I’m not for sure exactly how deep the OM layer may have been back in the beginning. Its not something that even crossed my mind back then. I just hooked up the disk and went to plowing. I suspect though, that it wasn’t too different than what I have now. Reason being…..a lot of what is going to effect the depth of that layer is the rooting depth of the plants your growing. A lot of what is creating that black layer of dirt is the rotting root systems of my previous crops. This field was mostly just bermuda grass and weeds back then. I’ve got the ability to now plant different deep rooted plant species that will take larger amounts of biomass deeper….things like the radishes for instance or sorghum. On a side note to this as well….other than the depth of the OM layer there’s also the concentration of the OM to consider. You want that layer to increase from 2% to 3% to 4%...etc. How fast and how much that increases depends on how you manage it.....how fast it decomposes.