PLS 671: Environmental Soil Chemistry

“Nitrification Studies/Iron and Manganese Extractions”
Lab #8 - 11/22/04

I. Objectives:

a.)  To follow changes in nitrogen over time after addition of ammonium (NH4+) on the three soils. 
      Extract iron (Fe) and manganese (Mn) from soils and soil fractions using the acidified oxalate method.

II. Nitrification Procedure

1.) Weigh out 2.0 g (in duplicate) aliquots of your air-dried soil into two 30 mL Nalgene plastic
      bottles.  Add a clean stir bar.

2.) Add 20 mL of deionized water.  Stir for 10 minutes.  Pull a 2 mL blank suspension sample while
     stirring and record initial pH.

3.) From a 0.1 M NH4Cl stock solution, add the calculated amount of stock solution needed to
     achieve an initial NH4+ of 1 mM without changing the solid:solution ratio.  The “time zero” will
     coincide with the emptying of the pipette carrying the NH4+ stock solution.

4.) Bottles will be shaken on the Big Bill shaker and 2 mL suspension samples removed at 1h, 2h,
     24h, 48h, 96h, and 168h (one week).  

5.) Measure equilibrium pH using a calibrated pH meter during sampling. 

6.) The reaction between NH4+ and soil will be stopped by filtration with 0.2µm pore size membrane
      filters using 25 mm filter holders and 10 mL syringes into labeled test tubes. 

7.) Store filtrates in labeled test tubes in the refrigerator.  The concentrations of NH4+, NO3-, and
     NO2- will be measured in the filtrates using colorimetic methods. 
III.  Fe and Mn Extractions (McKeague and Day, 1966).

1.) Weigh out 0.50 g (in duplicate) aliquots of your air-dried soil and 0.10 g aliquots of each fraction
      (feel free to readjust this weight depending on the amount of fraction remaining) in 40 mL
      centrifuge tubes (this should be a total of 8 tubes per group). 

2.) Add 10 mL of 0.2 M (NH4)2C2O4 buffered to pH 4.0.

3.) Wrap in aluminum foil and shake for 24 h.

4.)  Separate the solid from solution by centrifugation and/or filtration.

5.) Total Fe and Mn will be measured using FAAS.
IX.) References:

 McKeague, J.A. and J.H. Day.  1966.  Dithionite and oxalate-extractable Fe and Al as aids in differentiating various classes of soils.  Can. J. Soil Sci.  46:13-22.