--Analysis for K, Ca, Na, Mg, Fe, Mn, Cu, Zn, B, S, P
S.D. Carroll, C.G. Herron, D.C. Lafex
University of Arkansas Soil Testing and Research Laboratory
P. O. Drawer 767
Marianna, AR 72360 USA
Phone 870-295-2851
E-mail: soiltest@uark.edu
Contents:
Purpose and Background
Materials
Instrumentation
Extractant Solutions
Procedure
Quality Control Assessment
References
Purpose and Background
The Mehlich III method was developed by Mehlich in 1984 as a multi element soil
extraction procedure. This analytical procedure used for routine soil testing
attempts to estimate the amount of soil nutrients available to the plant during
its growing season. The procedure provides a simple and accurate determination
of Potassium, Calcium, Sodium, Magnesium, Iron, Manganese, Copper, Zinc, Boron,
Sulphur, and Phosphorus.
Materials
Spectro Analytical Instruments, Spectro Flame ICP Modula
Custom Laboratory Dynacrusher Soil Crusher with 20 mesh screen
20 mL filter tubes
Wooden rack constructed to hold twelve 20 mL filter tubes
11 cm Pre folded filters SAP-1F Laboratory Filtration Group
2 g scoop
Flat blade spatula-palette knife type
Custom Laboratory Styrofoam rack with twelve embedded 70 mL plastic extraction
cups
Brewer type pipetting machine or equivalent set to deliver 20 mL aliquots
Eberbach variable speed reciprocating shaker modified to hold 5 styrofoam racks
set to 200 cycles per minute
GRA Lab interval timer with electrical output
Saran Wrap or equivalent plastic wrap
20 L plastic carboy
250 mL volumetric cylinder
25 mL volumetric cylinder
600 mL polyethylene beaker
2000 mL volumetric flask
Thermolyne Nouva 11 magnetic stirrer
2" magnetic stirring rod
20 mL pipette
100 mL pipette
Pipetting bulb
Ammonium Nitrate (NH4NO3)- Reagent grade pellets
Ammonium Fluoride (NH4F) - Reagent grade pellets
Acetic Acid, Glacial (CH3COOH) - Reagent grade 99%
Ethylenediaminetetraacetic Acid (HOOCCH2)2NCH2CH2N(CH2COOH)2 - EDTA) - Reagent
grade
Nitric Acid (HNO3) - Reagent grade 70%
Spex Multi Element Standard
Instrumentation: Spectro Analytical Instruments, Spectro Flame Modula ICP
Using the #1 standard solution set the ICP to read 0 ppm for all elements.
Using the #2 standard solution set the ICP to read: K - 125 ppm, Ca - 125 ppm,
Na - 125 ppm, Mg - 125 ppm, Fe - 125 ppm, Mn - 125 ppm, Cu - 25 ppm, Zn - 25
ppm, B - 5 ppm, P - 125 ppm, S - 125 ppm
Using the #3 standard solution set the ICP to read: K - 250 ppm, Ca - 250 ppm,
Na - 250 ppm, Mg - 250 ppm, Fe - 250 ppm, Mn - 250 ppm, Cu - 50 ppm, Zn - 50
ppm, B -10 ppm, P - 250 ppm, S - 250 ppm
Extraction and Solutions
Standard Solutions: Standards are in solution in Mehlich III Extractant
#1 Mehlich III Extractant
Spex Multi element concentrate containing the following elements: K - 2500 ppm, Ca - 2500 ppm, Na - 2500 ppm, Mg - 2500 ppm, Fe - 2500 ppm, Mn - 2500 ppm, Cu - 500 ppm, Zn - 500 ppm, B -100 ppm, P - 2500 ppm, S - 2500 ppm
#2 Standard Solution:
Using the pipette add 100 mL of the Spex Multi element concentrate to the 2000
mL volumetric flask.
Dilute to the mark with Mehlich III Extractant.
Insert the plastic stopper into the volumetric flask and invert the flask 13
times.
Empty contents of the volumetric flask into the 20 L carboy.
Fill the volumetric flask to the mark with Mehlich III Extractant and empty
the flask into the 20 L carboy a total of 9 more times (total of 20,000 mL or
20 L).
Add a 2" magnetic stir bar to the 20 L carboy and place the carboy on the
Thermolyne stirrer.
Stir the solution overnight.
#3 Standard Solution:
Add 20 mL of Spex Multi element concentrate to a 2000 mL volumetric flask.
Adjust to volume with Mehlich III extractant.
Place plastic stopper in flask and invert 13 times.
Stock Solution: 3.75M NH4F - 0.25 M EDTA
Add 277.8 g of NH4F to a 2 L volumetric flask.
Add approximately 1200 mL of reverse osmosis (RO) water and swirl until dissolved.
Add 146.1 g of EDTA
Adjust volume to the 2 L mark with RO water.
Add a small magnetic stir bar and stir on magnetic stirrer for 2 hours.
Store in a plastic container (Provides stock solution for approximately 10,000
samples).
Extractant: Mehlich III
Add 14 L of RO water to a 18 L carboy.
Add a magnetic stir bar, place on magnetic stirrer and begin stirring.
Add 360 g of Ammonium Nitrate (NH4NO3).
Add 72 mL stock solution.
Add 207 mL Acetic Acid (CH3COOH).
Add 14.76 mL Nitric Acid (HNO3).
Adjust volume to 18 L with RO water.
Stir overnight.
Procedure:
Dry soil sample for 72 hours at 150 degrees Fahrenheit.
Crush sample in the Dynacrush soil crusher to pass a 20 mesh screen.
Scoop 2.0 g of sample into a 70 mL plastic extraction cup in the styrofoam rack.
Using the pipette machine add 20 mL of Mehlich III extractant to the extraction
cup.
Place the styrofoam rack containing the extraction cups in the Eberbach shaker.
Place a sheet of plastic wrap over the tops of the extraction cups.
Close and secure the shaker lid to hold the sample racks firmly in place.
Set the timer for 5 minutes.
Place a 11 cm filter paper in the filter tube in the wooden filter tube rack.
When the shaker stops, remove the racks and pour the samples into the filter
papers in the filter tubes.
When the samples have filtered, transfer the filtrate to the autosampler tubes
for analysis on the ICP.
Quality Control Assessment:
A quality control (QC) sample is placed in every 12 samples. This is either a check soil, high quality control, or a low quality control. The technician checks the results of these QC samples as they are printed out. An additional check is made once a week by a person other than the technician. All quality controls are checked to make sure they follow our criteria.
References:
Mehlich, A. 1984. Mehlich 3 soil extractant: A modification of Mehlich 2 extractant.
Commun. in Soil Science and Plant Analysis. 15:1409-1416