Please note that Mommsen uses the AUC chronology (Ab Urbe Condita), i.e. from the founding of the City of Rome. You can use this reference table to have the B.C. dates
From: The History of Rome, by Theodor Mommsen
Translated with the sanction of the author by William Purdie Dickson
Agriculture was no doubt practised in Gaul--for even the contemporaries of Caesar were surprised in the region of the Rhine by the custom of manuring with marl,(12) and the primitive Celtic custom of preparing beer (-cervesia-) from barley is likewise an evidence of the early and wide diffusion of the culture of grain--but it was not held in estimation.
12. "In the interior of Transalpine Gaul on the Rhine," says Scrofa in Varro, De R. R. i. 7, 8, "when I commanded there, I traversed some districts, where neither the vine nor the olive nor the fruit-tree appears, where they manure the fields with white Pit-chalk, where they have neither rock--nor sea-salt, but make use of the saline ashes of certain burnt wood instead of salt." This description refers probably to the period before Caesar and to the eastern districts of the old province, such as the country of the Allobroges; subsequently Pliny (H. N. xvii. 6, 42 seq.) describes at length the Gallo-Britannic manuring with marl.
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Reference address : https://www.ellopos.net/elpenor/rome/5-07-subjugation-west.asp?pg=16