The evidence from short term studies suggests that decompaction using an Airknife or Airspade has hardly any effect on soil bulk density or root growth parameters. The two docs below are ones I have easily to hand but there are more especially in JOA.
Soil_Fracture_Patterns_Impact_on_Bulk_Density_smiley.pdf Terravent_20ms_202.0.pdf
But we use an Airknife to decompact as often as we can, and at 230cfm we get quite impressive fracturing. It certainly seems to me to have a positive effect on the health of the trees I've treated, but maybe its just that I want to see it that way. We always impress on the clients that this is a treatment that needds to be repeated annually as the pores created will close again even with foot traffic.
I have not seen anything even approaching a tap root on any of the trees up here not even those growing on old beach dunes but I would accept that in very sandy soils with a lower water table you would expect to see significant sinker roots throughout the root system. What I have seen is adventitous roots on natives and exotics that have developed as the buried root system fails giving the distinct impression of a large tap root.
I agree with you Ekka roots will only grow into areas that are benficial to tree health no air, no nutrients, no water, no roots down there.
SF