Pam wrote:

...Working with wet soil (soil that is close to its water saturation level),
whether using a tiller or working by hand with with regular garden tools, leads
to compaction or the loss of pore space between soil particles. This reduces the
amount of oxygen in the soil and complicates drainage as well as making the soil
environment less hospitable to microorganisms, resulting in poor plant growth.
That is why it is recommended that one not work (or even walk on or allow
livestock to graze on) wet soils...



I agree with this. In addition, natural processes such as rain will
compact soil without the need for someone walking over it. However, I
would not describe the compaction as "damage", since it is correctable
once the soil dries out a bit. Plant roots can overcome moderate
compaction in good garden soils.

Subsoil compaction occurs even when walking in an apparently dry garden,
since the moisture level below ground can be significant. However, the
effect of walking is spread out as you go down further below the
surface, so the deep compaction will be a long-term thing rather than
the kind of compaction you can get on the surface when it's wet.

Clumping of wet garden soil can be remediated by tillage in dry
conditions. If the area under the plants has been compacted, tillage
next to the plants can restore drainage for proper growth.