Many Kansas farmers are getting concerned about yellow, off-colored corn, potential N loss, and possible need to apply additional sidedress N. In the May 15, 2015 issue of the Agronomy eUpdate we addressed the issues surrounding N loss from leaching and denitrification. See: https://webapp.agron.ksu.edu/agr_social/eu_article.throck?article_id=559
With 6 to 10 inches of rainfall in May and more predicted in many areas of Kansas, the questions now become: How can I determine if my corn actually needs additional N? Should I be considering sidedressing additional N as soon as it dries up? Should I call in a plane and fly on urea? If I do need more N, how can I determine how much is actually needed?
These are very important questions, and ones that are difficult to answer with certainty. But there are some tools available to provide some helpful information and guidance.
Probably the most important place to start is with how much N was applied early to the crop. If half or more of the intended N rate was applied early, there is probably enough N present in the soil in most fields to provide the N needed to set a reasonable sized ear. In these cases, we probably have some time to develop and execute a good plan. But if no N -- or very little N as starter only -- was applied, there will be more pressure to get N on quickly to ensure decent ear size. This may mean flying on N or some other means to get N into the plant.
The amount of N applied early is important because corn determines ear size and yield potential around the s6-leaf growth stage. If corn is under significant N stress at this point, it will set a small ear with a limited number of kernel rows. Even with adequate N later, yield potential will be low.
Assuming you have a little time to develop a plan, what are some of the tools you might consider using to figure out how much N you need to think about adding. The following are some of the things to consider.
Soil testing. Traditional profile nitrate-N soil tests are probably not the most reliable tool for assessing soil N supply at this time. Soil nitrate-N samples collected from growing crops will generally indicate less available N than is actually likely to be available. This is because some of your fertilizer N has been incorporated into an easily mineralizable N pool (temporary immobilization, incorporation in root mass, etc.) that is not reflected in the soil nitrate-N test. The test also doesn’t consider the ammonium-N, which is present from mineralization and from ammonium in fertilizer which has not yet been nitrified (and potentially denitrified). An additional problem is sampling, especially in fields where ammonia was applied. How frequently are you going to find those ammonia bands? So while a profile-N test is great for use in late fall or winter, prior to planting, it might not be the best tool now. It might provide some information, but will it be useful information?
A second type of soil test which has never been widely used in Kansas is the Pre-Sidedress nitrate soil test. This test was developed in New England and has been successfully used in areas where high soil organic matter or regular applications of animal manure are routinely applied. This would be a tool to consider for corn where chicken litter has been used, or other organic N sources have been applied. A number of states, such as Wisconsin, Michigan, Iowa and Indiana, have correlated and calibrated the test. But one reason it has never gained wide use is it can tell you if you need N, but it doesn’t tell you accurately how much N to apply. It can provide an estimate of N need, but is not as accurate as we would like. However, I would consider it strongly for fields with a history of manure application.
Two publications that describe the test and provide fertilizer rate recommendations are:
“The Pre-Sidedress Soil Nitrate Test for Improving N Management in Corn,” AY 314-W from Purdue University: http://www.agry.purdue.edu/ext/pubs/ay-314-w.pdf
“Pre-Sidedress Soil Nitrate Test for Corn,” Agronomy Facts 17 from Pennsylvania State University: http://extension.psu.edu/plants/crops/grains/corn/nutrition/pre-sidedress-soil-nitrate-test-for-corn
Plant analysis. Plant analysis is another tool which I really like for diagnostics and routine monitoring, but not for this purpose. Again, we can do a good job of telling if the crop has enough N, but the calibrations and correlations have not been developed for this purpose. Plus there will be lag time between sampling and analysis -- and when it dries out, we’ll need to roll. So this may not be the best option.
Chlorophyll meters and crop sensors. My first choice to determine where we are at today is the use of a chlorophyll meter such as the Minolta Spad meter. This technology has been around for a long time, and used successfully in many areas for planned late-season N applications, such as fertigation. It also has been used in dryland areas where N loss is common, but it is pricey. A SPAD meter cost about $2,000. In more recent years, attention has shifted to crop sensors such as the Trimble Greenseeker, AgLeader Optrx, or the Holland Scientific Rapid Scan.
Crop sensors are more attractive as a tool for making N recommendations because the system can be automated and applied directly to the applicator vehicle for on-the-go precision applications. But our knowledge base and experience with sensors is not yet as great as we’d like, and the algorithms, or rate-calculating equations, probably not as strong. But I would utilize either a chlorophyll meter or a crop sensor if it were available to me.
