Every fall, and often in the spring too, I would pick up those bags of leaves people would set out by the curb for pickup, take them home, and often shred them to make compost or use for mulch or sometimes leave them whole and use them for mulch. Shredding those leaves allows the bacteria to digest them faster, although they will digest them even if they are left whole eventually just as they do in the forest. Whole leaves spread on the garden as mulch do as much good, eventually, as shredded leaves put on the garden as mulch, but those whole leaves can suppress some plant growth just as whole leaves cause problems with some early spring flowers by restricting access to the sunlight plants need to grow. Tilling those leaves in to the soil could cause a temporary loss of Nitrogen as the Soil Food Web uses that N to work at digesting those leaves just aas they do with any material that has a high Carbon to Nitrogen ratio, leaves range from 40 to 80 C:N ratio. Double digging might be necessary once, but has no real benefit if done once the soil problems have been corrected. Here is a link that might be useful: About Double Digging

Where in the world you are can be of tremendous assistance to some of us, although my soil is sand while my sisters soil (6 miles west of me) is clay. Usually sand atop a clay base indicates that someone dumped some sand there in an attempt to have a soil kind of easier to cultivate then the clay would be. The clay, under that sand, will keep that sand from draining properly, two different soil types with vastly different drainage capabilities. I would talk with the people at your local office of your state universities Cooperative Extension Service to find out what your soil type shoulld be.

Cactus soil would be great. I can't help you on amounts. I water mine about every two weeks until water starts appearing in the tray/saucer/underpot. When I see the water, I stop, and don't water it again for another two or three weeks. It takes a real effort to change over to the 'other side' of watering. I am a committed underwaterer, that is why Sansevierias work so well for me. Anything that takes a lot of water, or humidity? Well, as soon as it enters my door, it's death warrent is issued. I'm working on it. I do have a fern (Leatherleaf) sitting on the front porch, and I did take it through the house. We will see if any paper got signed...

I was thinking about this thread as I was doing some reading in various places about humus. Even wikipedia puts it this way about stable humus: " this is organic matter that has been protected from decomposition by microbial or enzyme action because it is hidden (occluded) inside small aggregates of soil particles or tightly attached (sorbed or complexed) to clays. Most humus that is not protected in this way is decomposed within ten years and can be regarded as less stable or more labile." It also refers specifically to lignin being one of the main precursors to humus, and that black carbon (e.g. biochar) also seems to contribute a lot to stability. Interesting the way that OM gets attached to clays. I don't know if this is actionable information or what it tells us exactly about what to do in this case; but I'd be tempted to place an emphasis on woody/pulpy inputs (or composts from them) in adding organic material, or at least plant material (stems? corn cobs/stalks? etc) or paper/cardboards that have fair amounts of lignin and get some charcoal/biochar in there too. Just speculating somewhat, but hopefully logically. This will have implications for nitrogen and other nutrient availability - i.e. it might make growing some types of plants or getting good yields harder, at least in short term - but might have benefits in getting there to more stable soil in long term. FWIW.