HOW SOFT is soft? How hard is hard? Opinions differ when it comes to water. You are more likely to need a water softener tank system if you use well water, less if you are hooked up to the prime water sources in the Washington area.

The experts in charge of the area's water supply think the city water is soft enough. The people who make and sell water softener systems challenge this view. If you do want to soften your water, a number of firms are anxious to help you.

It may be wishful thinking but David McQuilkin of MESSCO, the area's rep for Bruner Water Softeners & Filters, believes that water is becoming harder in the Washington area."Our area is low in sales of water softeners compared to other areas in the country such as New York City and out West, but in the last 10 years we've noticed a big increase in sales. The more chemicals that are put in the water to fight bacteria, the harder our water."

Beverly warfield of the Washington Suburban Sanitary Commission (WSSC) says she doesn't think water is getting harder in the Washington area. "The only thing that would make it harder," she says, "is a dry spell that would increase the calcium and magnesium levels in the water."

Harder or not, McQuilkin says they've had a number of requests from restaurants for water softeners. "Many of our sales," says McQuilkin, Have been from restaurants where they need to keep the mineral build-up inside their dishwashers to a minimum," says McQuilkin. "Softer water also prevents silverware from spotting. It can drip-dry without spots. The same goes for washing a car. When you allow a car washed in hard water to drip-dry, it always leaves streaks. These streaks -- actually calcium builup -- are very hard on the paint."

The Water Quality Association's consumer information specialist, Judi Ford Johnson, believes the water in the Washington area is hard enough to require a mechanical softener system in every home. Johnson says soft water is a real money saver. (The Water Quality Association, an organizatin of water softener manufacturers, distributors, retailers and wholesalers, gives gold seals to members' products, and runs a program for water treatment technicians.)

Johnson says that "soft water feels better on your skin. It doesn't leave the calcium and magnesium buildup that hard water does. Soft water also block pores or cause itching, as hard water can.

"Soft water also leaves your hair shinier, because again no residue is left."

The Water Quality Association claims soft water makes stains wash out easier in clothes and ice cubes clearer.

Claims Johnson: "Hard water requires 10 times more soap than does soft water." Johnson illustrated this with two test tubes -- one filled with hard water, the other with soft. The hard water required 10 drops of soap to work up a good lather, while the soft water needed one.

At Messco, says McQuilkin, they will test your water for hardness for 70 cents to cover mailing the test tube of water.

"Hard water also makes a heating system boiler work harder," says Johnson, "because the mineral builds up on the pipes -- similar to the lime residue you'll find on a coffee pot after years of use. This buildup can eventually block the inside of the pipe."

Says Warfield, "In Montgomery and P.G. counties, as far as I can see, the water isn't hard enough to require a water softener. Father north, it may be necessary, but I don't think we need them here."

Water softening tanks come in one or two units that most often are hooked up to the main pipe where water enters the home -- usually in the basement. The tanks are round, about four feet high and two feet in diameter. They start at $350 and go up to $1200.

When water enters the tank, it is filtered through a bead-type screen made of resin. The resin holds onto the magnesium and clacium. As the water passes through it leaves these minerals behind, and becomes soft water. Salt water in another part of the unit is used to keep the resin clean, otherwise the minerals would build up and damage the resin's effect. The salt or brine solution is flushed through the resin. An ion exchange takes place. Sodium ions are exchanged for magnesium and calcium ions.

One of the biggest objections to water softeners come from physicians who warn that many people cannot tolerate the salt used in the softening process. Excess salt can be a health hazard. Salt or sodium is present naturally in the area's water, but at levels far below what is dangerous to someone watching their salt intake, according to Warfield.

"Adding sodium to your water worries some people who can't have salt in their diet," McQuilkin admits. "For these people we suggest hooking up the softener to everything except their drinking water." For instance, the softener could be hooked up only to the hot water supply. This complicated procedure should be discussed with your plumber.

Richard Foti, general manager for Rain Soft water conditioners in Rockville disagrees. This defeats the purpose of having a water softener he says. For instance, in a bath, when you mix softened hot water with hard cold water, the mixed water will be only half as soft.

Foti believes that the amount of salt in softened water is negligible even for the person on a low salt diet.

Dr. Robert Golden of the National Academy of Sciences says that the sodium chloride that comes from the ion exchange has some health complications. "Softened water contains a hidden source of salt that people trying to maintain themselves on a low salt diet have to consider, in addition to the food they eat."

In any case, when buying a water softening system, ask for an instruction pamphlet, warned McQuilkin, so you'll know how to maintain it.