A chlorophyll meter makes a measurement of leaf color. A person needs to walk or drive through the field, stop, clamp the instrument on a corn leaf, and take a reading. Since the color will vary across a field, one needs to take 20-30 individual readings and average those values to get a good estimate. Normally, the average from a series of readings is compared to a similar average reading made from a well-fertilized reference area. The reading from the target area you want to fertilize is then divided by the reading from the reference area. This value, or percentage of the reference, is commonly called a Sufficiency Index. An example calculation is given below:
Sufficiency Index = Average Spad Reading in Target area x 100
Average Spad Reading in Reference Strip
The sufficiency index is used to determine the needed additional N. The N recommendations based on the Sufficiency Index made by several states is summarized below.
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Table 1. N recommendations using a chlorophyll meter to determine a sufficiency index from several Midwestern states |
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Iowa |
Indiana |
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Sufficiency index |
N rate (lb/acre) to apply at 8- to 12-leaf corn |
Sufficiency index |
N rate (lb/acre) to apply at 8- to 12-leaf corn |
|
<88 |
100 |
<90 |
60 |
|
88-92 |
80 |
90-95 |
40 |
|
93-95 |
60 |
95+ |
0 |
|
95-97 |
30 |
|
|
|
>97 |
0 |
|
|
What is a reference area, and how can you establish one? A reference strip or area is simply a well-fertilized area where the corn is adequately supplied with N. Generally these areas are developed prior to or at planting by supplying 125% or so of the normally recommended N rate for the field. The areas don’t have to be large. Four rows, 50 feet long make a good reference strip for a chlorophyll meter or hand-held sensor. It is actually better to have multiple small reference strips scattered across a field than one big reference strip. The small strips can help you capture differences in soil texture and drainage, which will have an impact on nitrogen loss.
A simple way to establish a reference strip at this point in the season is to measure off an area in your field 4 rows wide (10-ft wide for corn in 30-inch rows) and 50 feet long. Mark the corners well with stakes or posts so you can find the areas as the corn grows, because you will want to come back to them. If you have already applied at least ¾ of the intended N to the field, weigh out and apply 2.5 pounds of urea to the reference area. This will be the equivalent of 100 additional pounds of N per acre. Now wait for the corn to take up the N and green up. This will likely take 2 weeks or longer. Don’t panic! If you had applied your normal N rate on the field you will still likely have enough N present to carry the crop though most of the vegetative growth stages with minimal yield reductions. But start taking your readings on a regular, perhaps weekly, basis.
What will the numbers tell you? If you had adequate N present and N loss was minimal, the difference between the target bulk field readings and the new reference strip readings may be small (a sufficiency index >95%). That’s good news. Additional N may not be needed, or a needed application will be small. If the difference develops quickly, and becomes large (<90%), N loss was severe and the needed rate will be fairly high. In that case, you should make the application as soon as possible.
In both cases, you should continue to make the readings and be prepared to make a second application later in the vegetative growth period. As corn grows, N is taken up and deficiencies are more likely to develop. So what appears okay today at 6-leaf corn may not be okay at 14-leaf corn. In Kansas we have a lot of experience with multiple application N management systems, especially with irrigated corn. We know we can get good responses to N applied in bands to the soil surface with a high-clearance ground rig prior to tassel. The only challenge is figuring how much N is needed.
Using a crop sensor is generally a similar process. Most of the rate calculators used by sensors require a reference strip, and are designed to use the difference in color and vegetation to make the recommendation. In general, the best window for use of most sensors is in the 8- to 12-leaf growth stage. K-State has algorithms you can download for free at the Agronomy Soil Testing Lab website for use with most sensors. The sensors can be used at later growth stages also, but variability can become an issue.
Some publications describing the use of chlorophyll meters are:
“Determining Nitrogen Fertilizer Sidedress Application Needs in Corn Using a Chlorophyll Meter” AY- 317-W from Purdue University: http://www.agry.purdue.edu/ext/pubs/ay-317-w.pdf
“Using a Chlorophyll Meter to Improve N Management” NebGuide G1632, University of Nebraska: http://ianrpubs.unl.edu/live/g1632/build/g1632.pdf
“In-Season Sensing for Nitrogen Stress in Corn” from Iowa State University, download online.
An excellent summary on using crop sensors is “Managing Nitrogen with Crop Sensors: Why and How” by Peter Scharf at the University of Missouri: http://plantsci.missouri.edu/nutrientmanagement/nitrogen/pdf/sensor_manual.pdf
Summary
While N loss through denitrification or leaching likely has occurred this year in parts of Kansas, not all of the N will have been lost even if the corn has N deficiency symptoms. As soils warm and dry, the root system will develop more extensively and N mineralization from organic sources will occur. The challenge will be making a reasonable estimate of the amount of N which will need to be added to ensure good yields. Several tools are available to help make that decision.
Dave Mengel, Soil Fertility Specialist
dmengel@ksu.edu
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