A great deal depends on soil type, structure, and conditions. Around these parts, best time to deep rip is the fall when ground is relatively dry. Subsoilers do their best when they shatter soil rather than simply ripping a groove through the hardpan. That shattering effect doesn't occur when soil is wet and muddy. If all works according to plan, there is just a little disturbance of the surface. Most of the action takes place underground.
In this area, common practice is to rip 30" to 36" apart IF soil conditions are right.
Subsoiling helps in many ways, primarily drainage. Excessive rain water runs THROUGH the ground into the water table, instead of running off. That run-off usually carries away your fertilizer and soil nutrients, as well as some of the top soil. As an added benifit, plants can root deeper once the hardpan is broken. That lets them reach that subsurface moisture in hot dry weather.
Pastures and hayfields are almost always compacted. Animals compact soil worse than tractors and equipment, but even simply running over a hayfield with tractor/mower, tractor/rake, tractor/baler, tractor/wagon, tractor/fertilizer spreader, ect, you're packing the soil more than normal freeze/thaw soil heaving can deal with. Pasture renovators and aerifiers have become popular in recent years. They do SOME of the job, but NOTHING like a deep subsoiling at 10"-12" maybe even 20".
