Yes Thailand, i put the shells on top of the soil, my tree's are in containers.
But i now know how they got in the container, i used natural soil from a special place...loam soil.
Now i have special soil from Germany in a bag, that will be sterili i think so i will use that and wash the container very good.
But i have many tree's in that natural soil, so i need to eat many shrimps and put that on top of soil, or do we need to mix it in the soil?
I also will grow french marigold around my tree's in ground, hope they go away but then they go to another fruittree.
Here is no lab, i have never heard of it. But i can use German soil with sand mixed, i think that's good but no cheap. Will do that for rare tree's, not for guava. I have more guava's, don't care much for chonburi white, is small fruit but taste good.