Soil samples for nitrogen tests are usually taken at a different time and using a different method than samples for the other nutrients (which are typically sampled to plow depth in the fall or spring).
In the humid regions of the U.S. there was no reliable soil test for N availability before the mid-1980s. The nitrate test commonly used for corn in humid regions was developed during the 1980s in Vermont. It is usually called the pre-sidedress nitrate test (PSNT) but is also called the late spring nitrate test (LSNT) in parts of the Midwest. In this test a soil sample is taken to a depth of 1 foot when corn is between 6 inches and 1 foot tall. The original idea behind the test was to wait as long as possible before sampling, because soil and weather conditions in the early growing season may reduce or increase N availability for the crop later in the season. After the corn is 1 foot tall, it is difficult to get samples to a lab and back in time to apply any needed sidedress N fertilizer. The PSNT is now used on field corn, sweet corn, pumpkins, and cabbage. Although it is widely used, it is not very accurate in some situations, such as the sandy coastal plains soils of the Deep South.
Different approaches to using the PSNT work for different farms. In general, using the soil test allows a farmer to avoid adding excess amounts of “insurance fertilizer.” Two contrasting examples follow:
- For farms using rotations with legume forages and applying animal manures regularly (so there’s a lot of active soil organic matter), the best way to use the test is to apply only the amount of manure necessary to provide sufficient N to the plant. The PSNT will indicate whether the farmer needs to side-dress any additional N fertilizer. It will also indicate whether the farmer has done a good job of estimating N availability from manures.
- For farms growing cash grains without using legume cover crops, it’s best to apply a conservative amount of fertilizer N before planting and then use the test to see if more is needed. This is especially important in regions where rainfall cannot always be relied upon to quickly bring fertilizer into contact with roots. The PSNT test provides a backup and allows the farmer to be more conservative with preplant applications, knowing that there is a way to make up any possible deficit.
Other Nitrogen Soil Tests
In humid regions there is no other widely used soil test for N availability. A few states in the upper Midwest offer a pre-plant nitrate test, which calls for sampling to 2 feet in the spring. For a number of years there was considerable interest in the Illinois Soil Nitrogen Test. The ISNT, which measures the amino-sugar portion of soil N, has unfortunately been found to be an unreliable predictor of whether the plant needs extra N.
In the drier parts of the country, a nitrate soil test that requires samples to 2 feet or more has been used with success since the 1960s. The deep-soil samples can be taken in the fall or early spring, before the growing season, because of low leaching and denitrification losses and low levels of active organic matter (so hardly any nitrate is mineralized from organic matter). Soil samples can be taken at the same time for analysis for other nutrients and pH.