Nitrate Sampling Protocol and Recommendations
Soil Nitrate Test Observation
Adapt-N’s Virtual PSNT (sometimes called LSNT) provides simulated nitrate levels in the top 12” of the soil on a daily basis throughout the season. This helps reduce or even eliminate the need for extensive PSNT sampling.
Users who nevertheless are sampling for soil nitrate levels in the field to enhance the Adapt-N simulations can enter those test results as an Observation into Adapt-N. This adjusts the soil nitrate levels for the date of the test, and subsequently adjusts Adapt-N’s simulations. Explicit nitrate PPM values can be utilized, which cause Adapt-N's nitrate value as of the date and depth of the test to be changed to the user-supplied value, with subsequent daily simulations continuing from that point forward. Or, relative values can be supplied using the Nitrate Adjustment factor. The Nitrate Adjustment factor allows the user to supply a relative percentage by which Adapt-N's inherent nitrate PPM calculations as of a given date and depth will be modified. These observations can be supplied either at the zone level, or via the Field Configuration tool.
Example single-zone observation setting an explicit nitrate value to 10 ppm:
Example single-zone observation using a Nitrate Adjustment Factor to increase the zone's nitrate value by 15% as of the date and depth supplied:
In the above example, if Adapt-N's inherent nitrate simulation on 2018-05-01 was 10 ppm, this would cause the nitrate value to be raised to 15ppm. 15ppm would then be the new baseline moving forward, for future-date simulations.
Recommended Sampling Methodology
Soil nitrate tests are inherently variable, so it’s important to follow guidelines to minimize sampling bias and to maximize the consistency of results. Sampling strategies should be well thought out, documented, and consistently used. Give consideration to the management practices to ensure the sampling effort accurately captures variability within the field.
- PSNT samples should be taken when the corn plants are between 6 and 12 inches tall.
- Weather - make sure necessary precautions or adjustments are made if sampling around rainfall events. Significant rainfall prior to sampling can cause spatial variability in results. This can inaccurately depict losses of applied N fertilizer resulting in excess N being applied. Try to avoid sampling while the field is very wet or within 3 days of a high rainfall event.
- Sample the top 12 inches of the soil profile (the standard depth for PSNT). With early season rainfall events, N below the 1 ft level could very well be lost before the roots reach it.
- Samples should be from representative areas within a field: take management zones, soil textures, or other forms of spatial variation into account when determining where to sample. If possible, log GPS coordinates of sampling locations for future reference.
- Each sample should be based on a composite of multiple soil cores. Collect at least 10 cores per sample to represent a sampling area: Establish a consistent protocol to determine how many cores need to be included in a sample, and where those cores should come from. Balance the cores among row and between-row areas.
- Applications of banded (vs. broadcast) fertilizer or manure applications cause high spatial variability of soil nitrogen -- especially when applied in high amounts (e.g., through a large pre-plant injection). It is very difficult to get accurate PSNT estimates, because the results are highly dependent on whether soil cores were collected from the band or not. A consistent sampling protocol is needed to avoid introducing bias. We suggest the protocol available on this help page to address this challenge. An alternative approach is provided by N Watch https://nwatchonline.com/protocols.
- Handling & Shipping Samples: Talk to your soil lab for instructions on how to handle and ship samples as your lab may have a specific protocol to follow to ensure accurate results. Samples should be sent off as soon as possible after sampling.
- Interpretation of Soil Analysis: We found that Adapt-N’s Virtual PSNT tool does a good job at estimating real field values. But due to the expected high variability of PSNT sample results -- especially with manure and large banded fertilizer applications -- you may see some discrepancies between the measured values and Adapt-N’s virtual PSNT estimates. Our field trail data has shown Adapt-N to generally be within 5 ppm of the measured value.
- Once you interpreted the results, you will need to be confident of their accuracy before implementing them into your N strategy. Adapt-N takes these results at face value and makes no determination on their accuracy. If you have concerns over sampling analysis, talk to your soil lab before utilizing results.