Nitrogen is a nutrient taken up by plants in the form of ammonium and nitrate ions for their green leafy growth. When ammonium fertilizers are applied to plants, the ammonium is converted to nitrate by nitrification process. Also, organic matter broken down in soil is transformed into ammonium first and then eventually into nitrate. These circumstances result in more nitrate in soils. Thus nitrogen tests measure nitrate rather than ammonium.
Plant tissue such as leaf and petiole contain high levels of nitrate, which has been shown over many years to be a good indication of the nitrogen status of the plant. Testing laboratories analysing for nitrogen content of dry plant tissue usually require ample time for collecting, transporting, drying, grinding and analysis. They deliver results after several days. With modern technology, quick tests for determining the nitrogen content in plant tissue like handheld nitrate meters with ion selective electrode, chlorophyll meter, and nitrate test strips have been developed.
As a general rule for nitrate testing, twenty petioles of most recently fully expanded mature leaves from different plants in a field are required for a representative sample. However, with onions, roots are used to determine nitrate levels.1
The LAQUAtwin nitrate ion meter provides the easiest way to measure nitrate concentration in fresh plant sap. The sensor requires only few drops of sap, which can be quickly extracted using a garlic press. The meter analyses the sap in just few seconds and displays reading expressed as either nitrate (NO3-) or nitrate-nitrogen (NO3-N) ppm units. Results can be obtained immediately in the field with much less effort and relatively low cost. These advantages over conventional laboratory testing are useful for onion growers in determining fertilization applications.
Select mg/L or ppm NO3-N in the LAQUAtwin nitrate ion meter set-up. Calibrate the meter according to manufacturer’s instructions using the 150ppm and 2000ppm nitrate standards included in the kit. The readings of
150ppm and 2000ppm nitrate standards will be calibrated as 34 ppm and 450 ppm NO3-N, respectively.
It is best not to take readings in areas of direct sunlight or high temperatures. Collect samples and bring them indoor for analysis. Allow the samples and standards to reach same temperature before proceeding with the analysis.
Refer to Technical Tip 2: LAQUAtwin Ion Sensor Maintenance Procedures for detailed information on conditioning, cleaning, and storing the nitrate ion sensor. The technical tip can be downloaded from the support section of our website www.horiba-laqua.com.
Onion roots display the greatest response to available nitrogen supply of any plant part.2 They are used in analysis because of the stable nitrate concentrations in the root xylem. The LAQUAtwin nitrate ion meter has been successfully used in determining nitrate-nitrogen (NO3-N) in onions and readings have been found correlated with lab results.3
When sampling, it is important to note the onion growth stage so you can determine the correct sufficiency guideline to apply with your analysis results. Sufficiency guidelines are generally expressed as nitrate-nitrogen (NO3-N) values. The NO3-N refers to the nitrogen portion of the nitrate molecule. The table below provides the general guidelines for acceptable root sap nitrate concentrations at various stages of onion plant growth.4 Notice that the nitrate concentrations decline as the plant matures. For 0.5 to 1.5-inch onion bulbs, the acceptable root sap NO3-N concentration range is 350 to 500 ppm.1 You may use the table in interpreting the LAQUAtwin nitrate ion meter results. Record the initial analysis results and use them to compare future results.
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