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
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.