USDA Natural Resources Conservation Service (NRCS) - This is the official webpage for the USDA Natural Resources Conservation Service in Washington, D.C. 

Click on the following link to access the Electronic Field Office Technical Guide (eFOTG) for any state in the nation.
 

Missouri Natural Resources Conservation Service - The Technical Resources tab on this website gives you access to various technical references including the current listing of approved soil test laboratories.

Missouri Soil Testing Association Accreditation Program - This webpage lists approved soil test labs for Missouri.