I have to concede that the design of essentially every crop circle so far could potentially be reproduced by man given enough time, since some of the formations that are known to be man made can be very complex and beautiful. Human artists are amazingly talented and are capable of pulling off great designs.
However this concept of ghost circles suggests that in addition to the known man made formations, there is also something anomalous happening in the "genuine" formations. Granted, this ghost phenomenon doesn't happen with every formation. If it did, then we could conclude that the pressure of the planks or rollers used by humans would compact the soil and alter the growth of the next crop planted in that field. But we don't ever see the return of formations that are known to be man made.
So there has to be some kind of energetic or chemical change happening in the soil of the "genuine" formations that affects the plant growth of the next year's crop.
In John Root's fantastic presentation "The Majesty And Mystery of Crop Circles", he shows that this phenomenon doesn't always manifest itself in the same ways. While it can lead to stunted plant growth in the next year's crop:
It can also lead to enhanced plant growth:
And also it can have effects on snow, like delaying the melting of the snow where the formation was:
Or even accelerating the melting of snow:
Here's a few other examples of ghost formations:
Also more ghost pics of the Cley Hill formation above, taken between the two pics in the pics above
