<PAGE>   1
                                                                   Exhibit 4(f)


                            FIFTH AMENDING AGREEMENT

     THIS AGREEMENT made as of the 28th day of September, 2000.

BETWEEN:

     THE BANK OF NOVA SCOTIA

     (herein, in its capacity as agent to the Lenders, called the "Agent")

     - and -

     THE BANK OF NOVA SCOTIA, ROYAL BANK OF CANADA, BANK OF MONTREAL, BNP
     PARIBAS, TORONTO DOMINION (TEXAS), INC., BANK OF AMERICA NA, CANADIAN
     IMPERIAL BANK OF COMMERCE, CITIBANK N.A., NEW YORK, CREDIT SUISSE FIRST
     BOSTON, RABOBANK NEDERLAND, NEW YORK BRANCH and COMERICA BANK

     (herein, in their capacities as lenders to the Borrower under the Credit
     Facility, collectively called the "Lenders" and individually called a
     "Lender")

     - and -

     POTASH CORPORATION OF SASKATCHEWAN INC., a corporation incorporated under
     the laws of the Province of Saskatchewan

     (herein called the "Borrower").

     WHEREAS  the  Borrower,  the Lenders  and the Agent  entered  into a credit
agreement made as of October 4, 1996, as amended by agreements dated November 6,
1997,  December  15,  1997,  October 2, 1998 and  October  1, 1999 (the  "Credit
Agreement") and pursuant to which the Lenders  established a certain term credit
facility in favour of the Borrower;

     AND WHEREAS the  Borrower,  the Lenders and the Agent have agreed to effect
certain amendments to the Credit Agreement upon the terms set forth herein;
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                                      -2-

     NOW THEREFORE THIS AGREEMENT WITNESSES that, in consideration of the mutual
covenants and agreements and for other good and valuable consideration (the
receipt and sufficiency of which are hereby acknowledged), the parties hereto
hereby agree as follows:


                                   ARTICLE I
                                 DEFINED TERMS

1.01  CAPITALIZED TERMS. All capitalized terms which are used herein without
being specifically defined herein shall have the meaning ascribed thereto in the
Credit Agreement.


                                   ARTICLE II
                                   AMENDMENTS

2.01  GENERAL RULE. The Credit Agreement is hereby amended to the extent
necessary to give full effect to the provisions of this agreement.

2.02  DEFINITIONS. Section 1.01 of the Credit Agreement is hereby amended by
deleting the definition of "CONVERSION DATE" and replacing it by the following:

     "CONVERSION DATE" means September 27, 2001, as extended pursuant to Section
     1.13.

2.03  ESTABLISHMENT OF CREDIT FACILITY. Section 2.01 of the Credit Agreement is
hereby amended by replacing "U.S. $778,000,000" in the last line thereof with
"U.S. $500,000,000".

2.04  DEPARTING LENDERS. The parties hereto confirm that each of Bank of
Montreal, Canadian Imperial Bank of Commerce and Credit Suisse First Boston (the
"Departing Lenders") hereby ceases to be a Lender under the Credit Agreement and
has no further obligations under the Credit Agreement. The parties hereto agree
that the Departing Lenders are signatories to this agreement solely for the
purpose of giving effect to this Section 2.04.

2.05  INDIVIDUAL COMMITMENTS. Schedule A to the Credit Agreement is hereby
amended by restating the Individual Commitment of each of the Lenders with
respect to the Credit Facility as follows:

<TABLE>
<CAPTION>
LENDER                                           AMOUNT
------                                           ------
<S>                                           <C>
The Bank of Nova Scotia                       $180,000,000
Royal Bank of Canada                          $ 80,000,000
</TABLE>
<PAGE>   3
                                      -3-

<TABLE>
<CAPTION>
LENDER                                                                AMOUNT
------                                                                ------
<S>                                                                 <C>
Bank of America NA                                                  $ 60,000,000
BNP Paribas                                                         $ 50,000,000
Toronto Dominion (Texas), Inc.                                      $ 50,000,000
Citibank N.A., New York                                             $ 30,000,000
Rabobank Nederland, New York Branch                                 $ 25,000,000
Comerica Bank                                                       $ 25,000,000
Canadian Imperial Bank of Commerce                                           nil
Bank of Montreal                                                             nil
Credit Suisse First Boston                                                   nil
</TABLE>

2.06  DELIVERIES PURSUANT TO CREDIT AGREEMENT. For the purposes of the Credit
Agreement, this agreement and any document or instrument referred to herein
shall be deemed to be delivered pursuant to the Credit Agreement and to be
referred to in the Credit Agreement.

2.07  EXTENSION FEE. The Borrower hereby agrees to pay to each Lender (other
than the Departing Lenders) on the date hereof an extension fee in an amount
equal to 0.025% of the Individual Commitment of such Lender (after giving effect
to this agreement).


                                   ARTICLE III
                         REPRESENTATIONS AND WARRANTIES

3.01  REPRESENTATIONS AND WARRANTIES. To induce the Lenders and the Agent to
enter into this agreement, the Borrower hereby represents and warrants to the
Lenders and the Agent that the representations and warranties of the Borrower
which are contained in Sections 10.01(e) to (l) of the Credit Agreement, as the
same may be amended hereby, are true and correct, and further represents and
warrants, as at the date hereof, as follows:

      (a)  STATUS AND POWER. The Borrower is a corporation duly incorporated and
           organized and validly subsisting in good standing under the laws of
           the Province of Saskatchewan. The Borrower is duly qualified,
           registered or licensed in all jurisdictions where such qualification,
           registration or licensing is required. The Borrower has all requisite
           corporate capacity, power and authority to own, hold under licence or
           lease its properties and to carry on its business as now conducted.
           The
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                                      -4-

           Borrower has all requisite corporate capacity, power and authority to
           enter into and carry out the transactions contemplated by this
           agreement.

