If you've ever hankered to own a farm -- a fertile, bucolic spread that will grow in capital value -- you can do it on financial terms this winter that well could be the lowest obtainable for many years.

That's the bittersweet message from America's rural heartland as l985 draws to a close. And the message is being heeded. Without fanfare, small groups of individual investors, plus some big-bucks foreign and domestic buyers, are beginning to make their moves into the rural-land marketplace. Brokers active in the land business report a significant rise in investor interest, thanks in large part to highly favorable combinations of prices and mortgage-money terms.

In dozens of prime agricultural areas across the country (with the notable exceptions of New England, Texas and New Jersey), acreage prices have begun bottoming out at dollar levels unseen since the mid-1970s. Data compiled this fall by the Department of Agriculture reveal that prices per acre have fallen by an average 12 percent in the past 12 months and by 19 percent nationwide since 1981.

Cumulative drops in average resale values of 35 percent or more have been racked up in the past five years in states such as Illinois, Ohio, Indiana, Iowa, Missouri and Nebraska, according to Agriculture. Prices of California farmland remained flat during those years, and only a handful of states registered net gains on average. They were Texas (up by 45 percent, including 10 percent the past year); New England (up by a cumulative 29 percent, 14 percent the last 12 months); and New Jersey (up by 9 percent the last year).

Analysts say what's now catching the attention of savvy small- and large-scale investors is the prospect that land values have reached their economic nadir and can only go up, at least in some key markets. Prices have stabilized for top-yielding prime farmland in parts of the Corn Belt such as central Illinois, according to Ralph Trail, senior vice president of Oppenheimer Industries Inc., the second-largest farm-management company in the country.

From his base in Kansas City, Trail monitors agricultural values throughout the Midwest. He also works with absentee investors "from California to New York" putting together land-acquisition and financing packages from banks. These include small groups whose members individually never could afford to buy a farm. Although a minority of current acquisitions are all-cash transactions, most purchases are made with the help of low-down-payment mortgages at below-market rates.

Let's say you and a couple of associates are interested in buying a 160-acre farm as a passive investment. The land is l00 percent leased at $125 per acre per year to the farmer who owns and works the 300-acre parcel next door. He grows corn or soybeans as the principal crops. The land is top quality.

Depending on its location, such a farm may carry a price tag of $275,000 to $385,000, or $1,800 to $2,400 an acre, according to Trail. Five years ago, the price would have been closer to $3,500 an acre. A group of small-scale investors anywhere in the country might acquire the farm on terms something like these: l0 to 20 percent down (say $30,000 to $65,000) and fixed-rate interest at l0 percent for five years or more.

Some investors have been able to acquire prime farmland simply by taking over the payments on long-delinquent mortgages -- for no money down -- according to Trail. "But no one should bank on that from the start," he said.

Oppenheimer, which manages 800,000 acres of farms for absentee investors nationwide, would take over complete on-site supervision of the property in the typical transactions arranged by Trail. For l0 percent of gross crop revenue, Oppenheimer representatives run the farm, sell the crops and prepare budgets, financial statements and even the tax returns for the investors.

Although the net after-tax returns to investors vary, most are in the 6 to 8 percent annual range, according to Trail. He says the real payoff comes five to seven years down the road "when land that once sold for nearly double what you bought it for moves back up in that direction" from current, depressed levels.

"This is no real estate tax shelter," Trail cautions would-be investors. "This is buying land at its low point, taking a modest economic gain every year and selling high when the market rebounds -- as it most certainly will for prime acreage."

If you're as certain about the coming rebound as Trail is -- and all analysts do not share his glowing optimism -- absentee ownership of "black gold" from the Corn Belt could be an intriguing real estate investment.