Nitrogen is an elusive nutrient. It can move and change forms. There are also many sources of nitrogen for your corn crop and many methods in which it can be applied. Some soils have more N naturally available, and previous crops can provide N as well.
After all this, how does one determine how much is needed?
Rate trials, university recommendations, and universal experience determine the amount applied in most fields. Results are quantified in crop appearance, tissue analysis and yield. Still, there is a desire to fine-tune rates.
The Pre-Sidedress Nitrate Test, or PSNT, is one of those tools slowly gaining ground in determining the needed rate of applied N. This test will measure residual nitrogen from the previous crop, soil type, and prior applied N, whether as commercial fertilizer or manure.
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Fields that particularly benefit from this testing are those where manure or other organic fertilizers have been applied, according to the Purdue Cooperative Extension Service. Springs with heavy rainfall seem to generate use of this testing to estimate the loss of pre plant nitrogen that was applied.
Danny Greene, of Greene Crop Consulting in central Indiana, says this is a timely application. Turn-around time on samples is generally less than a week, Greene says, but emphasizes that it is a busy time for all parties; farmers, consultants and the lab.
Timing is important. Soil samples should be collected when corn is in the 4 to 6 leaf stage, according to Purdue Extension. Keep in mind that these nitrogen results are only a snapshot of a particular point in time.
Once the results are obtained, weather conditions after the sample were taken should be considered also, as they can change the status of nitrogen in the ground.
While grid sampling is generally too expensive, most samples are based on the variability within the field. "If we have soil types that differ in organic matter, we prefer to separate them," Greene says.
If you apply manure regularly to your fields, PSNT is worth asking your crop consultant about.