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Soil testing for pH

Soil testing to determine whether aluminum toxicity or other issues related to soil acidification are the cause of poor crop growth should be done in the top 3". Only go to 5-6" if there has been deep banding of N fertilizer or major use of anhydrous N.

Either submit the top 3" to a lab for analysis or sample with a field probe as follows.

soil pH probe in soil slurry

Soil pH probe in soil slurry. Image by Clain Jones

  1. Scrape off the duff, which generally has higher pH, so exclude it.
  2. Dig a hole that is 2" deep with shovel or trowel, trying to make the bottom of the hole somewhat flat - parallel to surface
  3. Loosen the soil at the bottom of the hole
  4. Squirt in distilled water until water in hole is deep enough to fully immerse probe sensor (generally at or near bottom of probe)
  5. Place probe in slurry and give slurry a little stir with probe
  6. Wait until probe reading is stabile
  7. Repeat at 4"

Our best readings are in a beaker 1:1 or 2:1 soil:water slurries b/c can mix better. If you test soils using a container,  try to collect a 0-3" (if can easily remove duff) or 1-3" sample (if can't), since that's the root zone and most affected by low pH. There are no huge advantages of taking pH at deeper depths because either acidity is a problem in the rooting zone, or not. While things like duff layer and uneven surface are certainly issues and perhaps you're actually measuring pH at 1.5 " or 3" when want 2", we have never found pH to be low anywhere in that 1 to 3" depth, and not be low at 2", meaning it's low throughout that top 3" when it's low.

  • If pH < 5, you have verified that growth issues are due to pH
  • if pH is 5 to 5.5, it's possible that growth issues are due to pH, but might still have time to prevent further acidification
  • if pH is 5.5 to 6.0, acidity is coming and focus should be prevention
  • pH > 6, likely don't have a problem

It is good to sample in problem areas as well as areas that appear healthy, for comparison.

Soil samples to determine liming rates should be from thoroughly mixed 0 - 6” cores, or from averages of pH in the 0 - 3” and 3 - 6” depths.