Any amount is better than none at all in a very heavy soil (although Baltimore does not feature clay soils, it's silty sand--but can still be compacted).

Another thing you can do is feed organically through the season. Over time the soil will lighten and the worms do the aerating, turning, and lightening of the soil for you. I haven't tilled or turned my soil in nine years.

First, I am blessed with a good non-clay soil. I always mulch with material that will break down over the growing season and add to the soil composition, which has been improving annually to an already decent soil base. I am no-til, as well, and love not needing to disturb my worm population.

I have heard from growers in extreme clay areas that it is MUCH better to not mix the organic material into the clay as the clay will always win, but instead build raised beds to contain the good topsoil and compost, planting in that mix instead. One award-winning advanced grower used only mushroom manure in a raised dahlia bed a couple years ago, and had the best blooms ever. Personally, I'd be nervous to try that, but luckily, I have no need.

It sounds like your clay soil drains well enough, or else you'd be left with no tubers to dig. (No drainage equals drowned and rotted tubers) If you just want to lighten the soil, ANY additive should help, with no magic numbers to achieve. Do what you can, as you can. Try peat moss, mushroom manure, whole leaves applied in the fall after you have dug, coffee grounds by the five gallon pailful, garden compost.

Thanks for the responses. I am planning on raising the beds as best i can with 3" leaf gro. We are 25 miles west of Baltimore and DO have clay but not extremely heavy. The garden I am using for the dahlias has been in veg production for more than 40 years. It is LOADED with worms. My chickens had a feast when i was digging the tubers! The dahlias did great last year! I also do not want to till these beds annually. My biggest thorn will be the weeds. I covered the entire 30x60 bed with commercial weed cloth last year. Worked great but i know the weed seeds last forever!!! There are going to be 15 16'x3' beds with 4' paths that i will put cloth on again. But haven't decided how to deal w weeds in the beds besides hand weeding... Grass clippings? I don't have time to waste....thanks for any ideas!

A speaker at the last club meeting suggested four layers of newspaper under any type of mulch to keep weeds at bay through the growing season. The newspaper and mulch break down and add to the soil composition when you dig the tubers.

He added that color inks in newsprint are all made with vegetable dyes and not of heavy metals like they used to be, so color newspapers are safe to use in the garden.

Edited to add...

The speaker was a director of a University Agricultural Extension Office.