Exhibit B
                                                                       ---------

                       AGREEMENT CONCERNING JOINT FILING
                                OF SCHEDULE 13D


The undersigned agree as follows:

     (i) each of them is individually eligible to use the Schedule 13D to which
this Exhibit is attached, and such Schedule 13D is filed on behalf of each of
them; and

     (ii) each of them is responsible for the timely filing of such Schedule 13D
and any amendments thereto, and for the completeness and accuracy of the
information concerning such person contained therein; but none of them is
responsible for the completeness or accuracy of the information concerning the
other persons making the filing, unless such person knows or has reason to
believe that such information is inaccurate.

This Agreement may be executed in any number of counterparts, each of which
shall be deemed to be an original, but all of which, taken together, shall
constitute one and the same instrument.

Dated: December 21, 2000
                              TERRA NITROGEN CORPORATION

                              By /s/ George H. Valentine

                              Its Vice President and General Counsel


                              TERRA CAPITAL, INC.

                              By /s/ George H. Valentine

                              Its Vice President and Corporate Secretary


                              TERRA CAPITAL HOLDINGS, INC.

                              By /s/ George H. Valentine

                              Its Vice President and Corporate Secretary


                              TERRA INDUSTRIES INC.

                              By /s/ George H. Valentine

                              Its Senior Vice President, General Counsel and
                                    Corporate Secretary