Using the Soil Nitrate Quick Test (SNQT) for Guiding Nitrogen Fertilization 

The text below addresses the intricacies involved in using the SNQT to improve the efficiency of nitrogen fertilization practices. For specific details on how to use the quick test, click here or here. The nitrate test strips can be purchased here.

The challenges associated with nitrogen (N) management are mostly related to the significant fluctuations of soil N in a crop season. While all other essential plant nutrients present slight changes in the soil over a season, N can go from excessively high to deficient levels in a matter of few days.  These oscillations occur due to a combination of factors, including but not limited to: fertilization rates that differ from actual crop N uptake, irrigation inefficiency, uneven rainfall pattern, soil organic matter content, soil texture and numerous biological and chemical processes.

Understanding crop N uptake rate (pounds of N/acre/day), how that rate changes throughout the season, and making sure that irrigation amounts don’t often exceed the soil water holding capacity can have the highest impact on improving N use efficiency. If fertilization rates significantly differ from crop uptake, over time soil N will either vanish and result in drastic yield losses, or it will keep increasing to excessively high levels, which can lead to groundwater contamination and possibly crop quality issues.  The key is to apply the right fertilizer rate at the right time.

The SNQT is an important ally for improving N management as it provides a good estimate of how much N is in the soil, allowing for adjustments to the fertilization rates before and during the crop season. This inexpensive test (approximately $1 per sample in supplies) can determine soil nitrate status in a matter of 1 to 4 hours, depending on the soil texture.  The quick turnaround gives more flexibility in decision making without having to wait for laboratory results.  Being inexpensive also allows for testing multiple fields regularly. A field with excessive amounts of soil N could have the fertilization reduced or postponed, while a field with low levels could have the fertilization increased accordingly.    

The University of California Cooperative Extension in Ventura and Monterey Counties have conducted several field trials demonstrating that using the quick test can reliably identify fields in which N application can be reduced or postponed.

Considering the increasing scrutiny and regulatory pressure from the state regional water quality control boards to protect water quality, the SNQT is an important tool for aiding growers, farm managers and consultants to improve N use efficiency without compromising yields. 

Successful use of the SNQT requires training in soil sampling, test performing and results interpretation. The following is a quick description intended to demonstrate how practical and straightforward the SNQT test is: 1) collect a composite soil sample representative of the entire field at the depth of the active root zone. Homogenize the sample in a bucket and sieve a portion; 2) fill a 50ml centrifuge tube with 30ml of 0.01M calcium chloride solution and add the sieved soil until the total volume is 40ml; 3) shake it vigorously until the soil is dissolved, and refrigerate until the solution at top is clear; 4) dip a nitrate test strip into the clear solution to identify the nitrate content in ppm based on the color scale provided with the strip; 5) divide that number by a factor to correct for soil texture and moisture content.  If the sample was collected in the top 12” and the result indicates 20 ppm of nitrate-N, then that field has approximately 70-80 pounds N/acre available for crop use in the top foot of soil.

 

We will have a workshop on how to use the SNQT and interpret its results in the near future. Please email me (asbiscaro@ucanr.edu) for more information.