A belt tensioner assembly is described for a seeding machine that moves seed along a seed path with a seed-transport belt, the seed-transport belt being contained by a belt housing and supported by a fixed belt support. An adjustment mechanism may have an input member accessible from outside the belt housing and a positioning member disposed within the belt housing, the positioning member changing positions in response to user input applied to the input member. A transfer mechanism may be coupled to the positioning member and to a movable belt support, and may move in response to the change in positions of the positioning member. User input to the adjustment mechanism may vary the tension of the seed-transport belt by changing the positioning of the positioning member to move the transfer mechanism and drive the movable belt support along a centerline extending between the fixed and movable belt supports.
