Two very good books about gardening have been published recently. One is about composting and how to do it and the other is about plant propagation. Both can be very useful at this time of the year.

For at least 2,000 years, man depended on compost to sustain his croplands and to feed himself and others, according to "The Rodale Guide to Composting," by Jerry Minnich, Majorie Hunt and editors of Organic Gardening (Rodale Press, 405 pp., well-illustrated, $12.95).

Composting is a form of recycling and returning organic wastes to the use of the land from which they originally came. It is one of the most basic means of conservation. It builds good soil structure.

Good structure allows a soil to breathe and facilitates circulation. A heavy clay soil tends to become waterlogged quickly, preventing water and air penetration. Adding compost helps to loosen this packed soil by opening up pore spaces that, like little tunnels, carry air and water down into the soil. A crumb structure is built, and a thin film of moisture is held on each crumb of soil where plant roots can utilize it as needed, the authors say.

The higher the humus content, the more moisture a soil can absorb and retain. Soil with ample organic matter lets raindrops seep gently into it, instead of splattering and churning up soil particles.

In the case of packed, crusted soil, a muddy drop of water is formed which will run over the soil surface as the first stage of erosion. A heavy rainstorm may result in considerable runoff which carries away soil.

Sandy soils, which tend to let water drain away too rapidly, are also rebuilt by the addition of compost. The fine particles are united into large ones than can hold greater quantities of water in films on their surfaces. The higher the humus content, the more moisture a soil can absorb and retain.

Compost is an excellent vehicle for carrying nutrients to your soil and plants. With a good compost program, food crops and ornamentals will need no other form of fertilization. The nutrients occurring in compost are released slowly at a rate at which plants can use them; the compost is not only a source of nutrients, but a storehouse for them.

The book contains the latest information about composting with a wealth of photos and illustrations of materials and tools involved. It is a compilation of the whys and wherefores of what it is all about.

How to increase the number of plants in your home and garden by division, grafting, layering, cuttings, bulbs and tubers, sowing seeds and spores is completely covered in "Plant Propagation in Pictures," by Montague Free, revised and edited by Majorie J. Dietz (Doubleday, 253 pp., well-illustrated, $9.95).

Free's book, originally published in 1957, has been judged by experts to be one of the most thorough on the subject. Now it has been completely updated and expanded where necessary by Dietz. She is a former associate editor of the Brooklyn Botanic Garden's quarterly gardening journal, Plants & Gardens, is a former editor of Flower Grower magazine, which later was named Home Garden magazine, and is the author of several books.

Everyone who maintains a garden practices plant propagation to a certain extent, even though it may be only the scattering of seeds, according to the book.

But sometimes, raising plants from seeds is not so simple for certain seeds require special handling to ensure satisfactory germination. A knowledge of the various factors influencing germination is of immense value to the gardener whether in the backyard or professional category.

There are some plants which do not come true from seeds. A knowledge of the various ways of vegetative propagation enables us to increase our stock of these plants by division, cuttings, grating, or budding, methods which as a rule ensure the perpetuation of the desirable characteristics (and also,unfortunately, the undersirable ones) of the parent plant.

It may be maintained that these specialized methods belong in the province of the professional - the nurseryman and the commercial grower.

This is true in part, but there are many occasions when it is desirable for the home gardener to do his own propagating, and knowing how is essential.