In the second half of the nineteenth century steam plowing was enthusiastically tried. A stationary engine pulled the plow across the field with cables. Several could be used at once and the system was designed so plows could work in pairs, one set for turning the furrow to the left and the other to the right, the sets being used alternately. Forty or fifty acres per day could be plowed by an eight-furrow plow powered by two steam engines.
In 1936 Harry Ferguson developed a farm tractor equipped with a hydraulically controlled three-point linkage which enabled the plow to be directly coupled to the tractor, providing better and easier control.
Modern plows
The two main types are the moldboard plow, which is widespread in temperate climates, and the disc plow, which is more suitable for tropical conditions. In disc plowing three or four sets of discs are mounted vertically on an axle at a fixed angle of 45° to the soil surface. They do not completely bury the top growth, which makes them suitable for dry conditions. They are also resistant to damage from obstacles or misuse. There are also chisel plows, which loosen the soil without turning it and are suitable for cereal stubble.
Both disc and moldboard plows can perform fixed and reversible plowing. Fixed moldboard plows have one set of right-hand moldboards, coulters and skimmers. To use this type of plow involves setting out the field in such a way that sections can be plowed and eventually joined. Reversible, commonly known as one-way, plows have two sets of moldboards, coulters and skimmers, one left-hand and one right-hand set.
They are mounted one set above and one set below a central beam which pivots. At the end of each pass the plow is rotated mechanically or hydraulically and the tractor runs back alongside the newly plowed furrow. For surmounting obstacles without damaging the plow, each base may be connected to a mechanical spring-trip (release) or a pneumatic-hydraulic mechanism which allows it to spring back. However, some designs, particularly chisel plows, have shear bolts; these break under impact and can be quickly replaced.