'American Idol' Keith Urban NOT SIGNED ... But Almost

8/31/2012 10:15 AM PDT

'American Idol' -- Keith Urban Deal NOT SIGNED ... But Almost

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Keith Urban
is in deep negotiations with "American Idol" but the deal has NOT been signed ... TMZ has learned.

"Idol" sources tell us, the country singer is in the final negotiation stages with producers to take the third judging spot on the show -- alongside Mariah Carey and Nicki Minaj, who replaced Jennifer Lopez and Steven Tyler.

Keith will be replacing Randy Jackson -- but he hasn't officially signed the dotted line just yet. He's still working out a few contract kinks, but it's pretty much a lock.

He'll be receiving roughly $3-4 million for his hard work -- about half what Nicki Minaj is expected to make ... and a paltry fraction of Mariah's $18 million paycheck.

As we previously reported, there will be a 4th judge this season, but we're told no one has been picked for that spot.  But here's what we do know ... "Idol" honchos want a man for the 4th spot, because they don 't think this show will work with 2 women and a man.