There is a partial composer breakdown here
http://leaf.doorblog.jp/archives/3694156.html
I don't know if this is based on facts or guesses by music style, but accordingly the composers on SOIL would be
Maiko Iuchi (1,5)
Ryo Mizutsuki (2,7,10)
Soshi Hosoi (3,4,7)
Shinji Orito (6,8,9)
edit: this wiki entry further proves the composer credits:
http://www11.atwiki.jp/key_rewrite/m...9.html?guid=on - changing the entry based on this info