accurately testing pH by yourselfgreenspun.com : LUSENET : Countryside : One Thread
I have an Oakton pH tester that I bought for about $80 a few years ago. I also bought the 4.0 and 7.0 testing solutions. The testing solutions have an expiration date of a coupla years ago. Do they really expire? Do I *have* to buy new testing solution?
With this tester, I fill a clean glass container (usually a pint jar) with distilled water and put a couple of tablespoons of soil in the jar. I then stir with a clean spoon. When I put the tester in the solution, I get a number like 6.1, then it slowly rises and stablizes at 7.0. Almost all of my tests go like this.
Last august I had my soil tested by midwest labs. They say that my soil pH is 6.0 and my "buffer index" is 6.8.
My first thought is that maybe the number I see in, say five seconds on my meter is the pH and the number it eventually settles on is the "buffer index". But this is just a guess.
Is anybody more familiar with this sort of contraption and help me get a grip on this?
-- Paul Wheaton (email@example.com), April 15, 2002
Paul, did you read my response to you a while back about buffer index?
The standard soil pH test is determined with a glass electorde pH meter on a 1:1 soil to water suspension. Soluble salts in the soil sample can cause some interferences, like the one you describe, in the test. Consequently, labs in western states may use a dilute calcium choride solution instead of water for the standard pH test.
At any rate, in the standard test the sample is stirred vigorously for 5 seconds and then alllowed to stand for 10 minutes If the CaCl2 solution is used instead of water, the sample is allowed to sit for 30 minutes. The tip of the pH electrode is the slurry, not in the overlying liquid.
If the ph is 6 or lower, the buffer index is run. A special solution is added to the soil/water slurry from left over from the pH test which was just conducted. The sample is shaken and allowed to sit and another pH reading is taken. The lime recommendation is made from the buffer index result.
-- Cabin Fever (cabinfever_MN@yahoo.com), April 15, 2002.