"It's been a profound impact [on my songwriting] to meet somebody who offered me a life and helped me become the person I wanted to be," Urban, 42, tells PEOPLE Country for its March issue.
As Urban tells it, sometimes Kidman's influence is literal. Take "My Heart Is Open," a track from his 2009 Defying Gravity album. "My wife and I had been out a couple of times and I wondered how she felt about me," he says, describing the song's inspiration. "It came out as, 'How is your heart?' She said, 'It's open.' "
Says Urban, "My God, what a beautiful answer."
Kidman, also 42, has also helped her husband through dark times, including a stint in Betty Ford Center for his alcohol addiction, and the Aussie singer says that, and the birth of his daughter Sunday Rose in 2008, has opened him up.
"I felt the power of love in all its phases," he says. "There was an expansion of color and joy."
As for daughter Sunday, there's the possibility that she could be just as much of a muse for Urban as her mother. "The next few months will be all about writing," he says. "I don't know how she's going to be a part of that yet." But, he adds, she's already given him "a new joy, a new sense of purpose."
For more from Urban, Kellie Pickler and other hot country music stars – plus free iTunes downloads! – pick up the special PEOPLE Country issue, on newsstands Friday