Document:

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                                                                    Exhibit 4.11

               FIRST AMENDMENT TO TERRA CANADA CREDIT AGREEMENT
               ------------------------------------------------

          First Amendment (this "Amendment"), dated as of December 20, 2000, to
the Credit Agreement, dated as of December 31, 1997 and as most recently amended
and restated as of April 7, 2000 (as amended hereby and as the same may be
further amended, supplemented or otherwise modified from time to time, the
"Credit Agreement"), among Terra International (Canada) Inc., as the "Borrower",
certain Guarantors, the financial institutions from time to time party thereto
as lenders (the "Lenders"), and Citibank, N.A. ("Citibank"), as agent for the
Lenders and the Issuers (in such capacity, the "Administrative Agent").

                             W I T N E S S E T H:
                             --------------------

          WHEREAS, pursuant to the Credit Agreement, the Lenders have agreed to
make Loans to the Borrower; and

          WHEREAS, the Borrower, the Requisite Lenders and the Administrative
Agent have agreed to certain amendments to the Credit Agreement as more
specifically set forth below; and

          WHEREAS, the Borrower has requested the Requisite Lenders to consent
to permitting Terra Industries and certain of its Subsidiaries to repurchase
certain industrial revenue bonds;

          NOW, THEREFORE, in consideration of the premises and the mutual
covenants and provisions hereinafter contained, the parties hereto hereby agree
as follows:

          1.   Defined Terms. Capitalized terms used herein and not defined
               -------------
herein but defined in the Credit Agreement are used herein as defined in the
Credit Agreement.

          2.   Amendments to the Credit Agreement. As of the Amendment Effective
               ----------------------------------
Date (as defined in Section 4 below), the Credit Agreement is hereby amended as
follows:

                    (a)  Amendment to Section 1.1. Section 1.1 is hereby amended
                         ------------------------
by adding the following new definition of "IRB Repurchase" in the correct
alphabetical order therein:

                    "IRB Repurchase" means Terra Real Estate Corporation's or
          TI's repurchase or repayment of the industrial revenue bonds of series
          1992 issued pursuant to an agreement between Terra Real Estate
          Corporation, TI and the City of Sioux City, dated April 22, 1992.

                    (b)  Amendment to Section 7.14. Section 7.14 is hereby
                         -------------------------
amended and restated in its entirety to read as follows:

                    Section 7.14.  Hedging Contracts. Terra Industries and its
                                   -----------------
          Subsidiaries shall at all times maintain interest rate and natural gas
<PAGE>

          Hedging Contracts, on terms and with counterparties reasonably
          satisfactory to the Administrative Agent and in accordance with the
          hedging policy which has been and will continue to be adopted by the
          Board of Directors of Terra Industries and as in effect from time to
          time, to provide protection to Terra Industries and its Subsidiaries
          against fluctuations in interest rates and natural gas prices.

          3.   Consent. Notwithstanding Section 8.5 and Section 8.12 of the
               -------
Credit Agreement or any other provision of the Loan Documents, the Lenders party
hereto, constituting the Requisite Lenders, hereby agree that TI and/or Terra
Real Estate Corporation may consummate IRB Repurchases from and after the
Amendment Effective Date (as defined in Section 4 below); provided, however,
that the aggregate dollar amount of the IRB Repurchases shall not exceed at any
time $8,000,000.

          4.   Conditions Precedent to the Effectiveness of this Amendment. This
               -----------------------------------------------------------
Amendment shall become effective as of the date hereof on the date (the
"Amendment Effective Date") when the following conditions precedent have been
satisfied:

                    (a)   Certain Documents. The Administrative Agent shall have
                          -----------------
received, on or before the Amendment Effective Date, all of the following, all
of which shall be in form and substance satisfactory to the Administrative
Agent, in sufficient originally executed copies for each of the Lenders:

                    (i)   this Amendment, executed by the Borrower, the
     Guarantors, the Administrative Agent and the Lenders constituting the
     Requisite Lenders; and

                    (ii)  such additional documentation as the Administrative
     Agent or the Requisite Lenders may reasonably require.

