[snow white]

New to all this and a bit confused.  If it is recomended that you don't walk on the soil  after digging to avoid soil compaction, then what about  treading the  soil down hard for brassicas?  Doesn't it ruin the soil?

[mumofstig]

It doesn't 'ruin' the soil - it just compacts it.
Brassicas are about the only veg that prefers compacted soil the rest prefering something easier to get their roots into 

[Totty]

Stomping all over wet, poor draining soil will compact it, no veg likes that, treading it when the surface is dry will firm it which brassicas like. I also like onions to go into firm ish soil too.

Totty

[fatcat1955]

It is said that if you plant onion sets in firm soil, the sets will be pushed out of the soil by the roots .

[aelf]

An important component of soil is the air content. If the soil is lacking space for air (because it is over-compacted or waterlogged) then it's not a good environment for aerobic microbes to thrive or for insects/worms that help to develop the soil. Most plant roots need air as well. That is why it's generally best to avoid walking on or digging very wet soil - the air gets squeezed out. Also, when compacted soil dries out (especially if it has a lot of clay) the soil sets like concrete, nothing much can grow and any rain will not be absorbed - it will just run off the surface into puddles.