I've wondered the same thing myself about the test at home kits, but have never bothered with them. Yes the labs have very expensive but very accurate equipment, and I just send my samples to a lab. You don't really need a "kit" to have a lab do an analysis. Just take samples, and mail the soil in a plastic bag to the lab. It will need to be accompanied by whichever labs sample submission worksheet. There is actually correct and incorrect ways to gather a sample. A wrong way is to take single sample from a garden/yard/field. Another wrong method is to take a sample from the surface. The right way is to take multiple small samples at random from within the area you want tested. For example, gather 8-12 samples at random within the area you want tested. And in regards to gathering samples, one good method is to dig a hole 6 or more inches deep, then using a trowel or other small instrument like a tiny garden shovel, scrape a ribbon off the side of the hole you just dug, collecting a uniform amount from the surface to 6 inches deep. Repeat this process at every sample site. Then thoroughly mix the samples together in the bag so it will more accurately represent the soil test area as a whole. You might wonder why 6 inches deep: 6 inches deep ( I think technically it's like 6.7 inches, but no one gets that exact) over one acre is considered a furrowslice. In the world or farming, soil testing and fertilizer/amendment application, a furrowslice is commonly understood to weigh 2 million pounds. That number is used to calculate the weight of amendments so you can use math formulas to raise elements a known parts per million amount. Labs that analyze soil will often assume your sample depth is 6 inches unless otherwise noted.
I live in Tennessee and have used University of Tennessee's extension for soil testing. Their standard test is called a Mehlich 1. It lists most elements, but their sample doesn't list Cation Exchange Capacity (CEC) and also doesn't include copper or organic matter %. I currently use Logan Labs. They're an independent lab. Their standard test is called a Mehlich 3, which is a better testing method for acid soils (Not for high pH or calcareous soils). Soils in Tennessee are acidic. Their test includes copper, states the organic matter % and also lists the soils CEC. The university ag extension test costs less, like $15 if memory serves me correct, and Logan Labs charges $25, but worth it in my opinion. Knowing a soils CEC will let you establish guidleines to determine appropriate values and limits for the minerals on the test. Logan Labs will state in a separate column if some elements are in excess or if there is a deficit. It's easy to read and understand. Again, worth the $25 per sample in my opinion. Hope this helps!!
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