I wash the delta soil I use and it's a nice clay like material, the Organic material is fine......if you allow the soil to fully oxidize the OM and NH4.
You can boil the soil(10 minutes), you can soak in a shallow tray for 3 weeks etc, you can bake in a shallow pan for 1 hour at 450F.
All will do the same thing.
I do this anyway.
You can mix the soil with 3:1 sand(2-3mm) for the bottom 2" layer and then cap with 1"-2" sand.
This prevents a mess when you up root vs a solid layer of soil.
You can add a solid layer also if you do not move plants much and it'll be fine.
Continuous water changes makes life very easy with soil tanks.
This removes the NH4 and also stabilizes CO2.
We use that system for our lab vaults outside at the weed lab.
Simple and you can use any sediments without anything mucking up the water column, this also works well for high bioloads in small spaces(trout farms etc) or overloaded fish tanks like Alan's in our club.
But when you do not have such a system, then things can get a little bit troublesome. If you use CO2 or Excel, doing large water changes 2-3x a week for the first 1-2 months will help.
Thereafter you can reduce them.
There are few folks that fail with soil, however, the cost is well.......dirt cheap and it is a very successful time proven sediment source.
The goal I try to promote is success with soil, so figuring out why and where folks mess up is key and helping the newer folks learn from that also helps.
So the NH4, more than the OM is the key here, algae wise anyway.
Low O2 is the other thing that too much O2 will do the water column.
To the sediment itself, blackened smelly sulfur smell if you have too much often occurs. Adding lots of well rooting plants addresses that by pumping lots of O2 down there as well as mineralizing the sediment prior to use.
I wash through a screen and then let settle for a few days and decant. This seems to work well.
I typically look for a lake or stream where there is good plant growth, then dig up some nice thicker clay like sediment a little above the water line.
As the soil ages, you can re enriched the spots by using mud + ice cube trays=> "mud cubes". These can be inserted beneath the plants and will melt fast. No mess, cheap, easy.