IF THE AIR seems lighter today with music, perhaps it is because more than a few of us have Irving Berlin and his songs on our minds. He is 90 today. If the birthday were celebrated nationwide, a chorus of thanks would be heard across the land for this master who has been joining words to music and hearts to merriment since 1907.

The vast range of Mr. Berlin's songs takes us from the casual knock about lyrics of "Oh, How I Hate to Get Up in the Morning" to the patriotic peaks of "God Bless America." He wrote songs for lovers ("Always"), funsters ("You Can't Get a Man With a Gun"), strutters ("Alexander's Ragtime Band"), sentimentalists ("White Christmas"), romantics ("There's No Business Like Show Business") and even politicians, as in Gov. Al Smith's 1928 campaign song, "Good Times With Hoover, Better Times With Al." Those are among the better known of the 1,000-odd Berlin songs, but as for the best remembered, who knows what favorites lurk in the sunny memories of his fans?

President Carter has sent a birthday letter to Mr. Berlin - "God Bless Irving Berlin!" is part of the message - but, happily, recognition and honors do not come belatedly. In 1955, President Eisenhower presented Mr. Berlin with a congressionally authorized gold medal. The former general and the former infantry private enjoyed a lighthearted moment when Ike confessed that he was relieved Congress didn't dawdle in its approval of the medal: "I was so delighted it didn't take them two years to do it. I might have been gone." What appealed to the president was Mr. Berlin's success in rising the morale of the nation in two wars. "Yip, Yip, Yaphank" ran on Broadway in 1918 as a "musical cooked up by the boys at Camp Upton," and "This Is The Army" was an all-soldier hit during World War II: Mr. Berlin, the son of a cantor who fled the pogroms of eastern Russia, is known for his shyness. That means he will probably avoid being out front today to bask in the nation's applause. He has heard it all before, of course, in a career of amazing output and endurance. But if he does choose to take a curtain call today, he is entitled to let it last and last.