Do certain types of leaves make soil/compost acidic? I have an enormous cottonwood behind the house (and wanting to EAT the house), and it sheds bushels and bushels. I certainly won't cut it down (except maybe for the lurking branch over my son's room), but I'm interested to know if large amounts of these leaves will have an acidifying effect (or any other effect I might need to compensate for).
Also, we have a cedar on the north side. Any ideas of what to plant underneath that can survive the shower of needles? Do cedar needles make the soil acidic?

Good question! Acidity and alkalinity in the soil is a complex function of the balance and availability of sulfur, calcium, existing native soil minerals, humus, and even the microbes living in the topsoil around plant roots.

Leaf mold (100% decomposed leaves) is very acidic, because there were no "greens" in the original decomposition recipe. Greens plus browns plus air plus water makes good aerobic compost. Mature aerobic compost always has a near neutral pH.

Leaf mold is mainly decomposed by fungi rather than bacteria. Fungi tend to thrive under acidic conditions. True compost is a balance of not only greens and browns, but also aerobic bacteria, fungi, and other beneficial aerobic microbes. The hot thermal aerobic bacteria in most compost recipes tend to thrive under alkalinic conditions in the pile as the pile decomposed and undergoes extreme chemical, physical, and biological changes, as mature compost is being made.

Rich topsoil has 100% aerobic microbes living in it. Aerobic microbes love near neutral and alkaline local pH around plant roots. Stinky, anaerobic microbes can thrive in a strong acidic local pH near 3-4 !

That's why most composting experts preach aerobic composting, aerobic tea brewing, and aerating the soil.
It helps balance local soil pH around plant roots, plus it helps the right kinds of microbes and soil organisms to thrive in the topsoil too.

I add quite a lot of kitchen scraps to my compost--green stuff. So should I hold onto my leaves and slowly add them to balance the greens in my compost? Also, how much actual soil should I be adding to my pile? It's hard to add that this time of year because it's all frozen (though with the rain, my tulips are popping up!). So I usually just add kitchen garbage and cover with leaves in the winter. In spring/summer/fall I cover kitchen scraps with soil to lessen anaerobic stinkiness (and resulting raccoons, skunks, maggots).
Cap'n--how many piles do you have going at once? I'm about to start a new pile in order to finish my old one for use this spring.

You don't need any soil in your pile to make compost. That's from the "old school" OG books! (LOL)

All you need is free available greens and browns, moisture, air, and patience. Nature will do the rest.

I like to use approx. 2-3 times more browns than greens, and more plant waste than animal waste. This gives me a good balance of humus and microbes and odor control in my mature compost.

Browns supply carbon (carbohydrates) that gives microbes energy, makes humus, and controls odor, and helps neutralize mild toxins and pathogens. Browns are also fungal foods.

Greens supply nitrogen (ammonia or protein) will helps in microbial reproduction and internal heating. Greens are also bacterial foods.

I have a 3 acre farm, with about 1.5 acres in no-till beds. I have over 20 tons of compost stockpiles steaming in the woods in the backyard for myself and my customers.

I try to make about a ton of compost every 2-4 weeks. What I don't sell or stockpile, goes on my garden beds as a mulch on almost everything.

All of the research I've seen in the last 10 years into whether leaves will make soil acidic shows they do not. Woody mulches tend to aid soil acidity some because (according to Dr. Alex Shigo, retired chief of research USFS) woody mulches tend to encourage fungal activity which tends to make a lower soil pH while veggie waste (leaves) encourages bacterial activity which tends to raise soil pH.

Constant annual amending of the soil with humates from homemade compost, cover crops, and natural mulches, will eventually neutralize any soil beds toward pH = 7, regardless of any temporary acidicity or alkalinity from decomposing organic materials like wood ashes (alkakline), leaf mold (acidic), bone meal (alkaline), animal manures (alkaline), etc.