Extractable Cation Standards Preparation
Primarily follows Soil Survey Division Staff (2004).
The extractable bases (Ca2+, Mg2+, K+, and Na+) from the NH4OAc extraction (procedure 4B1a1) are generally assumed to be those exchangeable bases on the cation exchange sites of the soil. The term extractable rather than exchangeable bases is used because any additional source of soluble bases influences the results. The abundance of these cations usually occurs in the sequence of Ca2+ > Mg2+ > K+ > Na+. Deviation from this usual order signals that some factor or factors, e.g., free CaCO3 or gypsum, serpentine (high Mg2+), or natric material (high Na+), have altered the soil chemistry. The most doubtful cation extractions with this method are Ca2+ in the presence of free CaCO3 or gypsum and K+ in soils that are dominated by mica or vermiculite (Thomas, 1982).
Primary stock standards solutions (PSSS), high purity, 1000 mg L-1: Ca, Mg, K, and Na.
Working stock mixed standards solution (WSMSS), High, Medium, Low, Very Low, and Blank. In five 500-mL volumetric flasks, add 250 mL of 2 N NH4 OAc and the following designated amounts of Ca PSSS, Mg PSSS, K PSSS, and Na PSSS. Dilute to volume with RODI. Invert to thoroughly mix. Store in polyethylene containers. Prepare fresh weekly. Store in the refrigerator. Allow to equilibrate to room temperature before use. Prepare WSMSS as follows:
High Standard WSMSS: 90 mL Ca PSSS, 7.5 mL Mg PSSS, 20.0 mL K PSSS, and
100.0 mL Na PSSS = 180 mg L-1 Ca, 15 mg L-1 Mg, 40 mg L-1 K, and 200 mg L-1 Na
Medium Standard WSMSS: 60 mL Ca PSSS, 5.0 mL Mg PSSS, 10.0 mL K PSSS, and
50.0 mL Na PSSS = 120 mg L-1 Ca, 10 mg L-1 Mg, 20 mg L-1 K, and 100 mg L-1 Na
Low Standard WSMSS: 30 mL Ca PSSS, 2.5 mL Mg PSSS, 5.0 mL K PSSS, and 10.0 mL Na PSSS = 60 mg L-1 Ca, 5 mg L-1 Mg, 10 mg L-1 K, and 20 mg L-1 Na
Low/Low Standard WSMSS: 12.5 mL Ca PSSS, 0.25 mL Mg PSSS, 0.125 mL K PSSS, and 5.0 mL Na PSSS = 25 mg L-1 Ca, 0.5 mg L-1 Mg, 0.25 mg L-1 K, and 10 mg L-1 Na
Blank WSMSS = 0 mL of Ca, Mg, K, and Na PSSS.
Mixed calibration standard solutions (MCSS), High, Medium, Low, Very Low, and Blank. Dilute 1 part WSMSS with 19 parts of WLISS (1:20) dilution with resulting concentrations for MCSS as follows:
MCSS High Standard: 9.0 mg L-1 Ca, 0.75 mg L-1 Mg, 2.0 mg L-1 K, and 10.0 mg L-1 Na
MCSS Medium Standard: 6.0 mg L-1 Ca, 0.5 mg L-1 Mg, 1.0 mg L-1 K, and 5.0 mg L-1 Na
MCSS Low Standard: 3.0 mg L-1 Ca, 0.25 mg L-1 Mg, 0.5 mg L-1 K, and 1.0 mg L-1 Na
MCSS Very Low Standard: 1.25 mg L-1 Ca, 0.025 mg L-1 Mg, 0.125 K, and 0.5 4 mg L-1 Na
Blank = 0 mg L-1 Ca, Mg, K, and Na
When diluting samples if using the AA use a working lanthanum ionization suppressant solution (WLISS) as described beginning on pg. 185 of the Soil Survey Laboratory Methods Manual – Soil Survey Investigations Report No. 42 Ver. 4.0 (Soil Survey Division Staff; 2004).
The 1-mL pipette is for the MCSS and NH4OAc extracts. See above for reagents for preparation of the MCSS (High, Medium, Low, Very Low, Blank). Dilute 1 part NH4OAc sample extract with 19 parts of WLISS (1:20 dilution).
Soil Survey Division Staff. 2004. Soil Survey Laboratory Methods Manual – Soil Survey Investigations Report No. 42 Ver. 4.0. USDA-NRCS. Lincolin, Nebraska, USA. [Online WWW] URL: https://nrcspad.sc.egov.usda.gov/DistributionCenter/pdf.aspx?productID=886 [Accessed 10 March 2016].