Apparently Gibbon also mentions the degradation of the land
"Since the Age of Tiberius, the decay of Agriculture had been felt in Italy; and it was a just subject of complaint that the life of the Roman people depended on the Accidents of the wind and waves. In the division and decline of the Empire the tributary harvests of Egypt and Africa were withdrawn; the numbers of inhabitants diminished with the means of subsistance;and the country was exhausted by the irretrievable losses of war, famine, and pestilence."
Now does he mean an actual loss of soil fertility or just in the number and effectiveness of farms?