form of nitrogen amines rather than the more volatile nitrates as a precursor to nitrogen needs within a given site. The test itself involves distillation and diffusion—treating a soil sample with an alkaline solution, heating it and collecting amino sugar N in the form of gaseous ammonia. That ammonia level then indicates the soil’s N-supplying capacity. Mulvaney’s test has provided a soil test that is 90 percent accurate, a far cry from previous yield-based methods of calculating nitrogen needs in corn. With the success of the research in corn, Norman had hoped he could transfer the technology to a similar test in rice. His hope was to have an accurate soil test for rice within three years. The search continues Norman sent a team of graduate students to Illinois to be trained in Mulvaney’s anaerobic incubation method, and the two based their graduate work largely on making a similar organic N connection in rice. However, as has been the case for more than a generation, the soil test for rice remains elusive. “It is still very much in the preliminary stages, but we’re not seeing what we wanted to see,” Norman says. While Norman says the corn research certainly pointed research on a rice nitrogen test in the right direction, it doesn’t appear that with the silt loam soils in Arkansas and Louisiana, amino sugar will be the best form of organic nitrogen to determine plant available N. “Maybe we need to look more to amino acids, maybe we need to look at amides, we’re not sure yet,” he says. Norman says that more advance analysis procedures, such as chromatography, rather than traditional “wet chemistry” methods, might better isolate the form of nitrogen that could be accurately used in a soil N test in rice fields. “We need something that we can identify the fraction in the soil that is breaking down to make N available to the plant so we can analyze the level in the soil preplant to know how much is available. “I just don’t think this wet chemistry is going to find it,” he says. “I think we are going to have to look at chromatography. We’re going to keep pursuing it.” It’s a concept Norman and other researchers hope to continue looking at, because it’s one they think the industry demands. “Growers would really like to have a method to test for nitrogen requirements,” he says. “They know by experience that they don’t have to put as much on one field as they do another, but they don’t know why. They want to be able to apply a certain amount of N based on that crop and based on that specific field.” For questions or comments, contact Rice Farming editor Vicky Boyd at (209) 571-0414.
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