Answer: You can easily add a layer several inches thick of organic matter such as peat moss (fluff and dampen it first) or compost or rotted down autumn leaves. I would probably not recommend using humus unless you have very sandy soil as it tends to turn somewhat mucky when wet. Top soil is not a regulated material, so its quality can vary. For that reason I would suggest just add organic matter (if you dig down twelve inches you could theoretically add up to six inches or organic matter, but that is a huge cubic quantity) and if the native soil is very clay-y, perhaps a little bit of coarse builders' sand (not the fine play sand) or fine grit. Then use an organic mulch year round to help feed the soil slowly and provide additional organic matter on an ongoing basis as it breaks down over time.
You should also run some basic soil tests to check the fertility levels and also the pH in case you need to add lime. If you are starting a new flower bed and need to lime it is best to work the lime into the soil before you plant. Your local county extension should be able to help you with the soil tests and interpreting the results. Good luck with your flower bed this summer!
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