Cashing in

Late every afternoon a young boy paws through trash barrels in a Boston shopping mall, picking out returnable bottles and cans, which he converts to cash at a nearby market. In a suburb 25 miles away similarly enterprising youngsters keep once-littered streets container-free; they, too, return bottles and cans to supermarkets for 5 or 10 cents apiece.

Picking up discarded beverage bottles and cans is a new source of pin money for entrepreneurial youth in the nine states that have deposit laws. And in the process the landscape is losing much of its litter. Since the first of the year three states have begun experiencing this happy phenomenon: Delaware, Massachusetts, and New York - which joined only last month.

Nine states now have deposit laws. Proponents plan to push similar bills in several states next year, making the additional assertion that deposit laws save communities money by decreasing the amount of solid waste by 6 to 8 percent, as well as by trimming the costs of picking litter.

Opponents, who defeated some attempts this year, can be expected to counter as they have in the past that recycling containers costs jobs in container-manufacturing plants, a debated point.

In any case, only the heavy glass bottles are again used to hold a beverage, after first having been scoured and boiled. Thin glass bottles are crushed, but at the moment the market for crushed glass is weak.

Aluminum cans are melted and are sought for use in the manufacture of new aluminum. Plastic bottles are shredded, then become fiber fill in upholstery, among other uses.

Far better that empty containers should be put to those uses than become unwanted ''decorations'' across the land.