SP FAQ - Slot closure
Several. Another important function is controlling closure of the slot and also seeding depth. With Cross Slot openers both of these functions are undertaken simultaneously by the press wheels. They are located close to the seeding zone so they ensure the openers rise and fall (up to 45 cm or 18 inches if necessary) in harmony with changes in the soil surface.
But they also fold the soil flaps back over the seed so as to retain the integrity of a truly horizontal (inverted 'T' shaped) slot with all of its advantages discussed above.
90% retention of residues with Cross Slot
Tests comparing pressing on the top of the soil after covering, with pressing in the base of the slot before covering using hoe and double disc no-tillage openers showed a clear advantage for the latter. The only function that pressing after covering seemed to achieve was to control depth better than where no press wheels at all were used.
With Cross Slot openers, neither of the above pressing options had any effect. So long as the slot was covered, germination and emergence had been so good anyway that there was little if any room for improvement by pressing.
Strictly speaking, they are not press wheels so much as folding wheels. Their two main purposes are (a) to fold the flaps of soil and residue back over the horizontal shelves on each side of the slot, and (b) to control seeding depth. Any pressing action is incidental.