The coulters usually are operated at about the same depth or slightly deeper than the seed openers. Seed placement depth may be more consistent if the coulter is operated at the same depth or slightly shallower than the coulter, and the seed may be placed in firm soil. Openers are usually double-disk units with disks from 14 to 18 inches in diameter. Double-disk openers are well suited to shallow planting since they can be fitted with depth bands. Small disks require less downward force for penetration.
Large disks operate better in trash.
Offset disk openers usually open a narrower seed furrow, requiring less downward pressure for penetration and less soil moved back to cover the seed. Single disk openers may be mounted two to a linkage to balance the side-thrust. The most common depth control device is the combination gauge and press wheel operating behind the openers and attached to the same linkage. A press wheel narrower than the seed furrow is relatively ineffective as a depth control device. A few drills are available with gauge wheels alongside the opener, resulting in more accurate depth control under most conditions.