It is ironic that The Post laments the "ludicrous shortcoming" of too little school time being devoted to the teaching of the humanities {editorial, Sept. 6}. That American youngsters don't know elementary historical facts about their own country is only the latest in a long list of deficiencies that have been identified during recent years.

Studies have shown that they also need to know much more mathematics and science. They need to read more good literature, and they need to write much more. They need more exposure to foreign languages, and they need to add geography to their curriculum. Sufficient time for all the above could be found, according to The Post and the teachers' unions, by making more efficient use of the school day.

Could it? American youngsters are in the classroom fewer hours each day, and significantly fewer days each year, than those in any other country in the industrialized world.

And disturbing as their ignorance of their own history is (especially since, for American children, there is relatively little of it), it is far more distressing that otherwise intelligent and well-meaning adults do not even question why most of Europe, Japan and the Soviet Union, nations not known to be profligate with their financial resources, find it worthwhile to require school years of 200-240 days. If those countries haven't yet been able to figure out how to fit a quart into a pint bottle, what makes us think we can? GENEVE S. MAROON Washington