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.
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.
This method for testing is quick, easy, and inexpensive.
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.
Test kits are easy to use.
Readings depend on interpretation of color, which may vary for different people and are therefore subjective.
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.
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.
Portable pH meters are a convenient way to have laboratory accuracy in field testing.