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Nitrate extraction from soil

Nitrate-Nitrogen

Based on Method 52 in MAFF (1986) The Analysis of Agricultural Materials (3rd ed) HMSO

Extraction From Soil

Principle

Nitrate-Nitrogen is extracted from air dried soil with saturated calcium sulphate solution.

Apparatus

  • Bottles – 60 ml, wide mouth, screw cap.
  • Bottles – 125 ml, wide mouth, screw cap.
  • Scales – accurate to 1 DP.
  • Shaking machine with platform – approx. 275 strokes per min, length of travel 25 mm.

Reagents

  • Calcium sulphate solution, satd
  • – Shake 7 g of calcium sulphate, dihydrate, with 2 L of water at 20°C for 2 hr. Check that the temperature of the suspension is 20 °C and filter. Store the solution at 20°C.

Method

  1. Grind air-dried soil and pass through 2 mm mesh sieve.
  2. Transfer 20 ml of air-dried soil into a 125 ml bottle and add 50 ml of satd calcium sulphate solution at 20°C.
  3. Cap bottle and shake on the shaking machine at 20°C for 15 mins.
  4. Filter through a 125 mm Whatman No. 2 filter paper into a 60 ml bottle and retain the filtrate.

Determination

The concentration of nitrate in the extract is determined with a selective nitrate ion electrode.

Apparatus

  • m/V
  • meter – For use with a nitrate ion electrode and a calomel reference electrode.
  • Nitrate electrode –

Reagents

  • Nitrate stock standard solution, 0.2 mg/ml of nitrate-nitrogen
  • – Dissolve 0.722 g of potassium nitrate in satd calcium sulphate solution and dilute to 500 ml with satd calcium sulphate solution. Store in a refrigerator.
  • Nitrate working standard solutions, 1-100 µg/ml of nitrate-nitrogen
  • – Prepare solutions containing 1, 4, 10, 20, 50 and 100 µg/ml of nitrate-nitrogen in satd calcium sulphate solution.

Preparation of Standard Graph

  1. Set the mV meter and electrodes according to the manufacturer’s instructions.
  2. Immerse the electrodes into the 1 µg/ml nitrate-nitrogen working standard solution, keeping the solution stirred.
  3. Select the appropriate expanded mV scale and record the mV reading when a steady value is attained.
  4. Remove the electrodes from the solution and rinse.
  5. In a similar manner, record the mV readings corresponding to the other nitrate working standard solutions.
  6. Record the temperature at which the measurements are made (normally 20° C).
  7. Construct a graph on semi-logarithmic paper relating mV readings plotted on the linear axis to µg/ml of nitrate-nitrogen plotted on the logarithmic axis. The mV readings corresponding to 1 and 100 µg/ml of nitrate-nitrogen are approx. 140 and 30 respectively.

Examination of Extract

  1. Bring the temperature of the extract to that of the nitrate working standard solution.
  2. Immerse the electrodes into the extract, keeping the solution stirred.
  3. Select the appropriate expanded mV scale and record the mV reading when a steady value is attained.
  4. Remove the electrodes from the solution and rinse.

Calculation of Results

Read from the standard graph the number of µg of nitrate-nitrogen equivalent to the mV reading of the sample. Multiply by 2.5. The result gives the mg/l of nitrate-nitrogen, calcium sulphate extractable, in air-dried soil.

This calculation does not take into account two possible sources of error:

  • The nitrate ion activity coefficient can change from approx. 0.85 in satd calcium sulphate solution alone to approx. 0.79 in the extract from the soil having specific conductivity of 4000. With such soils the calculated results would be approx. 10% low.
  • The chloride ion has a sensitivity constant of approx. 4 x 10-3 with respect to the nitrate ion. At the 10 µg/ml nitrate level the presence of approx. 200 µg/ml of extracted chloride would give a positive error of approx. 5%.