Bill Jokela, formally Extension Associate Professor,
Department of Plant and Soil Science, University of Vermont
How
much nitrogen fertilizer does a corn crop need? That’s a difficult question
to answer because every situation is different. Use the Vermont Pre-Sidedress
Nitrate Test (PSNT) to answer that question for your farm. The PSNT recommends
the proper amount of fertilizer N for each corn field on the farm…and can
save you money.
You may be applying too much or too little nitrogen
on different fields. Applying too much wastes money on unnecessary fertilizer
and increases the possibility of environmental pollution by nitrate leaching.
Applying too little can reduce yields. The PSNT is a tool to get it right.
The Pre-Sidedress Nitrate Test started in Vermont, but it has been adopted
by most states in the Northeast and many in the Midwest.
Many Sources
of N for Corn
Corn is a big user of nitrogen. A corn silage crop
takes up 150 or more pounds of N per acre depending on yield. Much of this
can be supplied from nonfertilizer sources – manure, plowed down crop residues,
and soil organic matter. Nitrogen from these sources can reduce or even
eliminate the need for N fertilizer.
However, it is difficult to predict how much N will be supplied to plants by manure and other soil sources. This is because much of the N is in the organic form, a form not directly available to plants. Microorganisms in the soil must break down the organic N to ammonium or nitrate, simpler forms that plants can take up. Also, some of the nitrogen can be lost by leaching or by microbial conversion to N gas, a process known as denitrification.
To add to the complexity, all these processes vary greatly depending on rainfall, temperature, soil moisture, and other conditions. So nitrogen availability can be quite different in different fields or in the same field in different years.
That is why the PSNT is such a useful tool. It measures nitrate, the main form of nitrogen taken up by plants, shortly before the big demand for N by the corn crop in midsummer. By this time, much of the N from manure and other sources has been broken down and can be measured in the soil as nitrate. The amount of nitrate at that time also serves as an indicator of the potential for N release during the rest of the season. If the PSNT calls for additional N fertilizer, you can apply it as a sidedress or topdress application. This is the best time to apply N fertilizer on most soils in Vermont to maximize the efficiency of uptake by corn and to minimize losses of N.
PSNT
Based on Vermont Research
The Pre-Sidedress Nitrate Test was developed by
Fred Magdoff of the Plant and Soil Science Department at UVM. Results
of research from Vermont (Magdoff and Jokela) and four other northeastern
states (PA, CT, NH, and NY) are summarized in the figure. It includes data
from over 270 field sites, 150 from VT, covering a range of management
situations (manure, previous crop, tillage, etc.). At each field site,
a soil sample was taken for PSNT and the yield response to N fertilizer
was measured. In general, where the PSNT was low, especially below 15 ppm,
corn silage yields increased substantially from the addition of fertilizer
N. On the other hand, fields testing above the threshold of 25 ppm (no
fertilizer recommended) showed little or no increase from applying N.
As with any test or recommendation, there are some
limitations. The main one is that the PSNT cannot account for unexpected
changes in conditions after soil sampling. For example, if unusually heavy
rains occur a few days after sampling, significant nitrate can be lost
to leaching or denitrification. If this occurs, and if there is time, resample
the field. If not, adjust the recommended N rate based on best judgment
to account for suspected losses.
How to Use
the PSNT
Sampling for the PSNT is a little different than
routine soil sampling. Here’s what you need to do. Sample fields when corn
plants are 8 to 12 inches tall. Take at least 15 to 20 cores per field
to a depth of 12 inches – deeper than for the standard soil test. Avoid
sampling the starter fertilizer band near the row. Dry samples as soon
as possible to prevent changes in nitrate level from microbial activity,
unless samples can be delivered to the lab the same day. If drying will
be delayed more than a few hours, keep samples in a cooler. Once the lab
receives the samples, analysis will be done within one business day. Results
will be mailed or can be accessed via the Web. Check instructions in test
kit for more detail about sample handling and drying procedures.
To obtain PSNT kits (sample bag, form, and instructions),
contact the UVM
Agricultural and Environmental Testing Lab at 802-656-3030 or 800-244-6402
or your county UVM Extension office. For specific recommendations and more
information, check the extension publication Nutrient
Recommendations for Field Crops in Vermont.
This site is maintained by Sid.Bosworth@uvm.edu, Plant & Soil Science Department, University of Vermont.
Sponsored by:
and 
Issued in furtherance of Cooperative Extension work, Acts of May 8 and June 30, 1914, in cooperation with the United States Department of Agriculture. University of Vermont Extension, Burlington, Vermont.University of Vermont Extension and U.S. Department of Agriculture, cooperating, offer education and employment to everyone without regard to race, color, national origin, sex, religion, age, disability, political beliefs, or marital or familial status
Last modified March 20 2008 11:02 AM