Document:

<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;
<PAGE>   2
                                      -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
<PAGE>   4
                                      -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.

<PAGE>   5
                                      -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/
                                                      --------------------------

<PAGE>   7
                                      -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/
                                                      --------------------------

<PAGE>   8
                                      -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/
                                                      --------------------------<PAGE>   1
                                                                 Exhibit 10 (mm)

                     POTASH CORPORATION OF SASKATCHEWAN INC.
                               (the "Corporation")

                              Directors' Resolution

WHEREAS the Corporation on May 9, 1991 adopted a supplemental executive
retirement income plan (the "Supplemental Plan") which plan was amended May 9,
1996; and

WHEREAS  it is desirable to amend and restate the Supplemental Plan;

THEREFORE, BE IT RESOLVED:

1.       That the text of the restated and amended plan as set forth in Appendix
         I attached hereto be approved and adopted.

2.       That the actions of the Corporation in adopting the amended and
         restated plan shall not derogate from the rights of any executive
         arising from or pursuant to a written supplemental retirement income
         plan agreement entered into between the executive and the Corporation
         prior to the passing of this resolution.

<PAGE>   2
                                                                      APPENDIX I

               SUPPLEMENTAL EXECUTIVE RETIREMENT INCOME PLAN-2000

1.       The Corporation shall maintain a supplemental executive retirement
         income plan (the "Supplemental Plan") for persons employed by the
         Corporation in certain executive positions to be designated from time
         to time by the Compensation Committee of the Board of Directors.

2.       The Supplemental Plan shall be unfunded. Any benefits which become
         payable under the Supplemental Plan shall be paid from the general
         assets of the Corporation. The Corporation may purchase a letter of
         credit from a bank or trust company to secure the benefit of one or
         more members.

3.       Designated executives shall not be required nor permitted to contribute
         to the Supplemental Plan.

4.       The normal retirement age under the Supplemental Plan shall be age 65.
         However, members of the Supplemental Plan may begin to receive such
         benefits as early as age 55. Retirement benefits are reduced by 5% for
         each year that the benefit commencement date precedes the executive's
         attainment of age 62.

5.       No benefits are payable under this Supplemental Plan for members who
         leave the Corporation prior to age 55 for any reason.

6.       The annual retirement benefit payable under the Supplemental Plan shall
         be calculated as follows:

         (a)      2% of the executive's average three highest years' earnings

                                multiplied by

                  the executive's years of service with the Corporation as
                  credited under the Potash Corporation of Saskatchewan Inc.
                  Pension Plan (the "Pension Plan") to a maximum of 35

                                    minus

         (b)      the annual retirement benefit which can be provided
                  under the Pension Plan.

<PAGE>   3

                                      - 2 -

7.       For purposes of the Supplemental Plan only, an executive's earnings
         shall be defined as the executive's annual base pay plus:

         (a)      100% of all bonuses paid or payable in the year to the
                  executive pursuant to the Corporation's annual incentive plan
                  or any similar plan substituted therefor; and

         (b)      any other payment made in a given year to an executive which
                  payment is specifically designated to be included in earnings
                  by the Compensation Committee of the Board of Directors of the
                  Corporation.

8.       (a)      The normal form of the Supplemental Plan provides a reduced
                  initial pension to the executive with 60% of the reduced
                  amount continuing for the life of the executive's spouse (if
                  any) on the executive's death. Optional forms of benefit, such
                  as, but not limited to, providing 60%, 75% and 100% to the
                  spouse, with a corresponding lower initial pension to the
                  executive will also be available.

         (b)      At the direction of the executive supplemental benefits
                  payable under the Plan to the executive shall be paid in a
                  lump sum provided that such benefits shall be actuarially
                  calculated to reflect a cost neutral position to the
                  Corporation.

         (c)      For the purposes of calculating the lump sum payment payable
                  pursuant to paragraph 8(b) the:

                  i)       1983 GAM Mortality Table based on 50/50 unisex; and

                  ii)      annual interest rate on 30 year treasury securities
                           (as published monthly by the United States Federal
                           Reserve and sometimes referred to as the "GATT Rate")
                           for the second month preceding the month of the
                           executive's retirement
                                           or
                           such other mortality table, interest rate, lookback
                           period and stability period as may from time to time
                           be prescribed by the Compensation Committee of the
                           Board of Directors of the Corporation

                  shall be utilized.

9.       Should the executive die after age 55 but prior to retirement from the
         Corporation, the executive's spouse (if any) shall receive the annual
         retirement benefit the spouse would have received had the executive
         retired immediately prior to his or her death and elected a 60%
         survivor pension.

<PAGE>   4
                                      - 3 -

10.      The proper officers of the Corporation be directed to arrange for the
         appropriate documentation and communication of the Supplemental Plan
         and that the proper officers of the Corporation are further authorized
         to do all other acts and things as may be necessary or desirable to
         implement this resolution.

11.      Any two directors or officers of the Corporation be and are hereby
         authorized to execute on behalf of the Corporation any documentation
         required to establish the Supplemental Plan on the terms and conditions
         as aforesaid.

<PAGE>   5

May 16, 2000
                                                         PERSONAL & CONFIDENTIAL

Dear :

RE:    Long-Term Incentive Plan (LTIP) and approved amendments to Supplemental
       Retirement Income (SERI) Plan
--------------------------------------------------------------------------------

At its meeting on May 10, 2000, the Board of Directors of PCS approved a new
Long-Term Incentive Plan (LTIP) and also approved amendments to the Supplemental
Executive Retirement Income (SERI) Plan.

The new Long-Term Incentive Plan has changed substantially from the previous
plan and you should be receiving from Mr. Doyle information in that regard.
Among the changes is a provision that the LTIP award be paid in a lump sum at
the end of each three year cycle. Under the previous plan, the award was paid
annually on a 1/6, 1/6, 2/3 basis.

The SERI Plan was modified to define "earnings" as annual base salary plus the
Annual Incentive Plan payment. Prior to the amendments, "earnings" was defined
as annual base salary plus all bonus payments made in a year. As a consequence,
LTIP payments are not included in "earnings" under the amended SERI Plan.

The Compensation Committee of the Board considered the impact of these two
changes on existing SERI participants (of whom you are one). The Committee
concluded that it would "grandfather" existing SERI participants (so that the
amendments regarding the definition of earnings would not affect them) provided
that the participant agrees that payments under the new LTIP be deemed to have
been paid annually on a 1/3, 1/3, 1/3 basis for the purposes of calculating
"earnings" under the SERI Plan.

                                                                      . . . . /2

<PAGE>   6

Please signify your agreement with the foregoing by signing the enclosed copy of
this letter where indicated and returning it to me. This letter, when signed by
you, will constitute an amendment to your existing SERI contract.

If you have any questions regarding the foregoing, please call me.

Yours truly,

POTASH CORPORATION OF SASKATCHEWAN INC.

/S/ JOHN L.M. HAMPTON

JOHN L. M. HAMPTON
Senior Vice President, General Counsel
and Secretary

JLMH:klc
Enclosure

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