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Appendix 1. Protocol for measuring soil and biochar pH. Example data from plant growth experiment are included.
Why Measure Soil & Biochar pH?
pH is a good metric of the chemical environment of the soil. It is often considered a “master variable” in determining soil microbial community composition and activity (Fierer & Jackson, 2006; Wu et al., 2011). As part of the pre-experiment observation,
ask your students to measure and compare the pH of different soil types and biochar. You could also have students measure
soil pH at the beginning and end of the plant growth experiment (example data shown below) or soil respiration experiments.
Both plants and microorganisms acidify the soil environment through biological processes such as respiration Paul, 2014. We
would expect to see a decrease in soil pH over the experimental period. However, the pH of the biochar itself will alter the pH
of the biochar–soil mixture when added. Biochar is typically basic and often increases the pH of the soil, but results are variable
(Lehmann & Joseph, 2015). The effect of biochar on soil pH is often pointed to as a mechanism for observed plant and soil
responses to biochar addition. Use the pH results to help your students interpret the results of the plant growth and soil respiration experiments. Consider the question “How does biochar change soil pH, and how might this affect plant growth and soil
Using pH paper or a pH probe, measure the pH of 20 mL of deionized water. Record the pH of the water. Combine 10 g of soil
or biochar that has been dried at room temperature with the 20 mL of deionized water. Stir or shake the water and soil or biochar solution. Let the solution settle for 10 minutes before measuring pH. If using a probe, place the probe in the solution without letting it touch the settled soil or biochar. Compare the pH of the water to the pH of the soil or biochar solution. Determine
whether the soil or biochar increased, decreased, or did not affect the pH of the deionized water. Example pH results are given
in Figure A1.