Montgomery County is considering changes in its zoning laws that planners say would preserve the agricultural character of almost one third of the county from encroaching commercial and residential development.

Under the proposals, commercial ventures such as nurseries, riding stables and roadside produce markets -- now allowed by right in rural zones -- would be allowed only by special exception.

"These commercial ventures do have impact on rural areas, and the proposed zoning changes would mean the county would have more control," said Joan N. Yamamoto, principal planning analyst for Montgomery.

The proposed zone changes follow a controversial debate between the county and the commercial horticultural industry, whose spokesmen say it is becoming difficult to find affordable land for nurseries and landscaping centers in Southern Maryland because of zoning restrictions.

Yamamoto said the county's review of the agricultural zones predates the controversy and is part of a massive project to overhaul the county zoning ordinance. In the past few years, the county has redefined zoning for towns and corrected a number of inconsistencies in the zoning law, she said.

This year the county has taken up the question of rural zones with the idea of encouraging farming and making commercial ventures less attractive, said Melissa C. Banach, a planning coordinator for the county.

Current Montgomery law contains three zoning classifications -- residential, commercial and industrial. The proposed ordinance would add a new classification -- agricultural -- that would encompass three existing zones called rural, rural cluster and rural density transfer.

Jeff Zyontz, a data tracker in the planning office, estimates that the three zones account for almost one third of the county's surface area. The rural density transfer zone is the county's largest, encompassing more than 110,000 acres, he said.

The three zones, currently classified residential, allow various mixes of agricultural and low-density residential development, with the largest lot size limited to five acres.

"By grouping the three, their agricultural character would be emphasized," concluded a staff report on the proposed changes.

The most significant proposed changes would remove several permitted uses from the three zones and put them on a list of special exceptions permitted if the county finds the use compatible with the area.

Farm spokesmen interviewed this week said this proposal worries them because they are concerned that farmers may be caught in the net designed for commercial users.

"We need to be sure language relating to those enterprises does not affect a farmer," said William P. Anderson, vice president of the county Farm Bureau. "To say no commercial greenhouses is fine. But what about the farmer with a greenhouse raising products to be sold -- he has to be protected."

Yamamoto said county planners have had several meetings with farmers to work out problems with the proposed changes, as well as with county attorneys to determine the legality of the proposals.

"Sometimes it's difficult to explain to people in government what is a farm and what isn't," said Anderson, who rents his family's 450-acre grain farm near Dickerson.

However, only strictly commercial enterprises would need special exception permits, according to a staff report on the proposed zoning changes. For instance, large roadside markets where goods have been trucked in would need a permit. Small roadside stands, set up by operators of working farms, still would be permitted by right, but would have to observe some sanitary rules, according to the report.

"We can live with that," said Anderson. "We understand what the county is trying to do. We just are concerned that they get the differences between the two working farms and commercial enterprises right."

Anderson said farmers see the proposed zone changes as tying in with recent programs allowing farmers to sell development rights to the state. This allows farmers to receive compensation for their land value and continue a way of life.

"It is obvious that the county is taking a special look at ways to preserve its agricultural resources," he said.

The county planning board will continue work sessions with farmers and advisory boards until sometime this summer, when it will forward the proposed changes to the county council for final action.