The amount of fertilizer-nitrogen to be applied in-season can be estimated with the Late Spring Soil Nitrate Test, crop canopy reflectance sensing, or by use of crop model N tools, combined with fertigation.
Late Spring Soil Nitrate Test
LSNT has been available for more than 30 years and is used for corn production to assess the need for in-season N application. It has been less studied and used in Nebraska than in Iowa, and we advise the use of the Iowa State University guidelines. LSNT has been well-validated for medium and fine texture soils, but it is not expected to work well for sandy soil. Use this test as follows:
Collect a representative soil sample from the 0- to 12-inch depth when the height from the ground to the top of the corn plant whorl is 6 to 12 inches. The area represented by a sample should not be more than 40 acres with sampling zones defined according to soil properties likely to affect N availability or loss.
Each sample should be made from at least 15 cores, and more in cases of past manure injection. Avoid sampling in bands of fertilizer-N application. Samples should be collected at varying distances from corn rows. For example, three samples of varying distance from the row might be sampled at five sites for the management zone.
Refrigerate the samples or air-dry them in a thin layer on sheets of paper, or with the assistance of a fan. Alternatively, submit the sample so the analysis can be done within three days.
The laboratory analysis needs to be for nitrate-N only. The critical value for 2021 spring with slightly above-normal precipitation in Nebraska is 25 parts per million nitrate-N.
If nitrate-N is above the critical level, for example 27 ppm in 2021, do not apply in-season fertilizer-N. If nitrate-N is below the critical level, apply 8 pounds of N for each ppm below the critical level.
For example, if LSNT results are 15 ppm nitrate-N, the N rate = (25 − 15) x 8 = 80 pounds per acre N.
See the ISU publication “Use of the Late-Spring Soil Nitrate Test in Iowa Corn Production” for more discussion of LSNT considerations, such as for fields with manure application.
Sensor-guided in-season N application
Remote sensing of the crop canopy reflectance is the best option to quantify the need for in-season N if the plants are large enough. Remote sensing can be:
- with a hand-held sensor such as the made-in-Lincoln Rapid Scan
- with aerial sensing from drones, planes and satellites
- with sensors fitted on high-clearance N application equipment
Such remote sensing requires good canopy development such as the eighth leaf stage (V8; or with 10 horizontal or droopy leaves) or later. Remote sensing is best done with a reflectance index such as NDVI (normalized difference vegetative index). However, for corn which produces much leaf area, the NDRE index (normalized difference red edge) is preferred.
The crop N status for any given part of the field is determined by relating the NDRE for that part of the field with high NDRE readings from the field. The high NDRE readings are often from established “High N Reference” areas or strips in the field.
These areas can be small, such as 20-by-20 feet, with hand application of extra fertilizer-N, for example, at a rate of 1 pound of urea per 100 square feet. Data from sensor readings for other parts of the field are then compared to the high N reference with the ratio of the sensor reading from the field divided by the sensor reading from the high N reference equal to a “sufficiency index.” This sufficiency index (SI) equals target area NDRE divided by reference NDRE.
Mathematical algorithms developed for corn in Nebraska are used to convert the SI value to an N rate. The Solari algorithm is relatively simple and requires only the SI value: in-season N rate = 317 x the square root (0.97 – SI).
The Holland-Schepers algorithm requires additional information and assumptions. It determines the N rate from the shape of a typical yield response function and the optimum N rate for the yield goal established by the producer.
Producers provide this “optimum N rate” unless the economic optimum N rate is available. The Holland-Schepers algorithm also accounts for N credits such as due to the previous crop, manure application and nitrate-N applied in irrigation water.
The sensor-directed in-season N application is commonly done near the 12-leaf stage (V12 or with 13.5 horizontal or droopy leaves) of corn to correspond to a high rate of N uptake. The algorithms for calculation of in-season N rate are best calibrated for this growth stage. Early use is more likely to underestimate N need.
Crop model-based N tools
Crop model-based tools such as Maize-N, Adapt-N, Climate FieldView, Granular, FarmersEdge N-Manager and more are now widely available and being increasingly used for N management decision-making. Several of these models were developed at universities or in collaboration with universities, and are particularly well suited for in-season N applications.
Crop model-based tools have the capacity to assess the N losses of early-applied N fertilizers, and whether there is a need to compensate for these losses with an additional sidedress N application. These models take into account real-time, high-resolution rainfall and temperature data, management and soil characteristics for a location.
The success of these models depends on having accurate inputs, including soil characteristics, previous N management practices and being able to interpret the outputs correctly. Because of this, crop model-based N services in some cases may only be available through trained professionals that can help ensure data inputs and interpretation of the outputs.
The Nebraska On-Farm Research Network through the Precision Nitrogen Management On-Farm Research Project can support growers to implement and test these tools via on-farm experiments. Experimenting with these and other N management tools is an important part of an adaptive management process, through which we can collectively build greater confidence in our N management decision-making.
Fertigation is a common and cost-effective means of in-season N fertilization in Nebraska. As above, the use of LSNT, spot-checking of a field with a hand-held sensor, or the use of crop models can help determine if N should be applied by fertigation. If needed, 30 to 40 pounds per acre N can be uniformly applied.
The N need can be reassessed two weeks later using crop model or sensor information to determine whether an additional application of 30 to 40 pounds per acre N is needed. This procedure can be repeated with the last application no later than the R3 (milk) growth stage.
Iqbal is a Nebraska Extension nutrient management and water quality specialist. Puntel is a Nebraska Extension soil fertility and precision ag specialist. Maharjan and Wortmann are both Nebraska Extension soil and nutrient management specialists.