Board Thread:Rebirth: Applications/@comment-1875854-20141110174716/@comment-1875854-20141126185117

I believe Hashirama initially discovered natural energy enhancement on accident through his unique wood release KG. Wood Release is done through converting the combination of water and earth (or the combination of water and earth creates this) to the source of life. Using that as a basis (along with his KG's other unique abilities), I believe his wood constructs could absorb natural energy and become stronger which was not achievable by anyone else with his DNA because their power had been weakened.

This would allow his powerful wood techniques to grow in power. But at first it was uncontrollable and sometimes the plants turned unusable except through earth release as they turned to stone. But overtime, he was able to develop the ability to sense the natural energy following into his constructs and then control it's input so it didn't overload. This of course was done through his many years on the battlefield.

As a result, he was granted a weaker senjutsu that only worked through his techniques. But in time, enacting the process on himself (with perhaps training from elsewhere) he was able to enter Sage Mode, the highest form of senjutsu which allowed his power to increase much more then ever before.

And since he was self-taught to the point of instant Sage Mode, he was able to teach another person, Kachuu. Even though Kachuu was successful and became a master he was still turned into stone eventually.