Soil pH

For the entire detailed protocol, see the full downloadable manual

Soil pH is an important indicator of the soil’s ability to grow crops. Some crops do not do well in very acidic or alkaline soils. When the pH of soil is below 5, especially in tropical soils, aluminum toxicity to roots can be increasingly problematic, especially if sensitive crops and varieties are grown. Soil pH also affects the type and abundance of organisms in soil such as those that are responsible for nutrient cycling.

How-to video

YouTube video demonstrating the measurement of soil pH:

https://youtu.be/ULm_gbigMAc

Materials: to perform the soil pH test you will need:

● Field pH meter or pH “pen” meter, stored in storage solution

A pen pH meter. The pH meter should be kept in storage solution and calibrated before use.

● OR pH test strips accurate to at least 1 pH unit, 0.5 unit better (see the precautions on using pH strips below in the method)
● Small plastic cups or containers for 30-100 mL
● Balance to weigh soils
● Distilled or bottled water checked for impact on pH measurement, generally these will have low total mineral content. In some countries reverse osmosis water may be sold as bottled water and this works well. Total dissolved minerals should be checked on the label. If needed, readings can be checked using a bottled water alternative with soils, compared to known distilled water, to verify whether the use of bottled water makes a difference to the pH reading.
● (may improve response and stability if meter has not been used) Vinegar for pre-conditioning pH probe; tap water may also work.

Method

  1. Weigh 10 +/- 0.5 g soil into the cup
  2. Add 20 mL water (distilled or low-mineral content alternative)
  3. Mix the soil and water, stir up to a minute, let settle. Shaking briefly in a closed container is also effective, then transfer to the cup for measurement.
  4. Equilibrate 2 minutes in cup or container, swirling occasionally. Place pH probe in cup and swirl or move gently while measuring. Record the pH. The reading may vary slightly even when “stable”; one adequate measure of stability is less than 0.10 pH unit change in 30 seconds.
  5. The reading should be recorded to the nearest 0.1 or 0.05 units.  Differences of +/-0.01 pH units are not meaningful for soils.
  6. Using pH strips: pH strips can be dipped into the solution above the soil in the 2:1 mixture prepared above.  These pH strips should be purchased for the appropriate pH range, such as pH 4-7 or pH 5-8, and then compared to a color chart that comes with the strips. We are conducting tests to see how reliable this is, and so far it seems to lead to readings that are 0.5 to 1.0 pH units lower than with a pH meter (see the YouTube video above), which can be acceptable if users can verify that it is a consistent difference where a rough correction can then be applied.  To do this a number of samples should be measured with both methods to understand the relationship between paper and meter readings. In this way a conversion can be found, for example, adding 0.5 pH units to the strip reading.  This calibration of the relationship between the pH meter and pH paper strips should be done with the soils within a particular region since it may vary based on soil mineralogy or other soil properties.
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