The corn has been growing and, in spite of a brief cold spell, is about to begin its rapid growth phase and peak in its demand for nitrogen! A high yielding corn silage crop can easily require 150 pounds of nitrogen per acre. Much of the required nitrogen will come from manure applications, crop residues, and nitrogen tied to soil organic matter. These sources of nitrogen are in the organic form and are not readily available to plants as they first need to be broken down by microorganisms before a plant can use them. The exact amount of nitrogen that these organic sources of fertilizer will provide varies based on moisture, temperature, and substrate quality.
To help determine how much nitrogen from these sources are available to the corn crop, we recommend taking a pre-sidedress nitrate test (PSNT). This will help you determine the right amount of nitrogen to apply–for best yields, to prevent wasting money on unnecessary fertilizer, and to prevent leaching of excess fertilizer.
What is the Pre-Sidedress Nitrate Test (PSNT)?
The PSNT measures nitrate, the main form of nitrogen used by plants. By mid-season when you take a PSNT, much of the nitrogen that was in an organic form is being broken down rapidly to the nitrate form. The amount of nitrate measured by the PSNT can be used as an indicator of how much nitrogen will become available during the rest of the season and how much additional nitrogen fertilizer the corn may need. Additional fertilizer can then be applied as a sidedress or topdress application.
How to Use the PSNT
To take a PSNT, sample your fields right before the time of topdress — when the corn is 8 to 12 inches tall, in theV4 to V7 corn growth stage. Take 15 to 20 soil cores per field, each core 12 inches deep. You can use a soil probe (preferred) or garden trowel to take a sample. We have some soil probes at our St. Albans office available to borrow on a first-come, first-served basis. Take the soil cores in a random, zig-zag pattern throughout the field, avoiding spots where starter fertilizer was applied. Mix the soil cores together in a bucket and keep about 1 cup as a sample to send to the lab. Dry the sample right away in an oven at less than 150 degrees F in a glass dish until dry (2 to 4 hours should be fine) or by spreading it out on a paper bag in the sun. Or, keep the samples cool and deliver to the UVM Agricultural and Environmental Testing Lab in Burlington the same day it was collected. The nitrate concentrations will change in the sample if it stays at room temperature or has not been dried, giving inaccurate results.
If the PSNT results are low, especially below 15 ppm, it will be worth it to fertilize. Starter fertilizer rates greater than 20 pounds per acre should be subtracted from the recommended sidedress rates. Also, reduce the recommended rate by 30 pounds per acre where the previous crop was a well-managed stand of grass, legume, or mixed forage. If the PSNT results are 25 ppm or higher, there is little or no yield increase from applying more nitrogen. See Table 6 in Nutrient Recommendations for Field Crops in Vermont to find fertility recommendations based on your PSNT results.
A PSNT sample and submission form can be found at: www.uvm.edu/pss/ag_testing/nitrate_form.pdf. You can contact the testing lab at 802-656-3030 or AgTesting@uvm.edu, and/or visit their website at: www.uvm.edu/pss/ag_testing/. Feel free to call our office with any questions at 802-524-6501 or email us at firstname.lastname@example.org.