Q1: How Can I Reduce My N Expense This Year?

Most of Nebraska was drier than normal in 2020 with the less downward movement of soil water, nitrate leaching loss, crop N uptake and crop yield, particularly for rainfed crops. The residual nitrate-N from 2020 may be more than normal giving the potential to reduce fertilizer-N rates for 2021. To determine the residual nitrate-N credit, sample soil at 2- or 4-foot depth this spring and plug this credit into the equation for determining the N rate. Calculate the N rate according to Nutrient Management Suggestions for Corn or Nutrient Management for Agronomic Crops in Nebraska EC 117 or using the Corn Nitrogen Calculator.

Q2: How Can I Reduce N Loss From My Farm Fields?

There is potential for N loss due to ammonia volatilization when urea-N — from dry urea or liquid UAN — is not injected or incorporated and more than a ½-inch of precipitation or irrigation does not occur within a few days following application. The urea-based fertilizers can be treated with a urease inhibitor to delay the conversion of urea to ammonium giving more time for a rainfall or irrigation event to occur.

Typically, most downward movement of nitrate-N occurs in May and early June when there is much soil water, fertilizer-N has been applied, and the crop is not yet using much water or N. Splitting N application to <80 lb/ac pre-plant and the rest applied in-season can reduce N losses, especially for sandy loam and sandier soils.

Q3: What is a Better N Investment Strategy: In Season or Pre-Plant N?

Timing of fertilizer N application can be critical to crop yield, N use efficiency, economic returns and N losses to the environment. Less N use efficiency due to more N losses can occur if all N is pre-plant applied when there is little crop uptake of water and N. Since spring precipitation cannot be predicted, there is a high potential for N loss with heavy precipitation during late spring or early summer combined with all pre-plant N application.

Another disadvantage of applying all N pre-plant is that we can’t adjust for differences in actual nitrogen availability and changes in yield potential. Crop response to fertilizer-N can differ greatly in one year compared with other years for the same field. Variability in yield response to N can increase with variation in manure-N availability, cover crop effects on N dynamics and N immobilization by crop residue. To account for this variability and reduce N losses, consider applying some fertilizer-N in season to better determine N need and to synchronize N supply with N demand.

Q4: Which Tools Can I Use to Make In-Season N Management Decisions?

The amount of fertilizer-N to be applied in-season can be estimated with the Late Spring Soil Nitrate Test (LSNT, also called the Pre-sidedress Soil Nitrate Test or PSNT), crop canopy reflectance sensing, or by use of crop model N tools, combined with fertigation. These tools are briefly explained below.

Late Spring Soil Nitrate Test

The LSNT has been available for over 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: Use of the Late-Spring Soil Nitrate Test (LSNT) in Iowa Corn Production (an equivalent of LSNT is the pre-sidedress nitrate test). The 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.

1. Collect a representative soil sample from the 0-12 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.
2. 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.
3. The laboratory analysis needs to be for nitrate-N only.
4. The critical value for 2021 spring with slightly above normal precipitation in Nebraska is 25 ppm 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 lb 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 lb/ac 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 handheld sensor such as the made-in-Lincoln Rapid Scan
• with aerial sensing (drones, planes, satellite)
• with sensors fitted on high-clearance N application equipment

Such remote sensing requires good canopy development such as the 8^th leaf stage (V8; or with 10 horizontal/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 20x20 feet, with hand application of extra fertilizer-N, for example at a rate of 1 lb of urea per 100 sq ft. 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.”

Sufficiency Index (SI) = (target area NDRE / reference NDRE)

Mathematical algorithms developed for corn in Nebraska are used to convert the SI value to an N rate.

1. The Solari algorithm is relatively simple and requires only the SI value:

In-season N rate = 317 x the square root (0.97 – SI).

1. 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 assure 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

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 handheld sensor, or the use of crop models can help determine if N should be applied by fertigation. If needed, 30 to 40 lb/ac 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 lb/ac N is needed. This procedure can be repeated with the last application no later than the R3 (milk) growth stage.