If a man's home indeed is his castle then it stands to reason that his lawn must be the moat, or part of the castle grounds.

This vexing question of turf is troubling the U.S. Postal Service, which has enough problems without being hit by a nationwide keep-off-the-grass situation.

The problem is growing like crab-grass for the USPS. Last week, it was told by a U.S. District judge in St. Louis that letter carriers in that city must get permission from homeowners before they cut across the lawns on their appointed rounds.

This raises the prospect that the USPS may be affected by local lawn-crossing ordinances across the nation. That, in turn, could require mailmen to get written permission from homeowners to cut across their lawns, or perhaps even force the USPS to buy "rights of way" from customers who object to people, even the mailmen, treading on their sod.

The issue is much deeper than just grass. Jobs, safety and efficiency are at the bottom of the hassle.

For some years now the National Association of Letter Carriers, representing most of the country's outside mail delivery employes, says the USPS has been endangering members' lives, health and jobs, by pushing a "speedup" campaign. That speedup, a union alleges, forces letter carriers to cut across lawns, leaps hedges, fight off dogs and get tangled up in lawnmowers, kiddie cars and whatever else is left lying around the house.

The USPS, in its labor agreement with the union, says a "letter carrier may cross lawns while making deliveries if customers do not object and there is no particular hazard to the carriers . . ."

In effect, the USPS has been operating under the theory that the mailman can cross your lawns unless you tell him not to do so. But the ruling of U.S. District Judge J. H. Meredith says otherwise. He wrote:

"For a postman to beat a daily pathway across a householder's lawn is a violation of that householder's Fifth Amendment rights. Nor is allowing the householder the 'right' to object a sufficient protection of his Fifth Amendment rights. The government cannot choose to act on the presumption that householders will accept official shortcuts."

Gustave J. Johnson, secretary-treasurer of the Letter Carriers union, does not find the lawn-crossing issue a laughing matter. Johnson says nearly 65 percent of his members now retire on disability, and many of their medical problems stem from speedups and the hazards of delivering the mail to an estimated 63 million homes.

Johnson says that the USPS had to buy a Massachusetts homeowner a new "living fence" because the carrier rubbed a hole in it making regular deliveries.

In Florida, Johnson said, a letter carrier was seriously injured when he got tangled up in a barbed wire fence an irate customer put up one night.

Although the keep-off-the-grass ban now applies only to the St. Louis area, the union says it will push for more lawn-crossing ordnances, and for decisions to protect the grass. USPS lawyers say they are considering an appeal, hoping to head off the paperwork that would be involved in getting written permission from homeowners to cross lawns, or the expense and trouble of being required to buy federal rights-of-way for postmen.

Half Staff: The State Department's Foreign Service Institute in Arlington has been hit by an unusual bug that has kept an estimated 50 percent of its 200 instructors off the job Monday and Tuesday.

Union officials there say the employes are calling in sick to protest management's refusal to talk with them about higher job classifications and pay. Instructors now range from $9,959 to $21,604. That is less, they say, than linguists and supervisors get.

State Department officials say they hope the teachers will return to work today. Even if they do, representatives of the American Federation of Government Employes union say the point has been made. Classes have gone on, officials say, with "some obvious inconvenience and program changes."