Answer: 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. Improving soil takes lots of patience and several years worth of amendments. You can speed the process a bit by mulching over the top of the soil after planting, with the ultimate goal of turning the mulch into the soil at the end of the season. Another help is growing green manure cover crops which you turn under in the spring prior to planting again.
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