Board Thread:Naruto Discussions/@comment-3279282-20140413164418/@comment-28861328-20140413165434

The statement was made by Yamato, in reference to Wood Release, that one needed the chakra of water and earth in perfect unison to produce the wood element. In the same chapter, Kakashi adds that all advanced natures work this way and he explained this to Naruto by saying that Haku, a user of Ice Release, had to combine both water and wind to produce his advance nature. But that's all that's been said on that matter in the manga.

Edit: I should note that Yamato prefaces his remarks by saying that he is capable of using earth and water and then launches into the remarks I mentioned above.