A coulter or colter (Latin 'culter' = 'knife') is a vertically mounted component of many plows that cuts an edge about 7 inches (18 cm) deep ahead of a plowshare. Its most effective depth is determined by soil conditions.
Its earliest design consisted of a knife-like blade. Coulters with a flat rotating disc began being used c. 1900. Its advantage was a smoothly cut bank, and it sliced plant debris to the width of the furrow.
In his 1854 book, Henry Stephens used dynamometer measurements to conclude that a plow without a coulter took about the same amount of force to pull but using a coulter resulted in a much cleaner result. It softens the soil, allowing the plow to undercut the furrow made by the coulter.
A rolling coulter has an optional accessory called a "jointer". The jointer flips over a small part of the surface on top of the slice before the plowshare flips the main slice. It ensures that all of the plant debris gets covered by the flipped slice.
- Stephens, Henry (1854). The Book of the Farm Vol 1. W. Blackwood. pp. 271–272.
- Bacon, Charles Allen (1920). The Oliver plow book: a treatise on plows and plowing. Oliver Farm Equipment Company. pp. 160–162.
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