Every time I read about using human corpses to produce soil, I recall this article:
"Grass plants bind, retain, uptake and transport infectious prions"
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4449294/
In the past I read about this or similar startup and they mentioned that they don't accept corpses of people diagnosed with prion diseases. IMHO, this is not enough. For example, a person with early CJD could die of other cause and never be diagnosed. Besides that, sporadic CJD isn't as rare as "1 per million" (still rare though).
Here is an interesting video about prion diseases that, among other things, discusses epidemiology ("1 in 7000 US deaths"):
"Creutzfeldt-Jakob Disease and Other Prion Diseases - Brian Appleby, M.D."
https://youtu.be/4vyuby6gibs?t=328
Testing of corpses based on Protein Misfolding Cyclic Amplification (PMCA) might come in handy (if it's not too expensive). It is claimed to be very sensitive. Here is presentation by Dr. Rodrigo Morales (one of the authors of the article about prion uptake by plants that I linked above):
"Texas CWD Symposium: Pre-symptomatic prion detection"
https://www.youtube.com/watch?v=Q8ZX8sHAPUw
P.S.: Hopefully, this eco-burial fad wouldn't catch on. How about deep sea burials?