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).
Measure the pH (acidity) and EC (electrical conductivity) of a 1:2 soil:water (weight/volume) solution.
Label 50ml Falcon tubes (or other container) with sample ID using labeling tape (Use acid-washed Falcon tube or container).
Weigh out 10 grams (or 5, depending on the amount available) of air dried soil and add to labeled tube.
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.
At end of 30-min period measure the pH/EC using the Oakton PC700.
Also create a blank for the EC measurements; use the same nanopore or DI water used for the samples.
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.
For measurement instructions, see “Protocol for pH/EC measurements using Oakton PC700” below.
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Press “CAL” when screen is on pH mode
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
Repeat for pH 7.0 buffer and then for pH 4.0 buffer
When finished with the 3 buffers, screen should revert back to “MEAS”
Rinse probe with DI water and wipe clean, then start measuring samples
Replace probe storage solution every 3 months (can be found in fridge).
If a calibration goes wrong, clear the data points from the machine
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