A tillage coulter unit (22, 122) adapted for use with a farm implement (10) is provided which permits high speed tillage operations (e.g., field fertilization using liquid fertilizer such as ammonia, or seeding) without undue soil disruption or fertilizer vaporization. The coulter unit (22, 122) includes an upright, rotatable coulter blade (26, 130) with a gauge assembly (28, 132) operable to limit the depth of the coulter blade into soil and to minimize soil disruption. The gauge assembly (28, 132) preferably includes a pair of elongated, shiftable belt or chain loops (74, 212, 214) positioned in straddling relationship and astride the blade (26, 130). The loops (74, 212, 214) present lower runs (96, 212a, 214a) which substantially contact the soil as the unit (22, 122) traverses a field.