I want to add 3 inches of mushroom manure to the top of a small lawn with cardboard as a mulch underneath and a layer of leaves from the fall. There are other soils available that say that they are composted manure and sand but I wonder if they are actually city compost (which I don't want) and I'm not sure about horse/chicken manure. The mushroom manure has no sand but I'm wondering if it will be suitable. I don't have the energy to dig into the clay. We have quite a bit of rain where I live but not going to walk on it much.
If the question is whether it's okay to apply mushroom manure over a cardboard and fall leave mulch, over a clay soil then yes. Any organic material added to clay will eventually improve the soil and if you have earthworms, they will pull the material into the soil.
I think I'd prefer to use wet newspaper instead of cardboard though.
Purchasing soil amendments is, in my opinion, almost always more valuable than purchasing soil.
Clay soils do benefit from the addition of sand, which you mention is included in one of the “manufactured” soils. In my experience however these soils usually contain too much sand and not enough compost.
I agree with your idea of spreading mushroom compost over cardboard to improve your clay soil, with the following recommendations:
- Spread gypsum - 2 pounds per square yard / 1 kilogram per square metre;
- Place cardboard (or thick newspaper) and wet down;
- Spread mushroom compost - to thickness of 4 to 6 inches / 10 to 15 centimetres;
- Lay out mulch material - depending on proposed use, straw (or hay if affordable) for vegetable gardens, wood chip for ornamental gardens - to thickness 1 to 2 inches / 2.5 to 5 centimetres.
No need to do dig anything - that’s what earthworms are for. However if you’re able to borrow or hire a broadfork for a day and use this to begin to open up compacted clay, it will greatly accelerate the soil improvement process.
Note: Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO4•2H2O. Gypsum is often known as a “clay breaker” but in fact it does the opposite - it binds the tiny clay soil particles together to form clumps, thereby greatly improving water penetration and drainage. The calcium in gypsum also helps to “sweeten” (make less acidic) clay soils that are often acidic. A soil ph test kit can be purchased cheaply and may help you decide whether to add extra lime (calcium carbonate) to balance soil pH.