Soil Nitrate Measurement for Determination of Plant-Available Nitrogen

Introduction

Soil testing has been used effectively over the years in determining the availability of nutrients for plants. Nitrogen is one of these essential nutrients, which is converted to amino acids and then utilized in producing necessary enzymes and structural parts of the plant.  

The LAQUAtwin Nitrate Ion meter can be used to measure NO₃-N concentration in soil samples. It is an easy-to-use pocket-sized meter that provides quick results for on-site testing, thus eliminating the need to transport samples to a laboratory for colorimetric or chromatography analysis performed by trained analyst.

 
Method

Sample Collection and Preparation

1. Collect dry soil samples and pass through a 2 mm sieve.
2. Prepare soil extract by mixing soil and water or extractant in 1:5 ratio (e.g., 5 g soil and 25 mL water or extractant). Shake for 1 minute. Allow the sample to settle for 5 minutes or filter it using filter paper and funnel.

Calibration

Calibrate the LAQUAtwin Nitrate Ion meter using two NO₃-N calibration standards according to manufacturer’s instructions. Make sure that the meter measurement unit is set to ppm NO₃-N.

Sample Measurement

1. Place some drops of unused water or extractant into the sensor.
2. Record the reading as the blank. Blank is only required once for each batch of samples.
3. Rinse the sensor with water and blot it dry with tissue.
4. Place some drops of clear liquid taken from the top layer of soil extract (or filtrate, if filtered sample).
5. Record the reading once stabilized.
6. After each sample, rinse the sensor with water and blot it dry with tissue.

Results and Benefits

To express the results in NO₃-N mg/kg in soil, use the formula: NO₃-N mg/kg in soil = (soil extract reading - method blank) ppm NO₃-N × 5. If a different ratio is used in soil extract, substitute ‘5’ with the appropriate ratio.

Nitrate-nitrogen (NO₃-N) measures the amount of available nitrogen in the soil that can be absorbed immediately by plants. The amount required in the soil for specific crops varies from crop to crop, but in general the levels should not fall below 10 mg/kg and should not exceed 50 mg/kg. However, nitrate varies with soil water and so levels can fluctuate widely depending on soil water movement.

Figure 1: Guide to the interpretation of nitrate-nitrogen values for soils (Source: Soil health for vegetable production in Australia—Part 4)

To determine the amount of nitrogen fertilizer needed to meet crops’ demand, the NO₃-N concentration is being subtracted from the nitrogen requirement of the crop.

Supplementary Information

• Soil – Wet samples will need to be air dried by spreading thin layer of soil in a sheet of plastic under the sun. This is to eliminate the effect of the soil’s moisture content. Samples that have too much clay will need to be crushed.
• Extractant - Water or dilute salt solutions can be used to extract nitrate from most soils because essentially all the nitrate in soils with low anion exchange capacities is water soluble. The main disadvantage of water is its low ionic strength which can cause dispersion and result in cloudy filtrates. Extractants containing chloride cause problems if NO₃-N is measured by ion chromatography or ion selective electrode because chloride can interfere with analysis of NO₃-N by these methods. 0.04M ammonium sulfate, (NH₄)₂SO₄ is the preferred extractant for nitrate selective electrode (Griffin et al., 1995). Other extractants include 0.025M Al₂(SO₄)₃, 1% KAl(SO₄)₂, 0.025M Al₂(SO₄)₃ – 0.025M Ag₂SO₄ – 0.005M H₃BO₃ (pH 3), 0.01M CuSO₄, 0.2% or saturated CaSO₄, 0.01M sodium citrate and 0.05M K₂SO₄ (Thomas, 1986).
• Nitrate standard – Check the NO₃-N concentration indicated on the label of nitrate standard when calibrating NO₃-N (e.g., 150 ppm NO₃^-  standard is 34ppm NO₃-N).
• Measurement unit – The nitrate meter can be set to ppm (equivalent to mg/L) NO₃^- or NO₃-N. Refer to meter manual for the measurement unit setting. If the meter measurement unit is set to ppm NO₃-N and 150 ppm NO₃^- standard is placed onto the sensor, 34ppm NO₃-N reading will be displayed during calibration.

References

1. Bagshaw, J., Moody, P., and Pattison T., (2010) Soil health for vegetable production in Australia—Part 4: Measuring soil health. The State of Queensland, Department of Employment, Economic Development and Innovation
2. Geisseler, Daniel and Horwath, William. Sampling for Soil Nitrate Determination. Fertilizer Research and Education Program.
3. Griffin, G., Jokela, W., Ross, D., Pettinelli, D., Morris, T., and Wolf, A. (2009). Chapter 4 Recommended Soil Nitrate Tests. Recommended Soil Testing Procedures for the Northeastern United States. Cooperative Bulletin No. 493.
4. Thomas, J. (1986). Ion Selective Electrode Reviews, Volume 7. UK: Pergamon Press

REV 0, 4 AUGUST 2015