"Thatch is a spongy layer of dead, undecomposed grass roots and stems that form a dense tangle on top of the soil. Lawns get thatch either because they get too much nitrogen or because they've been treated with chemicals that have killed or repelled the microorganisms and earthworms that decompose the thatch.
"Dropping your grass clippings on the lawn does not - I repeat, does not - cause thatch. Those clippings are so high in nitrogen they decay in a jiffy, feeding the soil."It's easy to check for thatch. Just cut a slice of lawn from the yard and look at a cross-section. The thatch is a light brownish layer on the soil surface.
"A little bit of thatch isn't bad; it acts like a mulch to hold moisture in the soil. But once it gets to be more than about a 1/2-inch thick, grass roots start growing in it instead of the soil. Since thatch doesn't hold moisture as well as soil, the roots dry easily, so the lawn needs frequent watering. Thatch also is a perfect place for insects and disease to thrive."
Gardening Bookshelf is an occasional feature that excerpts advice from a newly released how-to book.