                    (b)   Representations and Warranties. Each of the
                          ------------------------------
representations and warranties made by the Obligors in or pursuant to the Credit
Agreement, as amended hereby, and the other Loan Documents to which any Obligor
is a party or by which any Obligor is bound, shall be true and correct in all
material respects on and as of the Amendment Effective Date (other than
representations and warranties in any such Loan Document which expressly speak
as of a specific date, which shall have been true and correct in all material
respects as of such specific date).

                    (c)   Corporate and Other Proceedings. All corporate and
                          -------------------------------
other proceedings, and all documents, instruments and other legal matters in
connection with the transactions contemplated by this Amendment shall be
reasonably satisfactory in all respects in form and substance to the
Administrative Agent and the Requisite Lenders.

                    (d)   No Event of Default. No Event of Default or Default
                          -------------------
shall have occurred and be continuing on the Amendment Effective Date.

          5.   Representations and Warranties. On and as of the date hereof, and
               ------------------------------
as of the Amendment Effective Date, after giving effect to this Amendment, each
of the undersigned hereby represents and warrants to the Lenders as follows:

                    (a)  Each of the representations and warranties contained in
Article IV of the Credit Agreement, the other Loan Documents or in any
certificate, document or

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financial or other statement furnished at any time under or in connection
therewith are true and correct in all material respects on and as of the date as
if made on and as of such date, except to the extent that such representations
and warranties specifically relate to a specific date, in which case such
representations and warranties shall be true and correct in all material
respects as of such specific date; provided, however, that references therein to
the "Credit Agreement" shall be deemed to include this Amendment;

                    (b)  No Default or Event of Default has occurred and is
continuing.

          6.   Affirmation of Guaranties. Each of the undersigned hereby agrees
               -------------------------
that the terms of this Amendment shall not affect in any way its obligations and
liabilities under the undersigned's Loan Documents, including, without
limitation, the Guaranty, all of which obligations and liabilities shall remain
in full force and effect and each of which is hereby reaffirmed.

          7.   Continuing Effect; No Other Amendments. Except as expressly
               --------------------------------------
amended hereby, all of the terms and provisions of the Credit Agreement and the
other Loan Documents are and shall remain in full force and effect. The
amendments and consents contained herein shall not constitute an amendment or a
waiver of any other provision of the Credit Agreement or the other Loan
Documents or for any other purpose except as expressly set forth herein.

          8.   Costs and Expenses. The Borrower and Terra Industries agree to
               ------------------
pay on demand all reasonable and documented out-of-pocket costs and expenses of
the Administrative Agent in connection with the preparation, execution and
delivery of this Amendment and other instruments and documents to be delivered
pursuant hereto, including the reasonable and documented fees and out-of-pocket
expenses of counsel for the Administrative Agent with respect thereto.

          9.   Governing Law; Counterparts; Miscellaneous.
               ------------------------------------------

                    (a)  This Amendment shall be governed by, and construed and
interpreted in accordance with, the law of the State of New York.

                    (b)  This Amendment may be executed in any number of
counterparts and by the different parties on separate counterparts, each of
which counterparts when executed and delivered shall be an original, but all of
which shall together constitute one and the same instrument.

                    (c)  Section captions used in this Amendment are for
convenience only and shall not affect the construction of this Amendment.

                    (d)  From and after the Amendment Effective Date, all
references in the Credit Agreement to the "Agreement" shall be deemed to be
references to such Agreement as modified hereby and this Amendment and the
Credit Agreement shall be read together and construed as a single instrument.

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<PAGE>

          IN WITNESS WHEREOF, the undersigned parties have executed this FIRST
AMENDMENT TO CREDIT AGREEMENT to be effective for all purposes as of the
Amendment Effective Date.

                              Borrower
                              --------

                              Terra International (Canada), Inc.

                              By: _______________________________________
                                  Name: Francis G. Meyer
                                  Title: Vice President

                              Guarantors
                              ----------

                              Terra Industries Inc.

                              By: _______________________________________
                                  Name: Francis G. Meyer
                                  Title: Sr. Vice President and Chief
                                             Financial Officer

                              Terra Capital Holdings, Inc.

                              By: _______________________________________
                                  Name: Francis G. Meyer
                                  Title: Vice President

                              Terra Capital, Inc.

                              By: _______________________________________
                                  Name: Francis G. Meyer
                                  Title: Vice President

                              Terra Nitrogen Corporation

                              By: _______________________________________
                                  Name: George H. Valentine
                                  Title: Vice President

                              Terra International, Inc.

