Hi Jen and Adam...
regarding frequency of sub-soiler use... it is possible that it would be a one time thing... depending on how effective the result was, your goals, and the effectiveness of subsequent management... the overall context is your guide... many crop farmers, for example, using tractors for a variety of passes, use the yeomans plow along with the final pass to de-compact soils compacted by all the heavy machinery...
the typical way it has been presented is that it may take 2 or 3 passes in a pasture renovation scenario, with each subsequent pass ripping a bit deeper than the last, allowing life to slowly filter activity deeper into the profile, and generate topsoil out of the underlying subsoils... this might happen over 3 years, or could be shorter too... conditions, context and goals will be decisive.
If the plow is used as one tool to help break an ineffective water cycle, achieves a good response, and is then followed up with good planned grazing management, this might be the only time using it...
Yeomans would sometimes re-rip pastures that hadn't seen any plow use for over 10 years, and see some re-improvement (he was a relentless experimenter).
as far as
tractor power to pull the yeomans plow... in general 15-20 horsepower are needed for every shank in the ground. In heavier soils with caliche or tough compaction, one is likely to need a full 20 horse per shank.
Where a tractor available doesn't have the 'oomph' one can always remove a shank and rip using one less...
The general costs are a couple of gallons diesel per acre, labor to drive the tractor, and any surveying costs for the topography used in analyzing how to best accomplish the keyline pattern cultivation on your specific ground.