Answer: The weed you describe is yellow woodsorrel, which shows up in lawns when they are weak from environmental stress (heat, drought, etc.) I think you put your finger on the problem when you said your soil is acidic and is heavy clay. If you can get your soil in shape, your grass will naturally out-compete the weeds. Soil improvement takes time, but it pays off in the long run. First have your soil tested, and concentrate on getting organic matter into the clay soil. Gardener's Supply Company (firstname.lastname@example.org, or call 1-800-863-1700) has a lawn care bulletin and a soil building bulletin, both of which include information to help you get your lawn back in shape. Your extension office can help you with a soil test. Their phone number is 803-283-3302.
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