This is my flower bed right now! The dirt is that old red clay mud and I'm having to bring in dirt or buy potting soil and I don't have the money for that.
Decomposed organic matter is the only thing you want to use to improve soil. Not to use to plant plants in or to use for fertilizer. 20 years means that sawdust is decomposed. Just putting this stuff on top of your beds/soil will feed the soil organisms who come up to eat (this is their one and only diet, decomposed organic material) then go back into the soil profile to poop it out mixing the organic material into the soil with no help from you.
Forget about bringing in potting soil or any other amendments to improve your soil. The only ONLY thing that improves ALL soils is decomposed organic matter. Read decomposed! You've got it. Just spread it on top of your plant beds. This feeds and causes your micro and macro soil organisms to multiply for a LIVE soil.
Clay is a wonderful soil. You do need to understand how to manage this soil. You need to have raised beds, not raised beds with lumber or cinder block sides, just double dug, turned over soil via the good old shovel. Never use a rototiller for your soil. Your decomposed sawdust is GOLD. As you double dig to fruff up your soil throw shovelfulls of this stuff into the soil. Leave plenty to cover the tops of your beds with this decomposed sawdust/organic matter.
Be sure to make 6"X6" trenches at the bottom of these beds (you'll have to rake smooth, compact using a piece of plywood before planting seeds or starts) dig down a good foot to foot and one half. Your beds will easily be 3 or 4 feet high, rake, flatten, and compact with the plywood with you jumping up and down on top...they will compact to less than a foot and over time might only be 8" high. Winter or spring cleaning out the trenches will bring them up a bit higher, again compact before planting.
Always fertilize with a balanced fertilizer. Always rotate your crops as in no tomatoes grown in the same soil (or eggplant or potatoes or peppers) for at least 2 years. Keep your walkways separate from your plant beds.
That decomposed sawdust is all you need to improve your clay soil. Seriously. Raised beds as I described, put more on top of the soil, add balanced fertilizer, get a soil test, water deeply and only water when the soil has had a chance to dry out.