Nitrate test strips are an affordable tool for quickly measuring nitrate (NO3) in soil and water, and can help farmers and crop advisers adjust fertilizer inputs to match the nitrogen (N) needs of various types of crops. There are now a variety of brands of nitrate test strips available, many of which are manufactured for testing the quality of aquarium water, but may also be suitable for soil testing. All of the brands of test strips are used in a similar fashion: the strip is briefly dipped into an extractant solution (for soil) or in water, and allowed to develop color during a standard interval of time, usually ranging between 30 and 60 seconds. After color develops on the strip, a color chart, calibrated to either parts per million (ppm) of NO3 or expressed in ppm equivalents of nitrogen (NO3-N), is used to determine the NO3 concentration of the sample. Multiplying Nitrate-N concentration by a factor of 4.43 converts the reading to NO3 concentration. Because the strips may continue to develop color with time, it is important to always read the strips at a standard time interval, or the measurements will not be accurate or repeatable. More detailed information on using the nitrate test strips for monitoring soil nitrate levels was presented in several of our past bulletins, newsletters, and blogs.
Depending on the soil type and crop nutrient requirements, vegetable farmers need test strips that are accurate for soil NO3-N concentrations ranging between from 5 to 30 ppm, which would roughly correspond to a range of 10 to 60 ppm of NO3 in the nitrate quick test extract solution. For strawberry production, and other crops that have a slower N uptake rate than vegetables, growers need test strips that are accurate over a narrower range of soil NO3 concentrations (5 to 15 ppm NO3-N in soil). Past studies have demonstrated that the Merckoquant test strip are accurate for measuring soil NO3-N in the range of 10 to 40 ppm. Because more brands of test strips have become commercially available in recent years with varying ranges of sensitivity, and the need to identify test strips that are accurate for measuring low concentrations of soil NO3-N (0 to 15 ppm), we evaluated the accuracy and ease of using six commercially available brands of test strips over a range of nitrate concentrations found in commercial agricultural fields.
A stock solution of a known NO3 concentration was prepared by dissolving a measured weight of sodium nitrate (NaNO3) into 1 liter of distilled water. This stock solution was further dilluted with distilled water to standard nitrate concentrations that matched the values of the color chips of the various test strips evaluated in this study. The NO3 concentration of each standard solution was confirmed by spectrophotometric analysis.
Each brand of strip was evaluated at NO3 concentrations corresponding to the color chips provided by the manufacturer. The Hach Aquacheck and Lamotte Instatest NO3/NO2 strips differed from the other brands because the color chips were calibrated in equivalents of NO3-N rather than NO3. For convenience of displaying and comparing the data, results for these two brands were converted to NO3 (by multiplying the NO3-N values by 4.43). The Merckoquant NO3/NO2 test strip was the brand originally tested by UC Cooperative Extension for use with the soil nitrate quick test, and was considered the standard in this evaluation. This strip measures to a maximum of 500 ppm NO3, but was only evaluated up to 250 ppm NO3 (56 ppm NO3-N) for this test.
Each brand of test strip was evaluated 4 times for each standard NO3 solution corresponding to the manufacturer's chip color chart. The procedure that we followed to determine NO3 concentration was to dip the strip briefly in solution, and hold it horizontally after removing it, allowing color to develop for the interval specified by the manufacturer. Most strip manufacturers recommended a 1-minute time interval between wetting and reading the strip color. The manufacturer for API 5-in-1 and LaMotte Instatest 5-Way recommended reading test strips after 30 seconds, but results appeared to be more accurate after a 60 second interval, therefore all results reported for these strips are from readings taken 60 seconds after placing the strip in the test solution. After waiting the specified interval, the color of the test strip was compared to the color chips provided by the manufacturer. If the test strip color matched one of the chips, then the value of the chip was recorded. In many cases, the color of the test strip was between 2 of the standard chips, and in these cases an estimate was made based on comparing the intensity of the color development with the 2 closest matching chips. Because this method relies on visual observations, all tests were made in a room with ample lighting and by one observer.
The mean NO3 values measured using different brands of test strips were compared to the standard solution values in Table 1. Some brands of test strips appeared to be accurate at specific ranges of NO3 concentration. The Merckoquant NO3/NO2 brand was the most accurate for the full range of NO3 concentrations (Table 1). The next most accurate brand over the entire range of NO3 concentrations evaluated was the LaMotte Instatest NO3/NO2. The Hach Aquacheck was accurate for the range of 10 to 90 ppm NO3 but measured NO3 lower than the standard solutions at concentrations above 100 ppm NO3. The remaining brands of test strips, LaMotte Instatest 5-way, API 5 in 1, Tetra 6 in 1 Easystrips, all measured less NO3 than the standard solutions over the range of 20 to 200 ppm NO3. These strip brands should probably not be used for the soil nitrate quick test and for assessing nitrate concentration in irrigation water.
Although the LaMotte Instatest NO3/NO2 also had good accuracy across the range of 20 ppm to 220 ppm NO3, it did not have a standard color chip for evaluating NO3 at low concentrations, and therefore may not be suitable for strawberries and other crops where soil nitrate is typically in the 5 to 15 ppm NO3-N range. Both the Merckoquant and Hach brands were accurate for measuring NO3 at low concentrations (10 to 40 ppm). Although the Hach Aquacheck strip had a color standard of 5 ppm NO3, the strip was not able to measure NO3 at a concentration below 10 ppm (Table 1).
With the exception of the Merckoquant NO3/NO2, all test strips were purchased online through Amazon.com. The price reported for the strips in Table 1 was the purchase price advertised at the time our study was conducted (January 2014). Some strips were available in larger quantities or from other vendors, for different prices. The Merckoquant NO3/NO2 can be purchased from Cole-Parmer (http://www.coleparmer.com) or at EMD Millipore (http://www.emdmillipore.com).
We identified 3 brands of test strips that accurately measured NO3 and can be used to quickly assess the concentration of NO3 in soil or water. Both the Merckoquant NO3/NO2and the Hach Aquacheck strips were accurate for measuring concentrations of NO3 as low as 10 ppm, which would roughly correspond to 5 ppm NO3-N in soil. No brand of test strip measured NO3 accurately below 10 ppm. Several brands of strips that measure NO3 in addition to other constituents in water were found to under estimate NO3 concentration, especially at high values. While laboratory analysis of NO3 is generally more accurate than using colorimetric test strips, the strips tested in this study appear to be sufficiently accurate to estimate the level of residual mineral N in soil samples and for determining the NO3 contribution from irrigation water, and should be useful for quickly assessing soil N status before making a fertilizer decision.
Table 1. Comparison of nitrate-test strip and standard solution values.