Wallaces Farmer

For Late Spring Soil Nitrate Test, the old rules don’t necessarily apply for this year.

Rod Swoboda

May 31, 2019

4 Min Read
young corn in field
DELAYED: With late planting and a cool spring slowing growth, collect soil samples for the LSNT in early June even if corn isn’t at recommended height.

Nitrogen fertilization for the 2019 Iowa corn crop has been complicated by frequent and sometimes excessive rainfall from late last fall through this spring. Many farmers who normally apply N in the fall couldn’t, and spring preplant N application and corn planting has been challenging in many areas with wet and extended cold soil conditions.

Thus, farmers are uncertain about the availability of N in the soil for corn. They also wonder about early postemergence assessment of the soil N supply, and the potential for using a supplemental sidedress N application.

Iowa State University Extension soil fertility specialists John Sawyer and Antonio Mallarino are fielding calls from farmers, fertilizer dealers and crop consultants regarding nitrogen management. The callers are wondering if they should use the Late Spring Soil Nitrate Test (LSNT) this spring since soil conditions are unusually cold and wet. Callers are asking how accurate the test is in these conditions.

New guide for soil nitrate test

The LSNT can be a useful tool for estimating crop N-availability in soils, say Sawyer and Mallarino. But they also say you may need to make some adjustments in a spring like 2019.

In 2017, a new ISU Extension publication, Use of the Late Spring Soil Nitrate Test in Iowa Corn Production, CROP 3140, replaced the previous publication, PM 1714. The new publication provides an overview of the LSNT, research on correlation and calibration, specific procedures for using the test, and interpretation of test results.

The basics and interpretation of the test are generally the same as the past, Sawyer says. Guidelines are now specific for interpretation in manured fields and corn following alfalfa, and an additional soil test category was added for those interpretations. Additional information was also added providing more detail on various aspects of the test, its reliability and precautions for use.

Adjustments for this spring

The calibrated LSNT sampling time — the time to sample soil for the test — is when corn is 6 to 12 inches tall (measured from the ground to the center of the whorl). That timing is usually late May to early June. This year, with the wet, cold conditions and delays in planting, many fields will likely not have corn reaching that height during late May through early June.

For this year, Sawyer and Mallarino recommend collecting LSNT soil samples in the first two weeks of June even if the corn is not the suggested height. This sample timing caveat is also described in the “time and depth of sampling” section of the new ISU publication CROP 3140.

The LSNT measures nitrate-N concentration in the top foot of soil, and the test result is used as a guide for making sidedress N application. “The measured N is a combination of nitrate residual from the prior year, mineralized N from soil organic matter, and any fall or early spring-applied N that has converted to nitrate,” Sawyer says.

Test reliability may vary

Due to the multiple sources of nitrate, and potential for nitrate leaching below the top foot of soil when rainfall is greater than normal, the test reliability may vary, Sawyer notes. This will be especially an issue this spring with the weather and timing of N application across Iowa.  

In general, research has shown that the LSNT is most reliable at predicting a lack of corn response to additional N at values above the 25 parts per million for the critical level, he says. In other words, if the LSNT shows that your soil has more than 25 ppm of nitrate nitrogen present, then you don’t need to apply more N fertilizer for this year’s corn crop. Reliability of the LSNT is lower for specifying the rate of N to apply when the test result indicates a potential deficiency (test results below 25 ppm).

Adjust soil test value

The transformation of soil organic N to nitrate may be delayed given the colder and wetter soils than usual, Sawyer says, and ammonium nitrogen applied close to sampling time (like recent anhydrous ammonia application) may not have all converted to nitrate.

The LSNT does not measure ammonium-N, and ammonium-N is not included because previous research has shown that in most conditions, ammonium measurement is not useful to assess and predict N supply for corn. Therefore, the reliability of the LSNT test is lower in these special conditions, and should be used with caution because an underestimation of future N supply for corn is likely.

Some areas of the state have had greater-than-normal rainfall, and generally soils have been colder than normal in Iowa this spring. Sawyer and Mallarino say you should consider using an adjusted soil test critical value of 20 to 22 ppm instead of 25 ppm, especially if you’ve had greater than 20% above normal precipitation between April 1 and time of sampling (for manured soils its greater than 5 inches of rain in May). The reason for this adjustment is described in the test results interpretation section of the CROP 3140 publication.

Resources available for nitrogen rate decisions include the following:




About the Author(s)

Rod Swoboda

Rod Swoboda is a former editor of Wallaces Farmer and is now retired.

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