Citrus Soil pH Testing Procedures1

Kelly Morgan 2

Figure 1. Collect soil sample.
Figure 1.  Collect soil sample.

Figure 2. Mix soil and distilled water in a 1:1 ratio by volume.
Figure 2.  Mix soil and distilled water in a 1:1 ratio by volume.

Figure 3. Continuously stir for 1 minute, then let stand for 15–30 minutes.
Figure 3.  Continuously stir for 1 minute, then let stand for 15–30 minutes.

Figure 4. Use testing equipment to gather soil pH readings.
Figure 4.  Use testing equipment to gather soil pH readings.

Facts

  • Maintaining the correct soil pH is essential to ensure optimal plant growth and crop yield.

  • Soil samples should be taken once per year, ideally at the end of the rainy season.

  • It is generally convenient to take soil samples when collecting leaf samples.

  • The accuracy of soil test interpretations depends on how well soil samples represent the area tested.

  • The slurry method allows you to get a representative sample and measurement of an entire area with just one test.

Soil Sample Method

  • Because soil pH can vary within a small area, be sure to take samples that are representative of the production block.

  • Take several soil samples to a 6-inch depth from under 15 to 20 tree canopies (irrigation zone) into a bucket.

  • Mix soil and place about one handful of soil in a sample bag marked with the sample location and date.

  • Samples should also be taken from between tree rows (bed tops or row middles) using the procedure above.

  • Keep these two samples separate.

Soil pH Measurements

  • Temperature fluctuations will cause measurement errors, so samples should be analyzed as soon as possible.

  • Add equal parts of soil and distilled or deionized (DI) water in a 1:1 ratio by volume.

  • Continuously stir the suspension for at least 1 minute.

  • Let the soil suspension stand for about 15–30 minutes to allow most of the suspended soil to settle out.

  • If electronic testing equipment is used, the electrodes must be thoroughly rinsed with deionized or distilled water between samples.

Test Strips

  • pH test strips, also known as litmus paper, are paper strips that have been saturated with pH-sensitive dyes.

  • When exposed to a damp substance, the strips will change color relative to that substance's pH. This color change corresponds to a color chart provided with the test strips.

Figure 5. pH test strips.
Figure 5.  pH test strips.

Advantage

  • This method for testing is quick, easy, and inexpensive.

Disadvantage

  • Test strip results are subjective because colors can look different depending on the lighting as well as from person to person. This leads to inconsistent results (+/-0.5 pH).

Chemical Test Kits

  • pH chemical test kits are like test strips one would use to determine the pH of pool water.

  • Using a soil test kit involves adding your soil, distilled or DI water, and some chemicals to a tube.

  • The chemicals react with the pH levels in your sample to create a color change.

Figure 6. Electronic pH tester.
Figure 6.  Electronic pH tester.

Advantage

  • Test kits are easy to use.

Disadvantage

  • Readings depend on interpretation of color, which may vary for different people and are therefore subjective.

Electronic Measurements

  • Each instrument/electrode system described below must be calibrated at a minimum of two points that bracket the expected pH of the samples and are approximately three pH units or more apart to obtain the best results.

Digital Pocket Testers

  • Soil pH pocket testers are digital, portable testing instruments that utilize a pH electrode.

  • The pH electrode takes a pH reading in your soil or soil slurry and displays it on an LCD screen.

  • Many testers also have a much higher resolution and accuracy than chemical options, generally between 0.1 and 0.01 pH units.

Advantage

  • The integration of a pH electrode in the durable casing of a tester allows for much greater accuracy than test kits or strips.

Portable Soil pH Meters

  • A bit larger than the digital pocket testers, portable soil pH meters offer many functions, including measuring salinity and data logging.

  • Most portable pH meters have automatic temperature compensation; they will come with either an integrated temperature sensor or a separate temperature probe.

  • Portable soil pH meter measurements have resolutions as low as 0.001 pH units.

Advantage

  • Portable pH meters are a convenient way to have laboratory accuracy in field testing.

Footnotes

1. This document is SL452, one of a series of the Department of Soil and Water Sciences, UF/IFAS Extension. Original publication date December 2019. Visit the EDIS website at https://edis.ifas.ufl.edu for the currently supported version of this publication.
2. Kelly Morgan, professor and center director, UF/IFAS Southwest Florida Research and Education Center, UF/IFAS Extension, Immokalee, FL 34142.