I'm trying to figure out if it is possible to configure a
small tractor so that no soil compaction occurs below the
depth of tillage.

We have clay soil and no significant freeze-thaw cycle.

Leading contender for tiller is a reciprocating spader.
Falc, Celli, Gramegna, ... (BTW as far as I can tell,
reciprocating spaders are only made in Italy, does anyone
know why?) We would want a unit 1.2 meters or narrower.
Depth of tillage varys greatly with model.

The lightest machine would be a BCS or similar
single axle "walking tractor". I get the impression
that the largest BCS is just barely large enough to
run the smallest spader made. Would this machine be beefy
enough to operate smoothly in clay, or would it be
a nightmare to control similar to walk-behind rototillers?
Are there other walking tractors sold in the US?
(I know there are other brands in Europe.)

Next step up are the small Italian tractors such as Ferrari,
Antonio Carrerra, etc. We've only started looking at these.
Are the Italian farm tractors more reliable than Italian
cars? Pros and cons of these? Any particular brand we
should look into? Are there very low pressure tires
available for these?

I have found a few graphs showing pressure below the
surface, such as:

http://ohioline.osu.edu/b301/301_2.html

I'd like to find more of these, with a wider range of weight
and inflation pressures, especially on the light weight
low pressure side. Most of the info I've found is
intended for megafarms, and talks about 15 ton axle loads.
Everyone says lighter causes less compaction, but how
light is light enough? I don't expect to eliminate
compaction at the surface, even raindrops cause surface
compaction. Is it realistic to expect to prevent compaction
below the depth of tillage?

How many psi does it take to create compaction?

Soil is naturally uncompacted by freeze-thaw cycles,
roots, earthworms, moles/gophers. Are there other
natural uncompacting forces?

Dieter