Most gardening books tell gardeners what to do but rarely tell them why they should do it. If the reason is known and its importance understood, gardening can be more interesting as well as more successful. One very good new book covers the why of gardening and another concentrates on the practical tasks and provides information about how to create and maintain a garden to be proud of.

"Why Does Your Garden Grow? The Facts of Plant Life," by William R. Van Dersal (Quadrangle/The New York Times Book Co., 210 pages, illustrated, $8.95).

Van Dersal, a resident of Arlington, was former deputy administrator of the Soil Conservation Service, USDA, and now is dean of the Management College, National Graduate University, here. His book is easy to read and understand.

"It is not a book about how to grow things, except more or less incidentally," he says, "but rather a book that attempts to provide some basic knowledge about plants for the benefit of practical gardeners. There are certain key scientific facts and ideas that are of direct and practical value in gardening. It is with these ideas that this book is concerned. It can bring about a better understanding of what's going on in the garden."

A number of scientists have been studying insect pest control and plant diseases, he says. The results of this study, especially in recent years, are leading them to think more carefully about applying poisonous chemicals.

"The use of such chemicals almost always has a number of side effects we really did not intend at all. Many insects are kept under fair control by parasites. If the chemicals we use kill them too, we've lost some helpful allies. We may, indeed, be killing an insect pest already parasitized which means that the parasite, unable to complete its life cycle, goes down the drain along with the insect pest."

"Practical Gardening Encyclopedia," is by a group of leading British horticulturists (Van Nostrand Reinhold, 350 pages, superbly illustrated, $19.95).

Among other things, the book covers types of soil, soil testing, climate and its effect on plants, design and planning of bedding landscapes, an explanation of how plants grow, all phases of cultivation, propagation, evaluations of ornamental plants, and how to take care of pests and diseases. The index includes more than 2,500 entries.

This encyclopedia is designed as a reference work, but the arrangement of information is thematic rather than alphabetical. This enables the reader who wants to pursue a subject in greater depth to read through an entire section, while the index may be consulted by a reader with a specific problem or query.

For example, if you are planning a big new garden project, perhaps a new vegetable and fruit garden, you should turn to the section on edible plants, where you will find information about how much room fruit and vegetables need, how long you will have to wait for your crops, how much trouble they are and so on. The information should help you decide which fruit and vegetables you want to grow and give you most details of how to grow them.