The Agriculture Department, which has spent billions subsidizing big farms, now has some free advice for the small-timer.

"It's called "Living on a Few Acres," a kind of soup-to-nuts on country life for the city dweller with an itch to get back to the land.

While not exactly a watershed, publication of the book represents the Agriculture Department's official awakening to a new constituency: people who earn their living in town but who prefer living in the country.

The 432-page yearbook, which can be acquired through congressional offices at no charge or from the Superintendent of Documents for $7, offers no Whitmanesque view of the land.

One chapter is headed, "Two-a-Day Milkings Kill Joy of Dairying for Most Families." Another chapter tells the would-be Walton Family that when the cold winter comes and the pipes burst, somebody has to go chop the ice to get water for the livestock.

The department decided to devote the 1979 yearbook to the part-time farmer 18 months ago after the Census Bureau had reported a back-to-the-land movement.

The census reported that between 1970 and 1975, non-metropolitan counties had added a net 4.3 million persons, after losing more than 3 million during the 1960s. Altogether 6.2 million persons left urban residences for life in the country during the first half of the decade, a historic reversal of the rural to urban migration.

But even the Agriculture Department doesn't know exactly how this has affected farming. Of roughly 2.5 million employed persons who moved from the city to the country during the early 1970s, only 114,000 took up farming as their primary occupation, according to USDA's Calvin Beale.

At the same time, the number of small farms has continued to decline, and the owners of those farms rely on other jobs for nearly 90 percent of their income.

But what Agriculture Department experts have discovered is that while the number of small farms has dropped steadily, the rate of decline has slowed somewhat. And they attribute this to the growing number of part-time farmers.

"Living on a Few Acres" is aimed at the family in farming as a hobby. They grow their own vegetables to lower their food costs, keep a few chickens for ambience and an occasional meal. They earn their living elsewhere and they want to keep their losses on the farm at a minimum.

This part of agricultural America has been largely ignored by the Agriculture Department, which has geared its programs to commercial farmers who keep the country fed.

Only a few years ago, it was virtually impossible to find information at the department for the part-time farmer, which is what James Peterson learned in 1976 after his family had moved from Silver Spring to a 26-acre farm near Frederick.

Peterson, who was at yesterday's unveiling with his wife, three boys and their rabbit Jumbo Jet, said when he asked for some help on getting started in chickens, he got a brochure on how to handle 10,000 broilers.

Thus "Living on a Few Acres" represents a shift in thinking at the department -- and it caused at least some minor problems in production. Jack Hayes, the editor, said he had trouble finding pictures within the department to illustrate some of the roughly four-dozen chapters. And he said finding good case studies was also difficult.