Determination of Soil pH

 

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 email soiltest@uark.edu

 

Contents

         Purpose and Background

         Extraction Procedure

         Quality Control                       

         References

PURPOSE AND BACKGROUND

            Soil pH is a measure of hydrogen-ion (H+) activity in a soil suspension. This property influences the many aspects of crop production and soil chemistry, including availability of nutrients and toxic substances, activity and diversity of microbial populations, and activity of certain pesticides. Soil pH is defined as the negative logarithm (base 10) of the H+ activity (moles per liter) in the soil solution. As the activity of H+ in the soil solution increases, the soil pH value decreases. Soils with pH values below 7 are referred to as “acid”; pH values above 7 are referred to as “alkaline”; soils at pH 7 are referred to as “neutral”. 

EXTRACTION PROCEDURE

1. Scoop 20 g of soil sample into a 3 1/2 oz solo cup in the styrofoam sample rack, leaving the first row of rack #1 & #2 empty.

2. Using the dispenser add 40 mL of Reverse Osmosis (RO) water to each sample cup.

3. Vigorously stir each sample with a glass, stirring rod.

4. Allow sample to stand for 15 minutes.

5. Insert racks into place on Robbie #2.

6. Check the level of saturated KCl in the electrodes. If not full then fill. Also check to see if bottom of electrode has build up of crystals. If so clean with warm RO water and fill with KCl solution.

7. Turn on stirrers.

8. Analyze the samples using the "Robot Table Data Acquisition Program".

QUALITY CONTROL

This laboratory participates in the North American Proficiency Testing Program (NAPTA) operated by the Soil Science Society of America for all soil analyses including soil pH.

During calibration a 7.0 buffer solution is read by the meter in millivolts. Next a 4.0 buffer solution is read in millivolts (mV).  The program calculates a calibration curve using these inputs.  A 6.0 buffer is read and after calculation should read exactly 6.0.  Four check soils are read and all four-check soils must read within ± 0.1 pH unit.  During this calibration there are several tests that have to be passed in order to continue to measure the pH of unknown samples.  Two performance tests are used to check electrode performance.  In the first performance test the reading of the 7.0 buffer (in mV) added to the reading of the 4.0 buffer (in mV) should equal to at least 172.0 mV.  If the value does not meet these criteria then the electrode will have to be recalibrated.  If it still does not meet the criteria then a new electrode should be placed on the meter.  The second performance test was designed to check the slope of the electrode.  The slope was calculated as:  Slope = (mV7 – mV4) / (pH7.0 – pH4.0).  The slope should be ± 5% of the theoretical Nernst equation slope (59.2 mV at 25°C).  If not the electrode is replaced.  Every twelfth sample is either a High QC, Low QC, or check soil and must read within ±0.2 of the specified value.

 
Modifications: We modified the soil water ratio from a 1:1 to a 1:2. The equilibration time was changed from 30 minutes to 15 minutes. We also have stirrers that stir the supernatant while it is acquiring a reading rather than hand swirling the sample. We do not use the CaCl2 step.
 

REFERENCES

Watson, M.E. and J.R. Brown. 1998. pH and lime requirements. North Central Regional Research Publication. 221: 13-16

 

Donahue, S.J. 1983.  Determination of Soil Water pH. Reference Soil Test Methods for the Southern Region of the United States.  p. 1-4.  Southern Cooperative Series Bulletin 289.  University of Georgia College of Agriculture Experiment Station.  Athens, GA.

 

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