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Soil KCl Extracts pH Protocol

Goal

Detail steps to measure pH from KCl extracted soil samples that were sampled for nitrogen mineralization.

Materials Needed

  • KCl Extracted Samples in labeled scintillation vials
  • Space to spread out
  • Datasheets
  • Pens/pencils
  • pH probe
  • pH standards- pH 4, pH 7, and pH 10
  • Deionized water
  • Kim-wipes

Protocol

Overview

We measured the pH of the samples that were taken for nitrogen mineralization. Soil samples were taken from each plot and put into a solution of 1M KCl, which was used to extract available soil nitrogen (ammonium, nitrate, nitrite). The supernatant was then pipetted off the soil/KCl mixture into vials. This solution was then analyzed for nitrogen content. The samples were then frozen until we measured pH. We take this value to be an approximation of soil pH.

First we thawed the samples. All pH measurements must be taken within 1 week from the day the samples were thawed. When not measuring pH, the samples should be kept in the fridge. We spread the samples out so that they could reach room temperature before we started the measurements. First, we calibrated the pH probe using pH standards (pH4, pH7, and pH10). We recalibrated after each set of 10 sample measurements. If the machine was not drifting, we recalibrated after every 25 samples. We made sure to rinse the probe with deionized water after every measurement.

Detailed methods

The samples that we measured the pH of are in labeled plastic scintillation vials. These are the 1 M KCl liquid end product of the nitrogen mineralization sampling. They have been analyzed for nitrogen content. We measured the pH of each plot. We take this value to be an approximation of soil pH.

Measuring pH:

  1. Thaw the samples. The samples can remain thawed for NO MORE than 1 week. So you must finish pH on all samples you have thawed within a week.

  2. Keep the samples in a refrigerator when not measuring pH.

  3. The samples have to be at room temperature before you start measuring. Acclimating the samples from refrigerator temp to room temp takes about 2-3 hours if you spread the vials out on a table (take them out of the box, if you keep them in the box, it will take much longer to acclimate to room temp).

    1. So, 2-3 hours before you start, take the samples out of the fridge and spread them out in the lab.

  4. Measuring pH.

    1. Record all pH’s on the provided datasheets.
    2. Calibrate the pH probe using all the standards (pH 4, pH 7 and pH 10 or some such thing) provided. Follow the direction provided with the probe. Recalibrate approximately every 10 samples, at least at first to make sure the machine isn’t drifting. If after every 10 it reads correctly, go ahead and do it after every 25.
    3. Swirl the vial to mix the solution and then stick the probe in the vial and let the pH stabilize, this could take a while (hopefully no more than 30 seconds). If the machine is having trouble stabilizing under a minute’s time, try recalibrating and then remeasure. If the hundredths decimal place is moving, that is fine. If the tenths decimal is moving more than +/- .1, try recalibrating.
    4. BE SURE TO RINSE THE PROBE WITH DI WATER AFTER EACH AND EVERY SAMPLE.
    5. Put the cap back on and go on to the next sample.
Last update: December 29, 2022