TO HELP PRESERVE agriculture and open space where land values are going up, some states - including Virginia and Maryland - give preferential property-tax treatment to farms, forests and the like. The concept is fine. Unless such laws are carefully written, though, they also benefit developers who can afford to buy large tracts, hold them long enough to get the tax breaks, and then turn the fields into a subdivision on shopping mall.

Such problems have developed in Prince William County. One landowner there, J.V. Elrod of Arlington, won a reduced assessment on a prime tract and then sold part of it for a proposed regional shopping center. Then Hylton Enterprises, Inc., the Dale City developer, enrolled 26 parcels totaling 2,685 acres in the land-use program, mostly as "forested" land, and cut the taxes on those properties from about $227,000 to around $10,000. According to the Potomac News, a Hylton spokesman said that showed how "ludicrous" the program is.

Developers and other corporations should not be barred from getting such tax breaks if they make long-term commitments to farming, forestry, public recreation or other non-intensive uses of their land. Virginia's law, though, is so broad that landowners can make hay without growing anything. The state's definition of "forests," for instance, is much less strict than that used in Maryland. Such laxity discredits the law and discourages its use in precisely those areas - such as Prince William, Loudoun and Fairfax counties - where a good conservation program would be most valuable. As Del. Raymond Vickery (D-Fairfax) and other have been arguing, the General Assembly should tighten up the law.