Widely used nitrogen fertilizers present more a threat to the earth's critical ozone shield than either of the more heavily publicized dangers from spray can propellants or supersonic aircrafts, scientists studying the problem say.

Unless some means is found to control the effect to nitrous oxide exhausts from the fertilizer, the scientists predict that ozone depletion over the next century coule cause as much as 60 per cent increase in skin cancers.

Nitrous oxides, as well as fluorocarbons from spray can propellants and nitric oxide supersonic aircraft like the Anglo-French Concorde, all act the break down the ozone layer that girdles the earth at a height of more than 15 miles. Researchers here are worried, however, that the growing amount of nitrogen fertilizer might break down ozone at a far greater rate than either of the other two dangers.

According to researchers, a small amount of nitrous oxide is released directly from the fields where fertilizer is used. Nitrous oxide turns into nitric oxide during a chemicasl reaction with ozone. Nitric oxide acts as a catalyst to speed up the breakdown of ozone in the atmosphere.

Depletion of the ozone layer allows the sun's ultraviolet rays to reach the earth's surface in increasing amounts. The rays are believed responsible for causing malignant and nonmalignant skin tumors.

According to a rule of thumb generally used by researchers, a 1 per cent decrease in the ozone shield is likely to cause a 2 per cent increase in the number of skin cancer in the United State alone.

Dr. John Gille, head of the Upper Atmospheric Project at the National Center for Atmospheric Research here, said in an interview that the maximum projected ozone depletion could reach 30 per cent within a century if the current increase of nitrogen fertilizer use continues.

Gille and other researchers emphasyed, however, that there are a number of crucial unknowns in their projections. For example, scientist don't know whether the depletion would speed up or slow down after initial large-scale atmospheric changes of as little as 10 per cent ozone depletion take place. They also don't know how fast the depletion process works because of the complex procedure that releases nitrous oxide into the atmosphere from fertilizers.

Nor do researchers know precisely whether the combination of fluorocarbons from spray can propellants and nitrogen from fertilizers will produce a slowdown or speedup in the ozone depletion process.

"We are playing with a very complex system that we understand very little of." said Michael McElroy, a Harvard researcher studying the problem. Nevertheless, he added, it will be difficult to avoid large-scale depletion unless some solution is found to the fertilizer problem.

At present the use of nitrogen fertilisers is increasing at the rate of 6 per cent annually with the sharpest increase in underdeveloped countries. But most researchers believe that growth level will slack off in part because of the heavy use of petroleum required to make nitrogen fertilizer.

McElroy, who is director of Harvard's Center for Earth and Planetary Sciences, estimated that present levels of fluorocarbons would cause a 7 per cent ozone reduction. An end to productioin of flourocarbon sprays has been recommended nationally and Oregon has already banned their sale.

According to McElroy, a conservative projection of the growth of nitrogen fertilizer use would deplete the ozone layer by 10 per cent over the coming century.

Skin cancer is not the only harmful effect that could be caused by ozone depletions, scientists said. A report by the Academy of Sciences in 1972 predicted that an ozone breakdown and the resulting increase in ultraviolet rays could affect world crop production and the growth of photoplankton, the ocean 's basic food source.

Unless some solution is found the alternative are becoming concerned the alternative of decreasing fertilizer use could pose equally serious problems.

"The thing that makes this more difficult," said Gille, "is that you can ban hairsprays and supersonic planes and not destroy civilization. But we need fertilizer to feed the world's growing population."