[dice]

[mustard]

I don't know how well mustard will play intermixed with
winter rye, hairy vetch, fava beans, oats, peas, crimson
clover, etc for winter cover crops, so I have not tried it
yet. Spring is pretty much out of the question, unless
I leave a bed fallow for a year and not plant any vegetables
in it. Mixing it into the winter cover crop seeds may be one
way to go, acknowledging that the first hard frost is going
to kill it.

[heavy clay soils]

It requires a lot of work and materials to turn these into
useful garden soils. You can till in a couple of feet of
hardwood chips. There will be some nitrogen drawdown the
first year, but these last awhile and provide drainage and
organic matter. Adding sand is useless unless you mix in
dump truck loads of it (adding 10% sand to a soil that is
70-80% clay makes it closer to concrete).

By far the cheapest, fastest way to go is to build raised beds
on top of it, and fill them with sandy loam, compost, leaves,
horse manure, etc. Tilling a foot of hardwood chips into the
soil underneath the raised bed probably helps making the
usable root space deeper, too.

There is what I call the "HoosierCherokee Method": fill big
burlap sacks with horse manure, line them up end to end
in long rows, cut a cross in the top to put transplants in,
supplement with a little epsom salts, gypsum, and soluble
fertilizers during the summer to fill in anything that the horse
manure may lack, and plant the plants directly in them. At the
end of the summer, build sides for raised beds around them and
fill in with leaves, grass clippings, and similar compostable
materials on top.

Then there is the Earl method: build tall sides out of hunks of
used wooden pallet, and fill in with compostable stuff all
summer (grass clippings, leaves from the previous winter's leaf
piles, manure, whatever else turns up that is useful for this).
The next spring, remove the pallets and build sides for raised
beds around it, spread it out a little, make "Earl's holes" in it.

Each method incrementally adds more raised beds as one
acquires materials to make them out of. HoosierCherokee
can use his beds the first year, because tomatoes will grow
in just horse manure pretty well. Earl's method requires a year
of seasoning before a bed is ready to plant in. Both would
benefit from tilling hardwood chips (or "hardwood fines") into
the hard clay soil under the raised bed first.

A variation might be to mark out a space for a raised bed,
till in the hardwood debris, add some high nitrogen fertilizer
like blood meal or fish meal or urea to compensate for nitrogen
drawdown the first year, and grow a crop of alfalfa in it. Then
mow it the next spring and build the raised bed on top of it.
(Alfalfa roots really deep, breaking up the subsoil a little.)