At the supermarket check-out today I saw a magazine with the headline "is your pH making you fat?".
On a more serious note, there is an article on the website of the National Gardening Association that gives more detailed and therefore usable instructions for the test under discussion, it actually gives measurements, unlike the first article. It is for one tablespoon of DRY soil, add some drops of vinegar. If it fizzes the pH is above 7.5. Take one tablespoon of WET soil, add a pinch of baking soda and if it fizzes the pH is below 5. Then they correctly suggest that if either of the tests is positive then get a proper pH test so that you can adjust the pH accordingly.
It might be helpful to put this in a little perspective. The pH scale is logarithmic, which means there is a big difference between points on the scale, ten times difference. So a pH of 5 is ten times more acidic than a pH of 6. A pH of 5 is 100 times more acidic than a pH of 7; a pH of 5 is 1,000 more acidic than a pH of 8, and so on.
Different plants have different preferred ranges. Some don't like to go above 6.5, for example. The vinegar/baking soda test isn't going to help with that because it's only purporting to tell you that your soil is somewhere below 5 or somewhere above 7.5. The comment about the pH of the water making a difference is correct, so if you're going to do the wet soil test it needs to be with water that won't change the result, or with soil that is already wet from the garden (which I would think is the better option).
It looks like a fun test to try but given the huge differences between points on the pH scale, and that the test doesn't cater for anything between about 5 and 7.5, and can't tell you how much amendment to use to fix any pH problem, it seems better just to get a proper pH test, whether with a meter, from the extension service, or from a private lab. Then you have a number with which to work.
