When looking at nitrogen and nitrogen loss, it is important to first have a good understanding of the nitrogen cycle. See our library article Nitrogen Basics before reading any further in this article. This way the reader will have a basic understanding of how nitrogen acts in the soil and can better determine what form the nitrogen may be in to make a better estimation on possible nitrogen losses.
Several Universities have developed score cards and other tools to evaluate nitrogen losses under heavy rainfall and losses due to denitrification. We have gathered information from three different sources from different years. With this information you should be able to make a better decision on the amount of additional nitrogen to apply if you desire.
The professors at Purdue University put together a calculator using the results of the PSNT (presidedress nitrogen test) taking into account the nitrate nitrogen as well the ammonium nitrogen result. This is based on a 12” deep soil sample. Basically the researchers have tried to estimate the amount of ammonium and nitrogen that should be in the soil under normal conditions, using this information you can make an estimated calculation of how much additional nitrogen should be applied. The following information comes from “Soil Sampling for Assessing Plant Available N in Previously Flooded Soils” by Jim Camberato and P.L. Nielsen. The recommendations are from several sources and complied in this publication.
If preplant or other band application methods of nitrogen have been applied, the taking of the soil sample is going to be more important than taking a regular soil sample in order to try to avoid the band. The band is going to create an area with a high concentration that can distort the results. Not coming into the contact with this band is going to be difficult unless it is a no til situation or one that the samplers can pinpoint the exact banded location. Since in most cases this is going to be impossible, Purdue developed a sampling method to help in the dilution of this hot zone if it is hit by one of the probes. The sampler needs to pull 3 cores from the area, 2 that are 3 inches from each corn row and then another in the middle of the corn row. See the figure below.
Recommended soil sampling pattern in relation to two corn rows when N fertilizer has been banded with the row. Always sample perpendicular to the direction fertilizer was applied. (Source of image: Brouder & Mengel, 2003).
The accompanying table contains estimates of expected soil NO3-N levels with different fertilizer rates assuming “normal” background levels of nitrate and ammonium at the time of fertilization and a “normal” amount of movement below the one foot sampling depth (approximately 1/3 of the fertilizer N is moved below the 1-foot sampling depth but retained within the root zone with normal rainfall). If the corn is healthy and the growing season typical from here on out, we would suggest applying no more than 10 pounds of N for every 2 ppm (This recommendation comes from Purdue University) reduction in soil sample N below the expected levels listed in the table.
|Expected levels of nitrate (NO3-N) and NO3-N plus ammonium (NH4-N) in the upper 1-foot of soil.
NOTE: Use the NO3-N column if this is the only form of N measured in your soil sample.
Add NO3-N and NH4-N levels together if both forms of N are measured in the soil sample and use
the last column to assess N supply
|Fertilizer N applied prior to rains||Nitrogen analysis|
|NO3-N 1||NO3-N + NH4-N 2|
|lb/acre||Expected N levels, ppm or mg/L N|
|1 Assumes background level of ammonium at 6 ppm and “normal” levels of soil N below the 1-foot sampling depth|
|2 Assumes “normal” levels of soil N below the 1-foot sampling depth|
|Source: Camberato and Nielsen, 2008|
Fertilizer N was applied at 160 pounds of N per acre in April as 28% UAN in southern Indiana. Only soil NO3-N analysis was requested because it was assumed that most of the urea- and ammonium-N had been converted to nitrate since temperatures were warm for many weeks prior to the late May/early June rain. The expected NO3-N level from the table below for a 160-lb N application is 35 ppm. Laboratory results indicated only 20 ppm NO3-N. The suggested N application rate would be: ((35 ppm – 20 ppm) / 2) × 10 = (15 ppm / 2) × 10 = 7.5 × 10 = 75 pounds per acre.
Anhydrous ammonia was applied at 160 pounds of N per acre in mid May in northern Indiana. Since temperatures were warm for only 2 weeks prior to the late May/early June rain, both NO3-N and NH4-N analysis of soil samples were requested. The expected NO3-N plus NH4-N levels listed in the table for a 160-lb N application is 41 ppm. Laboratory results indicated 15 ppm NO3-N and 20 ppm NH4-N for a total measured N level of 35 ppm. The suggested N application rate would be: ((41 ppm – 35 ppm) / 2) × 10 = (6 ppm / 2) x 10 = 3 × 10 = 30 pounds per acre.
(published 2004, may need slight adjustments based on current year)
|Fertilizer Management Factor|
|Nitrogen Source||Date Applied||Score|
|Anhydrous Ammonia||before November 1||5|
|Anhydrous Ammonia||Nov 1 to Dec 31||4|
|Anhydrous Ammonia||Jan 1 to Feb 28||3|
|Anhydrous Ammonia||March 1 to 31||2|
|Anhydrous Ammonia||April 1 to 30||1|
|Anhydrous Ammonia||May 1 or later||0|
|Urea||<4 days before excess water||4|
|Urea||4 to 14 days before excess water||3|
|Urea||>14 days before excess water||5|
|Ammoniun Nitrate or UAN solution||before April 1||5|
|Ammoniun Nitrate or UAN solution||April 1 to 30||4|
|Ammoniun Nitrate or UAN solution||May 1 or later||3|
|Note: This table has been may need to modified for the current weather patterns and dates to fit the current conditions.|
|Surface soil texture||Score|
|Flooded 6 or more days||5|
|Flooded 3 to 5 days||4|
|Flooded 1 to 2 days||3|
|Never flooded but saturated 3 or more days||2|
|Never flooded but saturated 1 to 2 days||1|
|Never saturated but sandy and excess rainfall||1|
|Never saturated and not sandy||0|
Nitrogen loss score = fertilizer management factor × soil factor × wetness factor
Reference: Nitrogen Loss Scoresheet 08
It is important to remember that nitrogen is the one of the most dynamic elements in crop production, there have been several generations of scientists studying nitrogen and trying to improve testing methods and recommendations for crop production. Soil for presidedress nitrogen of testing has been done for many years, but there is limited calibration on the use of the results and how much additional nitrogen to apply. The original test was to be used on fields that had previous manure applications with the soil sample being taken to a 12 inch depth when the corn is 10 to 12 inches in height. Here is a link to our guide for the PSNT test. Keep this in mind when you are talking with your growers and making recommendations on applications of additional nitrogen.