The farmer from the village of Ueken in canton of Aargau first stumbled upon the "green-tinged" coins earlier in July.
A total of 4,166 coins made of bronze and silver were collected in a find that "exceeds all expectations by far - something you experience as an archaeologist rarely more than once in a career," said archaeologist Georg Matter, who directed the search of the area.
Archaeologists have already started to examine the coins that date from between 270 and 294 AD.
Coin expert Hugo Doppler has examined 200 of the coins. The coins bear the images of different emperors, including Aurelian (270-275 AD), Tacitus (275-276 AD), Probus (276-282 AD), Carinus (283-285 AD), Diocletian (284-305 AD) and Maximianus (286-305 AD).
The coins have a silver content of five per cent and are well preserved because they were immediately withdrawn from circulation, Doppler said.