Board Thread:Theories and Speculation/@comment-5407586-20150624095943/@comment-3279282-20150704175948

The manga doesn't make a difference between normal and advanced natures that way. Both are called "seishitsuhenka". Mokuton is just not one of the base natures (基本性質, kihon seishitsu). Kakashi explains it as "By performing the "earth" and "water" seishitsuhenka at the same time, a new "wood" seishitsuhenka is brought forth" or so.