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.

Instructional video:

Materials 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, 1 g or 0.1 g precision; but see below on volume estimation of 10g soil if a balance is not available.
● Distilled water, OR bottled water checked for impact on pH measurement, generally this 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.

Method step by step:

  1. Weigh 10 +/- 0.5 g soil into the cup. If there is no scale, you can also estimate that a volume of 7 to 8 ml of soil will weigh about 10 g.
  2. Add 20 mL distilled water.
  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. Let stand 2 minutes, swirling or mixing from time to time.
  5. Measure with the pH meter: Place the pH electrode in the cup and stir slowly during the measurement, keeping the pH electrode in the supernatant or the top suspension in the cup. Record the pH after the value stabilizes. The meter reading can still be changing a small amount even when “stable”. The goal is to maintain a stable value that does not change more than 0.1 pH unit in about 30 seconds.
  6. Using pH strips: after allowing the solution to settle for a few minutes (so as not to excessively stain or color the paper strips with the color of the soil) the upper liquid or the supernatant of the soil suspension can be measured with pH color test strips of the appropriate range (for example, pH paper strips from 0 to 14, 4 to 7 or 5 to 8). This paper is then compared to a color chart. We are testing to see how reliable this is, and so far this seems to lead to readings 0.5 to 1.0 pH units lower than those of a pH meter, which may be acceptable if users can verify that it is a constant difference where a coarse correction can then be applied.