SP FAQ - Slot shrinkage
Shrinkage of this nature is unavoidable on occasions with almost all no-tillage openers. But even although the vertical portion of a Cross Slot slot may open just as all other slots do, a small humidity probe is placed down the vertical part of the slot and then into the horizontal part (as scientists have done) it will likely show a low humidity zone in the vertical slit but a high humidity zone under the soil/residue flaps in the horizontal shelves. Because the seeds are located in this horizontal zone they remain in a high humidity zone whereas with all other openers it is difficult to avoid seeds being placed in the low-humidity vertical zone.
Of course if the soil has adequate liquid moisture anyway, placing seeds in a low humidity zone may not then affect germination because the seeds can still get from contact with the sidewalls of the slot, regardless of whether these are vertical or horizontal. The main differences show up as soil conditions move from being damp (optimum) to dry (sub-optimum). They do not need to move far from optimum to show differences.
In friable soils, the emerging shoots simply push straight up and emerge through the soil flaps to one side of the central disc slit.
But in circumstances where the central slit of a Cross Slot slot shrinks open and the seeds are under a flap to one side that is not friable, the seedlings initially grow sideways towards the light in the open slit, and then they grow vertically. There are many examples and photographs (see below) showing that this is what happens. The end result is a perfectly healthy plant that would otherwise have died in other no-tilled slots.
Firstly if the soil is like this it will probably not shrink much anyway, so the central slit will not open up. In these circumstances most seeds will simply travel up through the overlying soil and emerge directly above where they were sown rather than take a pathway up through the central slit.
The differences in seedling emergence counts between openers get bigger as the soil conditions get more hostile. For example differences of 1,000 to 1,400% between Cross Slot openers and double disc openers have been reported in the scientific literature with dry soils. Differences of 20-50% are very common, even when soil conditions are only slightly sub-optimal. In wet soils, differences up to 400% in favour of Cross Slot openers have been reported for different reasons.