There are two problems here, one is the death of the earth from the heat of flame and fire, the other problem is an ugly black charred spot where fire turned wood into sooty coal and ash. Campfire remnants have a long life in the desert and are not easily absorbed into the earth. The accumulation of countless ground campfires quickly adds up to a desert that is damaged, dirty, and ugly.
In fact in Anza Borrego, California's largest state park, ground campfires are not allowed. In this case, the earth must be protected from fire and remnants that can seep into the earth. Which means your fire can not touch the earth.
There is a simple solution to protecting the earth while still enjoying a campfire. Use a steel trash can lid placed upside down on the ground. Then use rocks to build a protective wall around your fire, which help keep wood and debris falling down onto your trash can lid. This is a low cost and very convenient method that allows for an enjoyable campfire that leaves no trace.
Clean up is easy, which starts with ensuring your fire is completely out before going to bed at night. On the day of departure, double check that your fire is completely out with not a trace of hot coals. Disassemble the rocks surrounding your trash can lid and residual coals. Tip your trash can lid and contents into a transportable bag for hauling to an acceptable trash disposal site. For longer trips and longer distances of hauling fire pit debris, I use a five gallon bucket with sealing lid.