I have 8 30 gal pots all fed the same , do I core sample each one ? Or pick one looking weakest ? Can’t afford 8tests lol
take a core sample from all and send it in. if your soil is all built the same way with all the same amends and measures of 1/3rd parts and then feed same way. then your core samples will all be pretty much in line and over all the sample will be what your adverge is . when i send in my sample from my earth bed i walk and take a core sample at 4in deep every 5ft in a grid pattern, the bed is 15x15 then i take it all and mix it up , from that bucket of soil i then sift out all the large stuff , then fill my test vial and send it off. now there is a difrence between soil and potting mixs and you need to inform them of that, im saying this because i sent a sample to ohio state college and they couldnt do the test i needed because it wasnt soil , it was peat moss mix of compost and perlite. there are 3 difrent test they do and i cant remeber names right now, just got up , i buy those test kits off amazon for 30 bucks and send them in now, the test is usally back to me by email within a week or so soilsavy.com is who i use they are very good at referencing what you need to add if need be, they send a reference of both organic or conventional suggestions to fix your mix
In the long run might be cheaper to buy your own nutrient photometer and reagents and do your own testing. Nutrient Analysis Photometer - HI83325 (hannainst.com) 5 Best Soil Test Kits - May 2023 - BestReviews