pH and EC SOP

Owned by Linda van Diepen
Last updated: Jun 08, 2020
4 min read

Soil acidity is an indication of the H+ concentration of the soil water. The H+ ions can come from the soil mineral surfaces, disassociated molecules in the soil, and from soil organic matter. It is measured through pH (-log[H+]) of a soil water solution (Thomas 1996).

 Soil salinity is an indication of the soluble salts in the soil water. It is measured through the electrical conductivity of a soil water solution (Rhoades 1982).

  1. Measure the pH (acidity) and EC (electrical conductivity) of a 1:2 soil:water (weight/volume) solution.

  2. Label 50ml Falcon tubes (or other container) with sample ID using labeling tape (Use acid-washed Falcon tube or container).

  3. Weigh out 10 grams (or 5, depending on the amount available) of air dried soil and add to labeled tube.

  4. Measure out 20 mL (or 10ml if using 5 gram soil) of nanopore or deionized (DI) water in a graduated cylinder and add to vial with soil. Shake your soil slurry to break down chunks. If needed help the break down with a glass stirring rod. Shake/stir slurry several times during a 30-min period or place tubes horizontally in shaker for 30min on low setting. This 30-minute reaction period enables the soil and water to equilibrate.

  5. At end of 30-min period measure the pH/EC using the Oakton PC700.

  6. Also create a blank for the EC measurements; use the same nanopore or DI water used for the samples.

  7. You can leave tubes overnight (no longer than 1 night) in the fridge to have soil settle and measure the following day, but make sure to let the tubes equilibrate to room temperature before measurement as calibration buffers are at room temperature. Storage of tubes at room temperature can significantly affect the EC, and should be avoided.

  8. For measurement instructions, see “Protocol for pH/EC measurements using Oakton PC700” below.

Protocol for pH/EC measurements using Oakton PC700 in AG 1024

Before use:

  1. Plug in the correct pH probe: either “chemicals” or “soil slurries”

  2. Plug in instrument and turn on and press “Mode” until “pH” appears.

  3. The top of the screen should read “MEAS”. Take pH probe out of the storage solution, rinse thoroughly with DI water to remove storage chemical or soil from the probe, then wipe dry/clean with KimWipe. Measure the pH 7.0 buffer. If the reading is off by more than 0.1 units, recalibrate (see below under general maintenance).

During use:

  1. Insert the clean probe into the soil slurry and stir continually. Once the machine is happy with the measurement it will read “ready”.  You can record this measurement. This will be the same process for both pH and EC.

    1. You put the probe in and stir until it says ready. Some samples may give you a very hard time reaching this “ready” state.  If this is the case, and the measurement is fairly stable (i.e. not bouncing all over the place) you can record that number.

  2. After each sample, rinse thoroughly with DI water to remove soil from the probe, then wipe dry/clean with a KimWipe

  3. Measure one of the three buffers every 20 samples to watch for drift (off by more than 0.1 for pH or 10uS for EC). Re-calibrate if needed.

After use:

  1. Rinse and wipe probe, then place back in the storage solution and turn machine off

  2. Empty DI rinse bucket and clean up the area.

 EC measurements

  1. Press “Mode” until “uS” appears.

  2. Rinse EC probe thoroughly with DI water (also inside yellow cover), then wipe dry/clean with a KimWipe.

  3. Place probe in EC standard (1413 uS/cm); make sure both silver bands are submerged. Make sure reading is within 10uS from standard 1413 value. Rinse and wipe probe.

  4. Then measure samples; make sure both silver bands are submerged. Make sure that after each sample you rinse thoroughly with DI water to remove soil from the probe (also inside yellow cover), then wipe dry/clean with a KimWipe.

GENERAL MAINTENANCE:

  1. Calibrate the probe every 3 months or every 50 samples, whichever comes first.

·      CALIBRATION:

  1. Press “CAL” when screen is on pH mode

  2. First place the probe in pH 10.0 buffer solution, wait until screen says “Ready” and then press “ENTER”. Rinse probe with DI water and wipe with a KimWipe

  3. Repeat for pH 7.0 buffer and then for pH 4.0 buffer

  4. When finished with the 3 buffers, screen should revert back to “MEAS”

  5. Rinse probe with DI water and wipe clean, then start measuring samples

  6. Replace probe storage solution every 3 months (can be found in fridge).

  7. If a calibration goes wrong, clear the data points from the machine

  8. Keep buffers currently in use at room temperature, and bulk buffers in the fridge

References

Rhoades, JD 1982 Soluble Salts In: Methods of Soil Analysis, Part 2, Chemical and Microbiological Properties, AL Page, RH Miller, and DR Keeney (Eds) American Society of Agronomy—Soil Science Society of America. Madison, WI Pp 167-179

Thomas, GW 1996 pH and Soil Acidity In: Methods of Soil Analysis, Part 3, Chemical Methods, DL Sparks et al. (Eds) American Society of Agronomy—Soil Science Society of America. Madison, WI Pp 475-490