Measuring 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 soils, aluminum toxicity to roots can be increasingly problematic, especially if sensitive crops and varieties are grown, and nutrients such as phosphorus and calcium are less available. Soil pH also affects the type and abundance of organisms in soil such as those that are responsible for nutrient cycling.

Instructional video:

Materials

1. Two measuring options:
   • Portable field pH meter (normally this is more precise)
   • pH measurement strips, with a range from pH 4 to 8. It is recommendable to test these out and establish whether they read consistently the same in comparison to a well-calibrated pH meter as a reference. Some pH strips will give very incorrect measurements because they are not adapted for the low solution strength in soils.
2. Calibrant solutions for pH (buffers) with pH 7 and 4
3. Small plastic cups, beakers, or other containers with volume 30 to 100 mL
4. Small balance (1 g or 0.1 g precision) to weigh soils and water.
5. Distilled water, or some brands of bottled water:   If distilled water is not available, the total mineral content of bottled water (listed on the label as TDS or total dissolved solids) must be low, with <50 ppm (mg/kg) of calcium and magnesium, or even better, <10 ppm, so as not to affect the pH measurement. In the case of distilled water, keep in mind that it is difficult to measure its pH because it contains almost no soluble material and behaves in an unstable way with a pH electrode. If you have distilled water from a reputable source, there is no need to worry if you measure it and its pH is not 7.0; i.e. measuring it may not even be helpful.

Procedure

1. Weigh 20 +/- 0.5 g of soil in a small beaker (or just 10 g if you want to economize on the use of the soil sample). If you do not have a scale, you can estimate a volume of 14 to 16 mL of soil, which will weigh approximately 20 g.
2. Add 40 mL of distilled water, or water with low mineral content, or alternatively rainwater. You can use less or more soil and water, but the ratio should always be 2:1 water to soil, volume:weight.
3. Mix the soil and water, stirring for 2 minutes.

4. Measuring with pH paper: After letting the solution sit for a few minutes (so as not to stain the paper strips and alter the color reading), the upper soil suspension or supernatant can also be measured with the colored test strips to measure pH. This paper is then compared to a calibration color chart. Before widely using the strips with soils in a certain region, it is important to check that the value is comparable to the pH meter, e.g. within 0.3 pH units or at least consistently higher or lower in a way that can be adjusted.