Yes, my understanding is that the earth should be conductive, so in effect you need it to be wet and organic rather than sand or rock. I have lots of cunning plans in a 1950s crystal set book that show ways of attracting water to the earth conductor - ranging from digging a pit and filling it with coke, to filling a tin can with salt and burying that.
My 1950s Bush DAC34 and Pye P75A work very well with 2m of wire thrown out the window and no earth, but my Invicta AW75 is from an age when people were expected to have a decent aerial and earth at home if they were going to spend a fortune on a radio
