Documentation:Soil pH

What is Soil pH?

Soil reactions are described in terms of pH values. Soil pH is a measurement of the hydrogen ion (H+ ) concentration within the soil. It is an extremely important soil chemical property, which has a profound effect on the availability of nutrients and pollutants to plants and activity of soil microorganisms. The pH scale is defined by the equation:

$pH=-\log[H^{+}]$

Therefore, the pH decreases (or becomes more acidic) as the concentration of hydrogen ions increases.

The pH scale ranges from 0 to 14, while soil pH values usually range from extremely acidic (pH 2-3) to extremely basic or alkaline (pH 11-12). Because pH is defined as the negative decimal logarithm of [H+ ], each pH increase of one unit is a 10-fold jump in actual H+ concentration. Most soils pH range from 4.5 (considered strongly acidic for soils) to 8.5 (medium alkaline). The best plant growth takes place within pH 6 to 7 (i.e., in slightly acidic to neutral soils); however, some plants thrive in an alkaline environment while others prefer more acidic soils. Extreme acidity and alkalinity are conditions that usually need to be corrected if satisfactory crop production is to be achieved.

Types of Soil Acidity

1. Active acidity: amount of hydrogen ions (H+ ) present in the soil solution (i.e., soil water + solutes).
2. Exchangeable acidity: is represented by the quantity of hydrogen and aluminum ions present on exchange sites on colloids, easily replaced by a salt solution (such as 0.01 M CaCl2). Recall that cations in the soil solution are in equilibrium with adsorbed cations.
3. Residual or reserve acidity: amount of hydrogen and aluminum ions strongly bound to soil particles, not replaced by a salt solution but can be neutralized by limestone.

Methods for Determining Soil pH

Active Acidity (pH of the soil samples in water)

1) Prepare a 1:5 soil : water suspension by adding 50 mL of distilled water to 10 g soil in a clean paper cup. Mix thoroughly with stirring rod.

2) Let suspension equilibrate for 30 minutes, during which time most of the soil solids should settle out.

3) Insert pH electrodes into supernatant solution and when a stable reading is displayed (‘S’ appears on the screen) record the pH to one decimal place on the data collection table provided below.

4) Rinse the electrodes well using distilled water.

Exchangeable Acidity (pH of the soil samples in CaCl₂ solution)

1) Prepare a 1:5 soil : CaCl₂ suspension by adding 50 mL of 0.01 M CaCl₂ to 10 g soil in a clean paper cup. Mix thoroughly with stirring rod.

Note: the symbol M denotes moles of solute per liter of solution: M = mol/L

2) Let suspension equilibrate for 30 minutes, during which time most of the soil solids should settle out.

3) Insert pH electrodes into supernatant solution and when a stable reading is displayed (‘S’ appears on the screen) record the pH to one decimal place on the data collection table provided below.

4) Rinse the electrodes well using distilled water.

Field Test Kit

Follow instructions printed on the colour comparison card in the test kit:

1) Obtain 1/2 teaspoon of soil sample and place into a ceramic or plastic well.

2) Wearing gloves, add just enough Reagent N to absorb into your soil sample. If there is excess Reagent N, sprinkle a small amount of soil until fully absorbed.

3) Ensure the fine mesh sieve is placed on top of Reagent M bottle. Shake out a fine layer of Reagent M over your sample.

4) Wait for a a couple of minutes until the colour has developed and compare to the colour comparison card in the test kit.

5) Record your observation and discard sample.

References

1. Krzic, M., Grand, G., Ballard, T., Brown, S., and L. Dampier. (2020). Lab Manual for Introduction to Soil Science course. The University of British Columbia. Vancouver, BC. Retrieved from: http://lfs-sw.sites.olt.ubc.ca/files/2020/01/APBI200_Lab-Manual_2020_Jan2120_V2.pdf
2. Thomas, G.W. 1996. Soil pH and soil acidity. p. 475-490. In D.L. Sparks et al. (ed.) Methods of soil analysis. Part 3. Chemical Methods. Soil Sci. Soc. Am. Book Series No. 5. ASASSSA, Madison, WI