Soil Testing – My Rapid Test vs Lab Analysis Experience

Above, a snip of the lab analysis of my vegetable garden soil.

If you are familiar with the why’s and how’s of soil testing, feel free to jump to the bottom of this page for conclusions on which is best, based on my admittedly limited experience. Otherwise, read on.

Back in October 2019, following a not-so-great vegetable growing year, I decided to get a soil analysis to see what, if any, amendments would be helpful to add to our raised beds.  Fall is the best time to replenish and balance spent soils so they have several months to absorb nutrients before the next growing season. Here in my neck of the woods, King County, WA offers 5 free laboratory analyses of garden soils, something I had taken advantage of previously with good results. So, with trowel in hand, I followed their instructions and sent in my samples.  The results were promised via email within 2 weeks.  Yet for some reason, they never arrived.  I called a few times to try and track them down, but not surprisingly got only bureaucratic recordings.  Rats.  By January it looked like I would have to rely on an over the counter rapid test kit from the garden center instead.  So, once again with trowel in hand, I followed the instructions and performed the tests.  Matching my results with the color-coded chart provided with the rapid test, my soil pH looked ok; Nitrogen & Phosphorous didn’t show enough color to even register; and Potassium looked medium or low.  Puzzling over what to do next, I made one last ditch effort to contact the County office and amazingly, I got through!  My lab analysis from the Fall was in my inbox a few minutes later.  So, what I had suddenly was not only a highly in-depth evaluation of my soil’s health from the county, but also an interesting opportunity to compare the results from the two different testing methods.   And interesting they were in how they differed:

The color scale of the rapid soil test.

The most striking difference was in the Phosphorous (P) count, with the two results coming in at completely opposite ends of the scale.  Potassium (K) was also off but could be attributed to how inexact it is to determine a result based on a simple graded color scale.  The Nitrogen measure was the only result that was in agreement. Not even pH was close enough to be equivalent since the color ranges of the rapid test are so imprecise: Yellow zone for 6.0 – 6.9, Green zone for 7.0 to 7.9, etc.

The upshot is that the rapid test didn’t correspond to the lab analysis.  Granted, this was a one-time comparison, but going forward I would not be too keen to rely exclusively on results from a rapid soil test kit. Furthermore, the lab analysis provided much more in-depth and extensive assessments that better revealed how to amend my soils properly and without waste (where the CEC measure comes into play, for example).  It’s a no-brainer to go for a free lab analysis (even if it does get lost temporarily).  Paying more for a soil report would also be money well-spent.

Hope this experience of mine has been helpful for you.