                              By: _______________________________________
                                  Name: Francis G. Meyer
                                  Title: Senior Vice President
<PAGE>

                              Terra Methanol Corporation

                              By: _______________________________________
                                  Name: Francis G. Meyer
                                  Title: Vice President

                              BMC Holdings, Inc.

                              By: _______________________________________
                                  Name: Francis G. Meyer
                                  Title: Vice President

                              Beaumont Holdings Corporation

                              By: _______________________________________
                                  Name: George H. Valentine
                                  Title: Vice President

                              Terra Nitrogen (U.K.), Limited

                              By: _______________________________________
                                  Name: Francis G. Meyer
                                  Title: Director

                              Terra Nitrogen, Limited Partnership

                              By: Terra Nitrogen Corporation,
                                  its General Partner

                              By: _______________________________________
                                  Name: George H. Valentine
                                  Title: Vice President

                              Beaumont Methanol, Limited Partnership

                              By: Terra Methanol Corporation,
                                  its General Partner

                              By: _______________________________________
                                  Name: Francis G. Meyer
                                  Title: Vice President
<PAGE>

                              Beaumont Ammonia, Inc.

                              By: _______________________________________
                                  Name: George H. Valentine
                                  Title: Vice President

                              Port Neal Corporation

                              By: _______________________________________
                                  Name: George H. Valentine
                                  Title: Vice President

                              Terra International (Oklahoma) Inc.

                              By: _______________________________________
                                  Name:  Francis G. Meyer
                                  Title: Vice President

                              Terra (U.K.) Holdings, Inc.

                              By: _______________________________________
                                  Name: George H. Valentine
                                  Title: Vice President

                              Administrative Agent
                              --------------------

                              Citibank, N.A.

                              By: _______________________________________
                                  Name:  David Jaffe
                                  Title:  Vice President

                              Lenders
                              -------

                              Bankers Trust Company

                              By: _______________________________________
                                  Name:
                                  Title:

                              Citibank, N.A.

                              By: _______________________________________
                                  Name: David Jaffe
                                  Title: Vice President
<PAGE>

                              Fleet Capital Corporation

                              By: _______________________________________
                                  Name:
                                  Title:

                              Foothill Capital Corporation

                              By: _______________________________________
                                  Name:
                                  Title:

                              Hamilton Bank, NA.

                              By: _______________________________________
                                  Name:
                                  Title:

                              Jackson National Life Insurance Company

                              By: _______________________________
                                  Name:
                                  Title:

                              LaSalle Business Credit

                              By: _______________________________________
                                  Name:
                                  Title:

                              LaSalle National Bank

                              By: _______________________________________
                                  Name:
                                  Title:

                              Winged Foot Funding Trust

                              By: _______________________________________
                                  Name:
                                  Title:<PAGE>

                                                               Exhibit 10.1.6.a.

                               AMENDMENT TO THE
                             TERRA INDUSTRIES INC.
                              EXCESS BENEFIT PLAN

     Terra Industries Inc. desires to amend its Excess Benefit Plan as restated
as of January 1, 1992 (the "Plan"), all on the terms and conditions herein.
Accordingly, the Plan shall be amended by adding the following new Section 9
reading in its entirety as follows:

          "Section 9.  Legal Fees or Expenses.  The Company shall reimburse the
                       ----------------------
     Participant for any legal fees and expenses reasonably incurred in
     connection with the enforcement of Participant's rights under this Excess
     Benefit Plan; provided that the Company shall not be required to reimburse
     for such fees or expenses unless the resolution of any enforcement action
     taken by the Participant is substantially in favor of the Participant,
     whether by adjudication, settlement or otherwise."

     Except as provided herein or in the accompanying resolutions of the
Personnel Committee of the Terra Industries Inc. Board of Directors, the Plan
shall remain unchanged and in full force and effect.

     IN WITNESS WHEREOF, the Company has caused this Amendment to be executed by
its duly authorized officer as of the 26/th/ day of  July, 2000.

                                             TERRA INDUSTRIES INC.

                                        By   /s/ George H. Valentine
                                             --------------------------

                                        Its  Senior Vice President
                                             --------------------------

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