Answer: The very best thing you can do for yourself and your garden is to find out exactly WHAT kind of soil you really have. The best recommendation I can give you is to get your soil tested. This can usually be done through your University Extension Service office. Sometimes garden centers do this as well.
Here is the phone number and web site for UMass Extension is:(413)545-4800 web site: http://www.umass.edu/umext/
Generally when the soil test comes back they will include directions on exactly what you need to do to amend that soil making it wonderful for planting.
As a rule, clay soil does not drain very well so you can probably tell you have it if your soil "puddles". Pick up an handful of moist soil. If you can form it into a smooth ball, and it stays in that shape without crumbling, it's probably clay. If you've ever done any pottery--think about what the clay looks and feels like! Clay is made up of very fine particles, and packs down hard.
The best thing to do to loosen and lighten clay soil is to add lots of organic material like compost, well-rotted manure, grass clippings, leaves, etc. This allows the moisture to be absorbed and held by the soil for use by roots, and also improves drainage, allowing air to enter the soil making it more hospitable for worms and other "earth-healthy" creatures.
Clay soils are often acidic; the soil test should tell you if, and how much, lime you need to add to raise the pH. It takes some time to improve clay soils!
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