Oh, I'm not all pouty...
Ummm, we are really not qualified or have the equipment to test
souls.
However, we can do something with
soils... Many years ago, I used to buy those kits at the garden centers - still have the last one - it is the delux model in a steel case... Leaving the extraction method aside, the fundamental problem is that your are trying to compare the color of a liquid through
transmitted light with the
reflected light from a color chart. I never could get the colors to line up...
We have several colometric tests for various compounds/elements, but these are analysed with colorphotometer and spectrometer instruments which give us measurements to several decimal places or in parts per million. For other tests, we use an ICP instrument which reports in parts per million.
There is also the matter of sample preparation and, in our case, weighing. With other methods, a fairly accurate volume of sample is required (problems with that approach). We have soil grinders that reduce the samples down to a maximum particle size (20 mesh, I think) - an important consideration for extraction. I don't recall what extraction method the kits use - but probably the main one being the neutral Ammonium Acetate extraction, as it doesn't use strong acids for extraction. This method, as well as the strong acid methods depend heavily upon the soil type for an approximate analysis. This requires knowing the soil type and applying calibration formulas to obtain the analysis. As I recall, the kits don't mention that in the instructions.
The kits only test for N-P-K and maybe Ca. There's a whole lot more in your soil to be concerned with than just those things.
The reviews you read are accurate: The home kits are for selling - not using.
