Marc Anthony bought the farm ... AND FORGOT!

The singer was in court Friday ... testifying in his bitter child support case with Dayanara Torres. She wants support raised from $13K to $113K a month, claiming Marc is hiding income.

During testimony Marc was going over his assets ... explaining how he failed to account for a farm he had in upstate New York. Marc's answer was simple ... "I completely forgot I had a farm."

As for how he forgot .... Marc says he hadn't been there in 7 or 8 years.

Anthony says he thinks the farm was worth around $700K.

Marc has filed legal docs saying he makes $1.25 million a month, but he says he has trouble knowing exactly how much is coming in.