Soil pH

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 area soils, aluminum toxicity to roots can be increasingly problematic, especially if sensitive crops and varieties are grown.Furthermore, most of the organisms that are responsible for nutrient cycling are pH sensitive and to thrive in soil’s is limited by how acidic or alkaline a soil is.

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
● 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, or tap water may also work.

Method

1. Weigh 10 +/- 0.2 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 would also be very effective, then transferring to the cup for measurement.
4. Equilibrate 20 minutes in cup or container, swirling occasionally. Place pH probe in cup and swirl gently while measuring. Record the pH. You may want to test in the first few samples to assess whether you are getting a stable reading to within 0.1 pH unit — variation at a finer scale is not terribly relevant.
5. pH strips: In principle, the solution above the suspended soil can also be used with pH color testing strips in appropriate ranges, e.g. strips that read pH 4-7 or pH 5-8, and then compared to a color chart. 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, which is unacceptable (unless the relationship between paper and meter readings can be understood and characterized so that a conversion – e.g. always subtracting 0.5 pH units — can be found).

 

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