Tests for N
As detailed on the N cycle page, nitrogen can be temporarily locked in soil organic matter, available for plant uptake as nitrate (NO3-), and/or lost from the root zone via leaching or denitrification depending on many factors, including the time of year, supply of N, weather, and stage in the crop rotation to name a few. Tests to determine whether an adequate supply of N exists for corn production are critical for efficient use of N. The following provides an overview of the many tests available for N management in corn.
Pre-Sidedress Nitrate Test
General Description
The Pre-Sidedress Nitrate Test (PSNT) is a soil test for nitrate based on 12-inch deep soil cores when the corn is 6-12 inches tall. The test results indicate the likeliness to a response to additional N applications assuming that the nitrate that is measured was derived from the mineralization and nitrification of organic N. Use of this test can result in significant reductions of fertilizer N use on soils with high organic N contents either as a result of high soil organic matter, manure application and/or the plow down of a sod or cover crop within the past couple of years.
Results are used to see if a field will be responsive to sidedressed N or not. Most research has shown that when the soil nitrate-N level at PSNT time is above 25 ppm there is little chance of an economic response to adding additional N to the field. At soil nitrate-N levels below 21 ppm, sidedress N will be required to achieve optimum economic yield. Between 21 and 25, interpretations differ by state. Some states make fertilizer rate recommendations based on the PSNT while others use the PSNT to identify whether or not sidedress N is needed for optimum economic growth.
PSNT Sampling and Measurement Procedures
The basic approach is to determine the nitrate-N available in the soil from soil samples taken just before sidedressing; a timing that falls after the spring wet period but before the period of major N demand by corn. Thus, samples are taken over a depth of 12 inches, when corn is 6-12 inches tall . The average recommendation is 10 to 20 cores to a 12-inch depth. The combined samples are crumbled and thoroughly mixed before a sub-sample is taken. The sub-sample is immediately air-dried or frozen and then mailed to a soil-testing laboratory. Samples need to be dried or frozen because microbial activity can rapidly change the concentration of nitrate in soil samples at ambient temperatures and moist conditions. At the laboratory, samples are oven dried if needed, ground and then analyzed for nitrate.
Interpretations of PSNT Results in the Northeast United States and Ontario
Many states in the Northeast US and Ontario, Canada, offer guidelines for the PSNT. The table, below, presents the N recommendation for a crop with a yield goal of 20 tons of silage corn (35% dry matter) across the range of states.
Comparison of PSNT interpretations for NE USA states and Ontario.
State | PSNT beyond which fertilizer rates go to zero | Are rates based on PSNT (versus it being used as trigger only) | Rate for 20 ton silage crop if PSNT = 2 | Rate for 20 ton silage crop if PSNT= 10 | Any adjustments? |
New York |
25 | No | 110* | 110* | Yes, sod, organic matter, N uptake efficiency and manure corrections |
Pennsylvania |
21 | Yes | No, adjustments | ||
Delaware |
25 | Yes | 100 | 100 | No, adjustments |
Vermont |
26 | Yes | 110 | 90 | No, adjustments |
New Hampshire |
25 | Yes | 107 | 63 | No, adjustments |
Ontario |
25 | Yes | 223 | 151 | No, adjustments |
* This assumes fourth year corn (no past sod credits), a soil N supply of 70 lbs N and N uptake efficiency of 75%.
For additional information on the PSNT, consider the following fact sheets.
Pre-sidedress Nitrate Test for Corn - Cornell University.
Pre-sidedress Soil Nitrate Test for Corn - Penn State University.
End of Season Stalk Nitrate Test
Research in Connecticut concluded that the end of season stalk nitrate test was a powerful management tool that enabled corn producers to distinguish between optimal and excess applications of N. The corn stalk nitrate test was developed by scientists at Iowa State University as a tool to assess if excess nitrogen was available to the crop, especially late in the season. In the NE, Connecticut is the only state that recommends the use of this test (Hooker and Morris, 1999).
For corn silage, Connecticut recommends to collect stalk sections from one week before corn chopping until one day after harvest. The easiest method to collect a sample is to walk or drive across the field up to 24 hours after harvest and cut the stalk sections from the stubble. If stubble height is normally less than 14 inches tall, it is recommended to raise the corn head for about 50 feet in a few places in the field and to collect stalk sections from the stubble in those areas. For grain corn, the time for sampling is between one and three weeks after about 80% of the kernels of most ears have reached black layer.
To perform the end of season nitrate stalk test, an 8-inch segment of stalk found between 6 and 14 inches above the soil is taken. Leaf sheaths should be removed from the segments and stalks severely damaged by disease or insects should not be used. It is extremely important to cut the stalks at the 6- and 14-inch height above the ground. Cutting lower or higher may result in incorrect assessment of nitrogen availability: consistently cutting lower than 6 inches will result in higher than expected nitrate stalk concentrations and consistently cutting above 14 inches will result in lower than expected stalk nitrate concentrations.
Connecticut recommends that fifteen 8-inch segments be collected to form a single sample to be sent for analysis. These should be collected at random within an area not larger than 15 acres. Areas differing in soil types or management histories should be sampled separately. An easy method for collecting a sample is to collect 3 to 5 stalk segments from each of several small areas (less than an acre) that seem to be representative of larger areas within a field.
The Soil Testing Laboratory at the University of Connecticut tests the stalks for nitrate. For the best results, samples should be placed in cloth (not plastic) bags to enable some drying and minimize growth of mold. The test results can be incorrect if the samples are stored for more than one day before shipment to the lab.
