Answer: The best way to loosen and lighten clay soil is to add lots of organic matter. The best is compost or well rotted manure. Use organic materials as mulch, such as shredded leaves or grass clippings, and turn them under at the end of the season.
I would not bother to add sand. Though it may help improve drainage, it won't do anything to improve soil structure, which is really what you are after. Some people have had good luck adding gypsum to clay soils to help lighten them. Garden centers may have this soil amendment.
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