|For mountain laurel and violets in clay soil, is gypsum the best thing to add to lighten the soil?
|Clay soil is often called heavy and wet; the problem is that its particles are tiny and fit together tightly so that when it is wet it is sticky and when it is dry it is impermeable and hard. The best way to lighten it is to add a little coarse sand and plenty of organic matter, lots and lots of organic matter, on a regular basis. The relatively fluffy organic matter will increase the empty spaces in the soil so that it can hold air and allow water to both enter and drain more quickly -- in effect, lightening it. Over time, you need to keep replenishing the organic matter as it decays.
Mountain laurel (and other shallow-rooted acid-loving plants such as rhododendrons and azaleas) in particular require a well drained yet moist (not soggy) acid soil well enriched with organic matter. They will not grow in unamended heavy clay. Before attempting to growplants such as this , you might wish to perform some basic soil tests to help determine exactly which amendments would be best. (Your local UMASS Extension office can help you with the tests and interpreting the results. Their Eastern Region telephone number in Waltham is 891-0650.)
The violets on the other hand, should do well on a clay-based soil; prepare the bed as you would for a flower bed, working in a generous amount of organic matter and any other amendments as indicated by the soil tests.
For more information about evaluating your soil, amending it and selecting plants suited to it you might wish to look at a few basic gardening books; two I particularly like are "Gardening for Dummies" by Michael MacCaskey ISBN 1-56884-644-4 and "Perennials for Dummies" by Marcia Tatroe ISBN 0-7645-5030-6.