Tap water helps to undermine the accuracy of soil pH testing without any detectable indication. Chlorine can elevate pH readings by 0.3-0.5 units, while minerals in well water can falsely raise alkalinity. I have had to re-calibrate tests for clients using tap water because when I switched them to distilled water, the pH was 6.1 instead of 7.4.
Tap Water Contaminants
- Chlorine: Raises pH for 48-72 hours
- Dissolved calcium: Falsely alkaline readings
- Sodium bicarbonate: Common in municipal supplies
Water Prep Protocols
- Use distilled water for slurry tests
- Filter well water through reverse osmosis
- Wait 72 hours post-irrigation to test
Understanding regional water profiles is important. Limestone aquifers can provide 200+ ppm calcium to well water and if unfiltered this can skew pH tests towards higher alkalinity. A client's soil in the Midwest, labeled as "alkaline soil", actually was neutral soil with unfiltered well water falsely indicating a pH of 6.9 for years.
Timing resolves a large portion of the battle. Test the water 3 days after watering to avoid chlorine distortions in the sample. If testing sandy soils, use 5 days since their quick draining will concentrate minerals. I typically develop testing windows for all of my clients that work with their irrigation schedules and soil textures.
Read the full article: Soil pH Testing: 7 Essential Steps for Accurate Results
