The question has come in several times this year, probably due to the excessive rain, from folks wanting to know how much more nitrogen (N) to add. Recently the question came from a livestock operation.
For several years the use of the PSNT – Pre-Sidedress Nitrogen Test – has been recommended in manured or terminated legume situations. Ohio has done field test with PSNT and where soil test result exceed 25 ppm NO3-N additional N is unlikely to increase yield. Both Pennsylvania State University and Purdue University have taken the PSNT a step further providing guidelines to adjust N rates where the PSNT result does not exceed the no application threshold. We often point producers in those directions to learn more.
After a manure application or termination of a legume, nitrogen will become available for corn during the growing season. We can predict somewhat how much is available for the crop and recommend a reduction in added commercial fertilizer nitrogen.
- Sample 12 inches deep (or as deep as you can) in the field when corn is about 12 inches tall. Collect 15 to 25 cores per sample. More if there is a history of banding nutrients. Air dry the soil and send to a lab requesting the PSNT soil nitrate test.
To determine how much N to apply:
- From PennState University – if the soil nitrate test is above 21 ppm, then you would expect no economic benefit from added nitrogen.
- If the test shows less than 21 ppm NO3-N, then use this calculator to determine the need.
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1 |
Soil NO3-N level in ppm from PSNT |
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2 |
Yield expectations – in bu/A |
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3 |
Field history = The total of scores from three factors: Manure applied since last harvest + Previous crop + Manure history
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4 |
Recommendation = Yield – (PSNT x History score) |
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- For example: A recent PSNT test I was shown was 20ppm NO3-N, the yield last year was 165 bu/A, the field has a history of applying manure and is corn after corn – so the history score is 3.5 + 0 + 1.75 = 5.25. Our recommendation would be: N rec = 165 – (20 x 5.25) = 60 N/A
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- Purdue University suggests no additional N is required if the nitrate test is above 25ppm.
- Use a table to determine the level of N to add if the NO3-N level is below 25ppm. The table published with the AY-314-W factsheet was created prior to our current MRTN – Maximum Return To N – multistate developed corn N recommendation calculator (http://cnrc.agron.iastate.edu). The Corn Nitrogen Rate Calculator calculates the MRTN based on state, crop rotation, price of N and price of corn.
- A personal communication this past winter from Jim Camberato at Purdue suggested a modification to the factsheet table relying on the MRTN calculator.
|
Soil NO3-N ppm |
Adjustment to MRTN recommended N rate Lbs. N/A |
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0-10 |
Full rate |
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11-15 |
-30 |
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16-20 |
- 45 |
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21-25 |
- 90 |
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>25 |
No sidedress N |
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- For example: From the above grower with a 20 ppm soil nitrate PSNT test we use the 16-20 row and reduce our MRTN rate by 90 lbs. N/A. A quick run of the CNR Calculator suggests 174 lbs. N/A for Ohio at present – I subtract 45 and get a suggestion of 129 lbs. N/A. Yes this differs from the Pennsylvania recommendation but keep in mind that both are suggestions based on past history. One way to see how this year and your conditions may differ to provide a better corn nitrogen recommendation is to use a crop sensor.
Crop sensors
An alternative to using soil sampling is to use an in-crop sensor to determine corn nitrogen needs. Penn State suggests the use of a chlorophyll meter test; this would give you an idea of the nitrogen status of the crop based on the color. This requires a high-N reference strip although some suggest that with the variables of manure application perhaps (or perhaps not – Scharf in Missouri) you could use a virtual reference strip – meaning a place in the field where greater amounts of nitrogen are available. Using the chlorophyll meter you could determine the difference in N status between the low N and high N areas of the field.
Work done in Ohio, Oklahoma, Missouri and other areas suggest using an NDVI sensor to determine crop color. Work here since 2009 suggests that, yes, we can determine crop N needs with sensors such as the GreenSeeker and others. These would work in situations with manure or commercial fertilizer. Again as with the chlorophyll meter above we must have a high-N reference strip in the field. Wait until V8-V12 corn growth stage, compare the low and high N areas of the field and then use a calculator to determine any additional crop need for sidedress N. From work I am doing this year waiting until the V12 stage is a better indication of N status than the V8 growth stage, unless you had saturated soils and lost N due to denitrification. The calculator for Ohio resides on the Oklahoma State University Soil, Water and Forage Analytical Laboratory website, noted below.
Resources
From Pennsylvania State University: http://extension.psu.edu/plants/crops/grains/corn/nutrition/pre-sidedress-soil-nitrate-test-for-corn.
From Purdue University: https://www.agry.purdue.edu/ext/pubs/AY-314-W.pdf.
From the University of Missouri: https://plantsciences.missouri.edu/nutrientmanagement/nitrogen/pdf/sensor_manual.pdf.
From Oklahoma State University: http://soiltesting.okstate.edu/sensor-based-n-rate-calculator.