How dry should soil be when rice farmers are trying to schedule preflood nitrogen applications for their crop? Rice specialists say it’s best to put the first application of nitrogen on dry ground to prevent loss of the nutrient into the atmosphere.
“It should be dry enough that you can walk across it and not leave a track or carry any soil out of the field with you,” says Jarrod Hardke, Extension rice specialist with the University of Arkansas. “We would like it a little drier than that, but sometimes that’s the best we’re going to get.”
Hardke and Dustin Harrell, Extension rice specialist with the LSU Ag Center, spoke on fertilizing rice under the unusually wet conditions that occurred in their states in the spring and early summer of 2015. The LSU Rice Research Station where Harrell works received 26 inches of rain in the first three months of the growing season.
“If we can’t get that and continue to stay muddy, our second best option is to aim for a muddy soil with no standing water of any kind,” says Dr. Hardke. “If we can apply nitrogen on a muddy soil, treat it with an NBPT product, a urease inhibitor, and then allow that soil to dry, if at all possible, prior to actually flooding and incorporating that urea.”
In some cases this spring, farmers in Arkansas and Louisiana made those applications on soil that was as dry as they could get it “because we knew that rain was coming the next day,” said Hardke, “And there would be no opportunity to let those soils dry, and we went ahead to get the nitrogen in there.
Not applying any N
“In doing that, that application becomes about 20 percent inefficient,” he said. “So we know we are giving up some nitrogen, but the next worst case – as those plots clearly show – is if you put that entire application into standing water. You’re looking at losing the vast majority of your nitrogen, and your yield is likely to not be any different compared to if you hadn’t applied it at all.”
(Hardke was referring to a demonstration at the Rice Research Station that gave participants a side-by-side comparison of applying nitrogen on dry ground, into mud and into a flood. The application in standing water was comparable to a plot that received no nitrogen.)
Adding a urease inhibitor to nitrogen applied in standing water would not help, Hardke said, because the product would be washed off the nitrogen when it hit the water.
Growers who couldn’t get their fields to dry had to “spoon feed” the nitrogen on their plants, applying roughly 100 pounds of urea at a time to try to keep the crop from becoming too nitrogen-stressed, said Hardke.
The excessive rains left some growers unable to put up levees in their fields, which meant they were delayed in getting pre-flood nitrogen or a flood on their fields. The spoon feeding of nitrogen that resulted from such problems led to some difficult management decisions.
“The typical follow-up question, particularly when we got delayed waiting for dry soils, whether we got there or not, was ‘we finally got the flood established and within a week or two weeks I’m at green ring,’” said Hardke. “’What do I do now?’
Wait three weeks
“For most of our cultivars that we grow when we apply nitrogen in that optimum window, it’s three weeks or so until we get to green ring,” he noted. “What we have found is that when you hit green ring very soon after we finally get the pre-flood nitrogen on is that you need to wait at least three weeks before you apply that mid-season nitrogen application.”
The two criteria for the application is that it be three weeks after the preflood application and that the rice be at the green ring stage. “You’re better off to wait until those two situations are met,” says Hardke.
For more on the use of a urease inhibitor, go to http://link.springer.com/article/10.1007/BF00025060#page-1