What JMS said, in my typically wet clay soils, but prhaps I'm just resistant to change. :)
With a shank the soil fractures a bit, the gas can follow the cracks, and lose dirt falls back on top to seal some.
Does the disk go deep enough in your soil type, are their cover disks to seal the slot, and forth.
'Here' I see the blade used with liquid setups, haven't seen it with NH3.