Stalk nitrate concentrations can be divided into three general categories: LOW, OPTIMAL, and EXCESS. The LOW range indicates high probability that greater availability of N would have resulted in higher yields. Concentrations in this range apparently give little indication of the magnitude of yield increase that might be expected from more available N. Visual signs of N deficiency usually are present when nitrate concentrations are in this range. The OPTIMAL range indicates high probability that N availability was within the range needed to maximize profits for the producer. The EXCESS range indicates high probability that N availability was greater than when N was applied at rates that maximize profits for producers. The stalk nitrate concentrations for each of the three ranges are different depending on whether the stalks are collected from a field harvested for silage or for grain. The nitrate concentration ranges for are listed in the table, below.
University of Connecticut interpretation of the end of season stalk nitrate test1.
End of Season Stalk Nitrate Test (ppm nitrate) | |||
Low | Optimum | Excess | |
Grain |
<500 | 500-1000 | >1000 |
Silage |
<700 | 700-2000 | >2000 |
1. Hooker and Morris, 1999.
The concentration of nitrate in the stalk at the end of the season reflects all factors that influenced N availability and N needs during the growing season. Because many of the factors influence N availability after fertilizers are applied, it is unrealistic to expect any producer to attain OPTIMAL concentrations in all fields in all years. Indeed, experience has shown that the OPTIMAL range is difficult to consistently attain with existing management practices.
When interpreting the test, considerations must be given to weather conditions that occurred during the growing season. Rates of fertilization that have been most profitable for many years should be expected to result in LOW concentrations in some years and excess concentrations in other years. Lower than desired concentrations should be expected in years having unusually large amounts of in-season rainfall.
Fertilization rates should be increased for areas that usually test in the LOW range and decreased on areas that usually test in EXCESS range. The test does not directly indicate by how much N rates should be increased or decreased, but continued use the test of several years enables producers to make adjustments toward optimal rates. Concentrations in the EXCESS range indicate that use of the PSNT to guide N fertilization will probably increase profits of the producer.
Leaf Chlorophyll Meter for Nitrogen Management of Corn
Leaf chlorophyll meters offer a very rapid means to quantify leaf “greenness” in corn and have been shown in research from several Midwestern and Northeastern states to be well correlated with leaf nitrogen content. The chlorophyll meters are normally used to test for N sufficiency just before sidedressing time. If meter readings are below desired values, additional N should then be applied by sidedressing. Thus, this approach uses the chlorophyll meter in much the same manner as the PSNT to identify those fields that have sufficient N, primarily from manure applications, and thus need no fertilizer N, or considerably reduced rates of sidedress N to meet realistic yield goals.
The sampling period of the leaf chlorophyll test is the same as the PSNT test samples. The measurements need to be taken in time to apply sidedress N. Thus, readings should be taken when corn plants are 12-inch tall. A nitrogen sufficient reference area is required in each field where the test is used, in order to calibrate the chlorophyll meter to the particular field and corn hybrid. Prior to silking, the recommendation is to sample the uppermost-expanded leaf (top leaf with a collar that has emerged from the whorl). At and after silking, the ear leaf should be sampled. Reading should be done about midway between the leaf stalk and the leaf tip and halfway between the leaf edge and midrib. Most chlorophyll meters can store ~30 readings in their memory bank and compute an average value from these individual results. It is recommended to take 30 readings from both the field and reference area and use the average “relative chlorophyll reading”.
The test cannot be affected by luxury consumption; a plant will only produce as much chlorophyll as it needs regardless of how much available N is in the plant and corn producers can take readings as often as they would like. Results are very quickly obtained. By using the meter early in the season to monitor the leaf greenness, a farmer could signal the approach of potential N deficiency early enough to correct it without risking a reduction in yield. However, measuring when the corn leaves are wet gives an incorrect result and measurements should always be done during the same time of day or else the results aren’t accurate. Readings are very field specific, so a calibration area fertilized for N sufficiency is required in every field.
For more information about the Leaf Chlorophyll Meter Test, consider the following fact sheet.
The Early-Season Chlorophyll Meter Test for Corn - Penn State University
The Illinois Soil Nitrogen Test
Dr. Richard L. Mulvaney and Dr. Saheed A. Kahn developed the Illinois Soil Nitrogen Test, at the University of Illinois at Urbana Champaign. They noticed that the PSNT didn’t always detect non-responsive soils and felt that the ideal test for soil N would test a readily mineralizable organic fraction (amino sugars) of the soil N pool (Khan et al., 2001). This fraction of soil N would be much less variable in time and space than soil nitrate, yet still be an accurate indicator of plant available N in the soil. Through 10-15 years of research they developed a test that showed promise as a replacement for the PSNT and was both simple and inexpensive to use. Mulvaney’s team found that Illinois Soil N Test values correlated very well with plant uptake, and that it was accurate in predicting site responsiveness to N. At the University of Illinois, the new test was shown to be less susceptible to variation due to time and depth of sampling than the PSNT. The Illinois Soil Nitrogen Test is potentially an easier and more accurate test to use then the PSNT, as soil tests don’t need to be taken in the growing season and they can be taken at a shallower depth (8 inches) than PSNT (12 inches).
At Cornell University , the method is being tested on New York soils to determine if the test will be effective in New York . For more information about research on the Illinois Soil Nitrogen Test for use in New York , visit the Nutrient Management Spear Program site (http://nmsp.css.cornell.edu/projects/aminosugartest.asp).