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.

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.

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.