More than likely they were plowing under whatever green manure crop was there for somerfallow ground to build up nitrogen for the next crop. That was a common practice back in those days; we still did that into the early 60's.
Our black gumbo soil wouldn't scour during daylight hours some years so we ended up plowing at night. We were plowing under 6-7 ft. high sweetclover that was heavily infested with mosquitoes. No cabs on the tractors, just set there and get bitten all night. As soon as the sun came up the plows would quit scouring so we'd quit for the day and go sleep a few hours.