Answer: Clay soil responds best to additions of organic matter such as compost or chopped autumn leaves or milled spagnum peat moss. This helps loosen the soil and enables it to hold both air and water. You can till this in, or now that you have tilled, work it into the top ten inches of soil with a garden fork or spade. Then level the area and rake the surface smooth.
Using an organic mulch such as shredded hardwood bark or pine bark or straw other material available to you will also help feed the soil slowly over time as it breaks down. Mulch should be in place year round at a depth of about three inches spread out in a flat layer. Do not allow it to touch stems or trunks of plants. A pebble mulch does not help improve the soil and as you have noticed, tends to work down into the soil and eventually makes it difficult to dig there.
When moss grows it usually indicates an acidic soil that is compacted and/or poorly drained. Adding organic matter will help with that. You might want to run some basic soil tests and check the fertility, and also check the pH to see if you need to lime to adjust that.
Topsoil is not a regulated material so it is not usually recommended unless you have an excellent source for it.
Your local Penn State extension should be able to help you with the soil testing and interpreting the results. Enjoy your gardening!
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