Widespread heavy rains in the last weeks may require additional nitrogen applications for corn fields in South Dakota.
That’s according to South Dakota State University plant and soils scientist Ron Gelderman, who said soil type – along with rainfall – can make a difference in the amount of nitrogen lost.
“Most of our medium- to fine-textured soils can hold two inches of available water in each foot of soil,” Gelderman said. “Most of our soils were already full due to last fall’s precipitation, the winter snowmelt, and earlier spring rainfall. Therefore, additional rainfall that went into the soil will eventually push out that amount from the bottom of the root zone.”
Gelderman said the question producers face is how much of the available plant nitrogen (nitrate-N) went with it.
“In addition, we have a number of flooded low areas that formed in fields and may be there for some time, and in those places, the available nitrogen may be lost as a gas through denitrification,” said Gelderman.
Producers must consider several factors beyond soil water when evaluating nitrogen losses from leaching, including when the nitrogen was applied, whether or not a stabilizer was used, the forms of nitrogen, soil characteristics, and how much water actually entered the soil compared to the amount of water that exited through run-off.
“In general, leaching losses are more likely on sandy soils where water can enter and move quickly through the soil,” said Gelderman. “Most nitrogen in South Dakota is applied as either dry urea or liquid nitrogen, which is made up of one-half urea, one-quarter ammonium, and one-quarter nitrate-N.”
Gelderman said that if liquid nitrogen was used, the nitrate-N portion is immediately susceptible to leaching as is any carryover nitrate-N from last fall.
“How much of the urea portion converted to nitrate depends on how much time elapsed between the rainfall and application, and it depends on soil temperatures as well,” Gelderman said. “Pre-plant applied urea usually converts to nitrate-N within 4-6 weeks. Therefore, most of the nitrogen applied before May 1 should have been converted when the heavy rainfall occurred.”
Adding an nitrogen stabilizer could have delayed the conversion process by one to two weeks, thus resulting in less nitrate-N that would be susceptible to loss when the rainfall came.
“Because of the variables such as soil texture and rainfall amounts, it is very difficult to predict actual nitrogen loss,” said Gelderman. “Therefore, producers should soil-sample some of these areas or fields that are of concern to determine potential loss and possible corrections.”
Producers should select areas of the field that represent the major portion of the field area or divide the field into major landscape areas and sample separately. It’s best to wait until soil has dried sufficiently to support a probe truck or it can be done with a hand probe.
“Take at least 10-12 sample holes to depths of three feet and separate them by one-foot increments,” Gelderman said. “Avoid taking samples at too shallow a depth, because if you take shallow samples and find little nitrogen remaining, it still won’t determine if there is available nitrogen within the root zone.”
Denitrification usually occurs where water sets for a period of time and soil temperatures are warm. Research at the University of Nebraska estimated that nitrogen loss occurs from denitrification was 25 percent when soils were saturated for 10 days and when soil temperatures were between 55-60 degrees.
“That loss increased to almost 95 percent when temperatures were between 75-80 degrees. Our soil temperatures are about 65 degrees,” said Gelderman. “Therefore we could extrapolate that a loss of 30-35 percent is possible if the ground is saturated for 10 days.”
Applying supplemental nitrogen as a side-dress should be done before corn reaches the V6 to V8 stages, Gelderman said, especially if producers had not yet applied nitrogen or if their sampling indicates low levels of soil nitrate-N remaining.
“The safest application method is to inject either dry or liquid nitrogen into the soil to protect the urea from loss,” Gelderman said. “If soil conditions won’t allow this, producers should apply liquid nitrogen with drop nozzles or hoses to avoid leaf burn and also to limit urea nitrogen loss.”
Source: AgBio Communications Unit at SDSU
Posted by Kaci Switzer