      (b)  AUTHORIZATION AND ENFORCEMENT. All necessary action, corporate or
           otherwise, has been taken to authorize the execution, delivery and
           performance by the Borrower of this agreement. The Borrower has duly
           executed and delivered this agreement. This agreement is a legal,
           valid and binding obligation of the Borrower enforceable against the
           Borrower by the Agent and the Lenders in accordance with its terms,
           subject to the qualifications of the nature contained in the opinion
           of the Borrower's counsel delivered pursuant to Section 12.02(d)(vii)
           of the Credit Agreement.

      (c)  COMPLIANCE WITH OTHER INSTRUMENTS. The execution, delivery and
           performance by the Borrower of this agreement and the consummation of
           the transactions contemplated herein do not conflict with, result in
           any breach or violation of, or constitute a default under the terms,
           conditions or provisions of the charter or constating documents or
           by-laws of, or any unanimous shareholder agreement relating to, the
           Borrower or of any law, regulation, judgment, decree or order binding
           on or applicable to the Borrower or to which its property is subject
           or of any material agreement, lease, licence, permit or other
           instrument to which the Borrower is a party or is otherwise bound or
           by which the Borrower benefits or to which its property is subject
           and do not require the consent or approval of any Official Body or
           any other party.


                                   ARTICLE IV
                                  MISCELLANEOUS

4.01  FUTURE REFERENCES. On and after the effective date of this agreement, each
reference in the Credit Agreement to "this Agreement", "hereunder", "hereof", or
words of like import referring to the Credit Agreement, and each reference in
any related document to the "Credit Agreement", "thereunder", "thereof", or
words of like import referring to the Credit Agreement, shall mean and be a
reference to the Credit Agreement as amended hereby, and each such related
document is hereby amended accordingly. The Credit Agreement, as amended hereby,
is and shall continue to be in full force and effect and is hereby in all
respects ratified and confirmed.

4.02  GOVERNING LAW. This agreement shall be governed by and construed in
accordance with the laws of the Province of Ontario.

4.03  ENUREMENT. This agreement shall enure to the benefit of and shall be
binding upon the parties hereto and their respective successors and permitted
assigns.

4.04  CONFLICT. If any provision of this agreement is inconsistent or conflicts
with any provision of the Credit Agreement, the relevant provision of this
agreement shall prevail and be paramount.

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                                      -5-

4.05  FURTHER ASSURANCES. The Borrower shall do, execute and deliver or shall
cause to be done, executed and delivered all such further acts, documents and
things as the Agent may reasonably request for the purpose of giving effect to
this agreement and to each and every provision hereof.

4.06  COUNTERPARTS. This agreement may be executed in one or more counterparts,
each of which shall be deemed to be an original and all of which taken together
shall be deemed to constitute one and the same instrument.

     IN WITNESS WHEREOF the parties hereto have executed this agreement.

                                                  THE BANK OF NOVA SCOTIA,
                                                  AS AGENT


                                                  By: /s/
                                                      --------------------------

                                                  By: /s/
                                                      --------------------------

                                                  POTASH CORPORATION OF
                                                  SASKATCHEWAN INC.


                                                  By: /s/
                                                      --------------------------

                                                                            c.s.

                                                  By: /s/
                                                      --------------------------


                                                  THE BANK OF NOVA SCOTIA,
                                                  AS LENDER


                                                  By: /s/
                                                      --------------------------


                                                  By: /s/
                                                      --------------------------


<PAGE>   6
                                      -6-



                                                  BNP PARIBAS


                                                  By: /s/
                                                      --------------------------


                                                  By: /s/
                                                      --------------------------


                                                  ROYAL BANK OF CANADA


                                                  By: /s/
                                                      --------------------------


                                                  By: /s/
                                                      --------------------------


                                                  CANADIAN IMPERIAL BANK OF
                                                  COMMERCE


                                                  By: /s/
                                                      --------------------------


                                                  By: /s/
                                                      --------------------------


                                                  BANK OF AMERICA NA


                                                  By: /s/
                                                      --------------------------


                                                  By: /s/
                                                      --------------------------


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                                      -7-


                                                  BANK OF MONTREAL


                                                  By: /s/
                                                      --------------------------


                                                  By: /s/
                                                      --------------------------


                                                  TORONTO DOMINION (TEXAS), INC.


                                                  By: /s/
                                                      --------------------------


                                                  By: /s/
                                                      --------------------------


                                                  CREDIT SUISSE FIRST BOSTON


                                                  By: /s/
                                                      --------------------------


                                                  By: /s/
                                                      --------------------------


                                                  CITIBANK N.A., NEW YORK


                                                  By: /s/
                                                      --------------------------


                                                  By: /s/
                                                      --------------------------


                                                  RABOBANK NEDERLAND, NEW YORK
                                                  BRANCH


                                                  By: /s/
                                                      --------------------------


                                                  By: /s/
                                                      --------------------------


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                                      -8-

                                                  COMERICA BANK


                                                  By: /s/
                                                      --------------------------


                                                  By: /s/
                                                      --------------------------


     The undersigned, being a guarantor of the indebtedness, liabilities and
obligations of the Borrower to the Lenders, hereby consents to the foregoing
amendments to the Credit Agreement.

     DATED as of the 28th day of September, 2000.


                                                  PCS NITROGEN, INC.


                                                  By: /s/
                                                      --------------------------

                                                                           c.s.

                                                  By: /s/
                                                      --------------------------