Joe Jamies wrote:

About 10 cubic metres of fresh fir sawdust was just delivered to my
garden site. The soil is basically clay and very hard to work with. I
would like to use this sawdust to improve the soil structure.

Should I compost the sawdust in a pile (or several piles),

My money would be on composting it ASAP with an accelerant to get it going
quickly. Garotta cut with ammonium sulphate is pretty good. You may have
enough bulk there that it is self starting anyway. But it will rot down
much faster with some twiggy stuff to provide air and some extra nitrogen
accelerant.

Don't leave a big compost pile too close to fences or your garden shed it
can get very hot.

or can I spread the sawdust over my soil so it can compost "in place"
(due to
the large amount of sawdust)?

If you mix it into the soil directly it will rob nitrogen rather badly.

As a nitrogen source, I am thinking of using urea. How long before this
soil is ready for planting
vegetables?

I'd plant spuds as a way of breaking up the clay without bothering too
much about soil structure.
Best time to fight clay is in the Autumn when you can add clay breaking
chemicals and leave the frosts to do their work. Even so composted lawn
mowings and anything else organic and bulky you can get are more useful.

My vegetable garden will take up half the site, so I would like to
prepare that soil ASAP. I would like to use the other half of the site
for trying other ways of improving soil, eg. cover crops, etc.

This year I'd live with the heavy soil and grow the vegetables in it.
Improve modest areas by a large amount rather than spreading it too thinly
to have any measurable effect.

My soil is extremely heavy clay and I have only improved it where
absolutely necessary. Clay soils are pretty fertile ground even if in
winter they stick to your boots and tools in inch thick slabs and set like
concrete in summer. And many vegetables will thrive in it provided you
keep them watered.

Regards,
Martin Brown