Is there a method to accurately test for the presence of ancient amino acids in soil?
I know it can be done with residues of foodstuffs from pots etc, where the proteins have been preserved in a ceramic matrix, protected from the elements, but it seems like a very long shot to hope that even if the ground around stonehenge had been covered in wheat for centuries, that enough identifiable protein would have survived in a form that would make it disctinct from the levels of amino acids you'd find anyway as a result of the grass that's been there for such a long time.
Say there is a method of identifying the ratios of a.a.'s in the soil, and that there is a particular a.a. that is present in higher proportions in wheat than in ordinary grass (e.g. glutamic acid, from all the gluten in the wheat). In the (unlikely imho) event that you found evidence that there's more glutamic acid around the stones, you'd have no way of proving it was because of people grinding wheat. It could have come from all the glutamic residue left over from the bio-fuel used to power the UFOs when they were taking off. You'd have to be looking for particular peptide sequences that map onto the wheat genome, and the chances of polypeptides surviving will be even lower than for individual amino acids. I reckon that in the environment of Wiltshire soil, the bacteria and the elements would have long ago trashed any biological molecules. I could be totally wrong like.
Would you not be better off trying to get people to sift through the soil for remains of husks or somesuch?