# su does not update path
 

when I 'su' to root, the $PATH is not set . How to fix this ?

$ su // $PATH is unset
Password:
# echo $PATH
/sbin:/bin:/usr/sbin:/usr/bin:/usr/bin/X11:/usr/local/sbin:/usr/local/bin

# su - // $PATH is set fine here
# echo $PATH
/usr1/bin:/opt/Software/Multimedia/RealPlayer:/opt/Software/Development/Java/jsk/bin:/root/bin:/sbin:/usr/sbin:/usr/local/sbin/:/usr/local/bin:/usr/bin:/bin:/usr/bin/X11:/usr/games:/root/kde3.2.3/bin

What do you want to fix, it's running perfectly:
"su" = change only the user, without a name it's root
"su -" = change user and load his environment, without a name it's root

when doing just 'su' , it should inherit the user's environments and variables - correct ?

Yes, but I wouldn't say inherit, just "su" changes nothing except the username and allow you doing things in his name with his permissions.