The University of Missouri Extension provides soil testing for those in the St. Louis area. Written instructions are listed on their website as well as an instructional video for how to take your soil samples.
There are fees that depend on how many soil samples you have and what you are testing for.
Once you have your soil sample(s) ready they can be dropped off at the location below.
MU Extension in St. Louis County (Kirkwood)
132 E. Monroe Avenue
Kirkwood, MO 63122
Monday-Friday, 9:00 am-4:00 pm