A proposal to protect about 50,000 acres of prime Montgomery County farmland from suburban development by allowing only one dwelling unit per 25 acres was defeated yesterday on a tie vote of the Montgomery County Planning Board.

"The inability of the board to act places us in the position of not being able to support agriculture in this county," said Chairman Royce Hanson, a former farmer who led a 90-minute campaign in favor of the plan.

"I feel a real sense of loss and desperation."

But planning commissioner Richmond M. Keeney argued that the measure would deny property rights by reducing the value of the land without adequately compensating owners.

The idea also got little support from uncounty farmers.

More than a third of Montgomery County's 316,8000 acres still is in agriculture, but constant development pressures have converted more than 7,600 acres of former farmland to subdivisions in the past six years.

Hanson's citation of these figures and his remark that "the only thing I'm certain of is, once it's gone, it's gone, and you aren't going to raise many soybeans on it," failed to win the necessary votes.

Commissioner Helen Scharf voted with Keeney, and Mable Granke agreed with Hanson. The fifth commissioner, George Kephart, who had indicated support for the idea earlier this week, disqualified himself yesterday because he owns a farm.

The idea took most developers and farmers by surprise this week.

"I think they're crazy as hell," said Frank Jamison, a realtor in the western Poolesville area. "Everybody wants a couple of acres of ground for a horse or two and maybe a cow, and tis effectively would slim down that American dream."

"These large lot developments usually have just the opposite effect from what they're supposed to," said Billy Anderson, a Farm Bureau leader. "You just eat up the ground with mini-estates."

In 1973, the planning board and county council converted the two-acre rural zone into five-acre zoning in an attempt to minimize development of agricultural land and were accused of favoring a "no-growth" plicy.

The latest proposal "harks back to that exclusionary zoning," said Ed Crowley of Kettler Brothers, developers of Montgomery Village. "At the same time, the hue and cry is for lowcost housing," Crowley said. "Maybe this is fine -- if you live in a tent."

The planners proposed the zoning change as an interim, 24-month measure until more permanent approaches to agricultural preservation can be developed.