Ok this is meant to be cheap and easy. It is not meant to be an engineered wall, nor a wall that a city or agency is going to inspect. It's meant to be a rural garden wall.
Our dirt is decomposed granite and in pockets it has gravel, sand and a little loam. We discovered that someone used it as mortar on a wall in the 1930's and the wall was still up. I was using some concrete mix from the local quarry and it made awful concrete, no finish and it cracked easily. I figured this out and tried out dirt, great concrete!