I have acid clay soil in usda zone 3. There is no sand and precious little organic matter in my soil. I am in the process of trying to start a food forest. I mulch and compost to the extent of my available resources, but most things grow slowly if at all. However there are some things which actually are growing. Is there a way I can improve my soil passively without having to dig everything up or disturbing existing trees?
Acid clay is my soil type as well, with a layer of sand on top from from where all the topsoil has washed away. I made one hugel bed two years ago, which was evidently over a rock vein and about killed me to dig out. So last summer, in the bed next to the hugel, I piled a lot of old half rotted hay right on top of the grass then continued through the year adding anything else I came across, leaves, compost, tons of sweet gum balls, etc. I haven't actually looked at the soil underneath but I'm about to right now because after reading these soil threads I have high hopes!
I concur with Alex, hay or straw, either one will provide what that clay needs to become good soil. To either of these "covers" you will want to pour on fungal slurries (whizzed up mushroom caps from anywhere) and when you have it ready compost teas.
By making the additions of compost teas and mushroom slurries you are adding microbiome organisms and those will do the work of turning the medium (the hay or straw) into soil nutrients by being eaten by the good microbiome you are adding.
When using hay or straw, deeper is better, I use full, rectangular bales of straw, these measure around 4' long, 24" wide, and 28" tall. If you use full rectangular bales stand them open ends (of the individual straws) up so liquids can pass through the straws.
Don't be afraid to heap stuff on, I like to use spent coffee grounds, collected urine, manures and anything else I know will rot to nothing. Once I even used some out of date fish emulsion mixed down to 25:1 then poured it all over a set of bales. The clay soil that was under those bales is not recognizable as clay based today (two years after the first bales were set). The current bales are on their last year of looking like bales of straw, most of them are only around 6 inches tall today. I forgot to mention that if you set bales so the two or three strings are facing your legs, you have set them correctly so water will soak down into the interior and start the decaying process.