turnplough

A turnplough is a plough built around a curved mouldboard that lifts soil, rotates it through roughly 135 degrees, and deposits it inverted into the furrow behind. This action buries surface vegetation, crop residue, and weed seeds while breaking up compacted layers. The mouldboard is the working heart: a steel or cast iron surface shaped to roll soil cleanly without excessive resistance. Unlike simpler scratch ploughs that merely cut a furrow, the turnplough fundamentally transforms the tilth and structure of the worked ground.

The mouldboard's curve and length determine how completely soil is inverted. A longer, gentler curve produces a wider furrow and more gradual turning; a shorter, steeper curve concentrates the inversion into a narrower path. Mouldboards are typically 18 to 28 inches wide on single-furrow implements, though larger gang ploughs may have multiple bodies working in parallel. The angle of the mouldboard to the line of draft, called the lay, is adjustable on quality machines; a greater lay angle increases the turning action but also the draft requirement.

Turnploughs come in three broad categories: fixed ploughs, which work one direction only; reversible or balance ploughs, which can be flipped hydraulically to work either direction without turning the implement; and mounted ploughs, which attach directly to a tractor's three-point linkage. Wheel-mounted trailing ploughs have become less common since the 1960s as three-point mounting gained dominance, but trailing ploughs remain in use where heavier work and finer control of depth are needed.

The chief wear points are the landside (the long vertical blade that guides the plough sideways in the furrow) and the sole or frog (the horizontal base supporting the mouldboard). As these surfaces contact abrasive soil, they wear convex, causing the plough to drift sideways and requiring periodic replacement or welding of new strips. Loose soil, stone, or clay that sticks to the mouldboard degrades the turning action; farmers historically addressed this by polishing the surface or coating it with compounds. Modern high-carbon steel mouldboards resist sticking better than cast iron.

The term turnplough distinguishes this type explicitly from skim ploughs (which use a small blade to cut turf into the furrow without deep inversion) and from disc ploughs (which use rotating concave discs instead of a mouldboard). In organic and conservation agriculture, turnploughs remain standard for primary cultivation because their burial action controls weeds without herbicides, though the cost in fuel and structural damage to soil biology is now debated. The implement sits at the boundary between small-scale craft and industrial-scale tillage: a 14-inch single-furrow plough pulled at 4 miles per hour covers roughly one acre per hour, whereas modern four- or five-furrow gangs working at 6 or 7 miles per hour cover 4 to 5 acres in the same time.