Carwashes have pits to catch mud and grime washed off cars. If you dig
them out the goop smells very bad and is black and green. Spread it in
the sun and in a short time it is again dirt, dirty dirt but dirt.
Anaerobic bacteria live in Oxygen free environments and give off H2S,
Hydrogen Sulfide gas which smells like rotten eggs. A very toxic
chemical and in those mud pits they live and eat up all grease and
such, good action!. I suspect that is what kills orchid roots, bad
action!, in rotten medium. I can't test that theory now "cause I ain't
got no rotten roots"! All my plants are in S/H, leaving no room for
these little ABs to live and grow. Next time you have rotten roots lay
them out in the sun, the smell and color will leave.
Joe T