MEHLICH 3 SOIL EXTRACTION (1:7 Dilution Ratio) -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
Phone 870-295-2851 email soiltest@uark.edu
Contents

Purpose and Background

Materials

Instruments

Extractant Solution

Procedure

Quality Control Assessment

Reference

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 Schleicher and Schuell #1 filter paper #10308543
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 14 ml aliquots
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
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
Erbach variable speed reciprocating shaker modified to hold four styrofoam racks set to 200 cycles per minute
 

INSTRUMENTATION: Spectro Analytical Instruments, Spectro Flame Modula ICP

1.      Using the #1 standard solution set the ICP to read 0 lbs/A for all elements.

2.      Using the #2 standard solution set the ICP to read:

K = 500lbs/A
Ca = 500 lbs/A
Na = 500 lbs/A
Mg = 500 lbs/A
Fe = 500 lbs/A
Mn = 500 lbs/A
Cu = 100 lbs/A
Zn = 100 lbs/A
B = 20 lbs/A
P = 500 lbs/A
S = 500 lbs/A

EXTRACTANT AND SOLUTIONS

Standard Solutions: Standards are in solution in Mehlich III Extractant

#1 Mehlich III Extractant

#2 25 ppm K 5 ppm Cu

25 ppm Ca 5 ppm Zn

25 ppm Na 1 ppm B

25 ppm Mg 25 ppm P

25 ppm Fe 25 ppm S

25 ppm Mn

#2 Standard Solution:

1.      Add 20 ml of Spex Multi Element Standard to a 2000 ml volumetric flask.

2.      Adjust to volume with Mehlich III extractant.

3.      Place plastic stopper in flask and invert 13 times.

Stock Solution: 3.75M NH4F 0.25 M EDTA

1.      Add 277.8 g of NH4F to a 2 L volumetric flask

2.      Add approximately 1200 ml of reverse osmosis (RO) water and swirl until dissolved

3.      Add 146.1 g of (EDTA)

4.      Adjust volume to the 2 L mark with RO water

5.      Add a small magnetic stir bar and stir on magnetic stirrer for 2 hours

6.      Store in a plastic container (Provides stock solution for approximately 10,000 samples)

Extractant: Mehlich 3

1.      Add 14 L of RO water to a 18 L carboy

2.      Add a magnetic stir bar, place on magnetic stirrer and begin stirring

3.      Add 360 g of Ammonium Nitrate (NH4NO3)

4.      Add 72 ml stock solution

5.      Add 207 ml Acetic Acid (CH3COOH)

6.      Add 14.76 ml Nitric Acid (HNO3)

7.      Adjust volume to 18 L with RO water

8.      Stir overnight

PROCEDURE:

1.      Dry soil sample for 72 hours at 150 F.

2.      Crush sample in the Dynacrush soil crusher to pass a 20 mesh screen.

3.      Scoop 2.0 g of sample into a 70 ml plastic extraction cup in the Styrofoam rack.

4.      Using the pipette machine add 14 ml of Mehlich 3 extractant to the extraction cup.

5.      Place the plastic rack containing the extraction cups in the Eberbach shaker.

6.      Place a sheet of plastic wrap over the tops of the extraction cups.

7.      Close and secure the shaker lid to hold the sample racks firmly in place.

8.      Connect the shaker power cord to the power output plug on the GRA Lab timer.

9.      Set the timer for 5 minutes.

10.  Place a 11 cm #1 filter paper in the filter tube in the wooden filter tube rack.

11.  When the shaker stops, remove the racks and pour the samples into the filter papers in the filter tubes.

12.  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

 
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