su'ing not sourcing .bash_profile

[Daniel Underwood]

When I am logged in as a non-root user and I try to become root by
entering "su" and typing the root password, the resulting bash prompt
does not reflect the contents of /root/.bash_profile

My /root/.bash_profile contains (among other things):

export PS1="[\e[1;31m\]$(tput bold)\u$(tput sgr0)\[\e[0m\]@\h \w]\$ "

The point is to make the username ("root") display in BOLD and RED
text.  After su'ing, the text is not bold nor red.  If I then enter
"source ~/.bash_profile", however, the prompt displays correctly,
showing "root" in bold and red text.

How come su'ing doesn't seem to effect everything in the
/root/.bash_profile file?

Thanks,
Daniel