Simple test kits can be a big help for a gardener. However, they are often not very good for fine measurements such as the difference between a pH value of 6.0 and 6.5 Even when measuring with a scientific instrument capable of determining a pH difference of plus or minus 0.1 unit, the most accurate measurements would be taken after the solution has been filtered and there are no solid materials remaining in it. The age of the test kit from the date of manufacture (not how long you have had it) can sometimes cause a problem with a test kit results by developing a weak color development, in some extreme cases, an incorrect color.
Many "clay" soils in the southeastern part of the USA are very acidic. It is not unusual to obtain pH readings in the 5.5 - 6.0 range when testing them. Many gardeners in that area must use lime (agricultural grade ground limestone) to bring the pH into the 6.5 - 7.0 range even after adding various organic materials to allow good drainage. If you want to test your test kit, put water in two test tubes and add a drop of vinegar to one and a small amount of baking soda to the other. When you add the colorizing chemical to the tube with the drop of vinegar, it should indicate being acidic (it should turn the water yellow) while the tube with baking soda should indicate being alkaline (a strong blue color or perhaps even a purple).
I hope this helps,
Larry
