How do you keep 'em down on the farm when net income for a farmer can average less the $5,000 in a good year or less than $1,500 in a bad year, such as 1980?

How do you keep 'em down on the farm when an acre yields $350 a year in corn sales, but the same land can bring $8,000 to $10,000 if it is sold to a developer?

These are tough questions, and they are being asked in many states where farmland is slowly disappearing as farmers find they simply can't afford to stay on the land. In Virginia, the questions have a special urgency.

Only three states -- Florida, Texas and California -- are converting agricultural land to urban uses faster than Virginia, according to new and preliminary estimates by the federal government's National Agricultural Lands Study.

At first glance, the study seems to be at odds with recent statistics on Virginia, which showed that farmland has increased in the last decade, especially in areas near urban centers.

But Berkwood M. Farmer, chief economist for the Virginia Department of Agriculture and Consumer Affairs, believes the federal analysis is probably closer to the mark.

Farmer contends the more optimistic statistics are flawed because they are based on 1974 figures, which underestimated the amount of Virginia land being used for agricultural purposes. Even when land is converted to agricultural use, Farmer says, it tends to be marginal and therefore less productive than the prime, rich-soil acreage that is relentlessly claimed for subdivisions.

The disappearance of farmland used to be an issue that was urgent only to do-gooders who also wanted to save wells, old buildings and redwood trees. But, as the United States begins relying on agricultural exports to prevent its trade ledgers from being swamped by the black ink of petroleum imports, many observers believe the loss of farmland will become an issue that is important to everyone.

Ten years ago, according to Robert J. Gray, executive director of the federal land study, only one out of six acres was used to produce exported food. Today, he says, the figure is one out of three acres, and by the year 2,000, he predicts, the ratio may be one out of two.

In Virginia, these figures are beginning to sink in. The General Assembly has appointed a panel -- the Joint Agriculture Land Preservation Subcommittee -- that is exploring a range of proposals designed to halt the loss of prime farmland.

While the committee is moving cautiously, it has sent out a clear message at its workshops and meetings so far: We will look at every technique to save farmland, even those that seem outside the "Virginia way."

In sessions held recently in Leesburg, in the center of Northern Virginia's agricultural country, and in Lexington, in the farm-rich Shenandoah Valley, the committee listened intently to proposals that, in the conservative Old Dominion, sound positively subversive.

One is the concept of transferable development rights. Under that plan, farmers who agree to keep their land for agricultural uses can sell the land's potential to a developer, who then can apply that potential to another piece of land that has been set aside for urban uses.

TDRs, as the transferable rights are called, are a direct attempt to deal with the problem of equity: Is it fair to tell farmers that they can never realize a profit from the sale of their land for a more intense use? Although the idea gets to the heart of an important question, only 184 acres have had their development rights transferred under programs in effect in the United States since 1972.

Montgomery County, in suburban Maryland, has agreed to try the idea, but given the less conservative legal and planning atmosphere there, it is not too clear how much of an example can be set for traditionally conservative Virginia.

Another idea being considered by the legislative committee involves state tax credits for farmers who don't sell their land for development. Tax credits, which are being tried in Wisconsin on a local-option basis, go far beyond the land-use tax being tried in some Virginia counties (including Loudoun and Prince William). Under the land-use tax, agricultural land is assessed at a lower rate, which is voided retroactively if the land is sold. i

Still another proposal -- and probably the most controversial -- is agricultural zoning. Such zoning would flatly prohibit farmers from allowing their land to be used for anything but agricultural use.The closest Virginia has come to agricultural zoning is setting up agricultural districts, which offer a series of incentives for keeping land in agricultural use but do not prohibit landowners from exercising their right to develop the land.

The problem with all the proposals the committee has heard so far is that they are so new that legislators have little evidence to judge whether the proposals would work. Moreover, most of the plans are being tried in states with stronger traditions of controlling development than Virginia.

These thoughts were probably on the mind of subcommittee chairman Ford Quillen (D-Pound) when he said at the Leesburg session, "I'm not so sure if we will do anything at all" in preparing recommendations for more state laws in protecting farmland.

Despite a reluctance to plunge ahead too hastily, the Virginia legislators are getting a message that is unmistakably clear, whether it's being delivered by preservationists, local officials or even farmers such as John Rocca, of Loudoun County.

"Rome is burning today," Rocca recently told the committee. "There is just not any more time left. . . We need something done to save our farmland. I don't know what it is. . . But we've got to do something."