<PAGE>   1

                                                                    EXHIBIT 4(E)

                           FOURTH AMENDING AGREEMENT

     THIS AGREEMENT made as of the 1st day of October, 1999.

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, BANQUE NATIONALE DE PARIS -- CHICAGO, TORONTO
           DOMINION (TEXAS), INC., BANK OF AMERICA NA, CANADIAN
           IMPERIAL BANK OF COMMERCE, CITIBANK N.A., NEW YORK, CREDIT
           SUISSE FIRST BOSTON, DEUTSCHE BANK A.G., NEW YORK AND/OR
           CAYMAN ISLAND BRANCH and RABOBANK NEDERLAND, NEW YORK
           BRANCH

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

           -- and --

           COMERICA BANK

           (herein called "Comerica")

           -- 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 and October 2, 1998 (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;

     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.
<PAGE>   2

                                   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 as
follows:

     (i)   the definition of "Applicable Margin" is deleted and replaced by the
           following:

           "Applicable Margin" means, at any time, the applicable rate per annum
           set forth in the table below for the applicable S&P rating and the
           applicable Utilization Rate:

<TABLE>
<CAPTION>
                                                                 UTILIZATION RATE
                                              ------------------------------------------------------
                                                                     greater
                                                                     than or
                                                                   equal to 1/3          greater
         S&P's Corporate Credit or Unsecured       LESS              and less            than or
         Debt Rating of Borrower                 than 1/3            than 2/3         EQUAL TO 2/3
         -----------------------------------  ---------------    ----------------    ---------------
         <S>                                  <C>                <C>                 <C>
         A- or above....................       .40% per annum     .475% per annum     .55% per annum
         BBB+...........................       .50% per annum     .575% per annum     .65% per annum
         BBB............................       .60% per annum     .675% per annum     .75% per annum
         BBB-...........................       .75% per annum     .825% per annum     .90% per annum
         BB+ or below or unrated........      1.00% per annum    1.075% per annum    1.15% per annum
</TABLE>

     (ii)  the definition of "Conversion Date" is deleted and replaced by the
           following:

           "Conversion Date" means September 29, 2000, as extended pursuant to
           Section 1.13.;

     (iii) the definition of "Standby Fee Rate" is deleted and replaced by the
           following:

           "Standby Fee Rate" means, at any time, the applicable rate per annum
           set forth in the table below opposite the applicable S&P rating:

<TABLE>
<CAPTION>
         S&P'S CORPORATE CREDIT OR UNSECURED DEBT RATING OF BORROWER   STANDBY FEE RATE
         -----------------------------------------------------------   ----------------
         <S>                                                           <C>
         A- or above.................................................  .10% per annum
         BBB+........................................................  .125% per annum
         BBB.........................................................  .15% per annum
         BBB-........................................................  .175% per annum
         BB+ or below or unrated.....................................  .25% per annum
</TABLE>

     (iv)  the following definition is added:

           "Utilization Rate" means, at any particular time, the fraction
           obtained by dividing the amount of Accommodation outstanding under
           the Credit Facility at such time by the amount of the Credit Facility
           at such time.

2.03.  Establishment of Credit Facility.  Section 2.01 of the Credit Agreement
is hereby amended by replacing "U.S. $925,000,000" in the last line thereof with
"U.S. $778,000,000".

2.04.  Term-Out Premium.  Section 7.01 of the Credit Agreement is hereby amended
by adding the following at the end thereof:

           "After the Conversion Date, the Applicable Margin shall increase by
           0.15% per annum."
<PAGE>   3

2.05.  New Lender.  Comerica hereby agrees to be bound by the terms of the
Credit Agreement as a Lender with an Individual Commitment of $25,000,000 and
the following address for notice for the purpose of Section 15.02 of the Credit
Agreement and Schedule A to the Credit Agreement:

     500 Woodward Avenue, 23rd Floor
     Detroit, Michigan 48226

     Attention:  Marian Enright
     Telefax:     (313) 222-3377

2.06.  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.....................................  $190,000,000
Royal Bank of Canada........................................  $120,000,000
Banque Nationale de Paris -- Chicago........................  $113,000,000
Canadian Imperial Bank of Commerce..........................  $ 75,000,000
Bank of America NA..........................................  $ 60,000,000
Bank of Montreal............................................  $ 50,000,000
Toronto Dominion (Texas), Inc...............................  $ 50,000,000
Credit Suisse First Boston..................................  $ 40,000,000
Citibank N.A., New York.....................................  $ 30,000,000
Rabobank Nederland, New York Branch.........................  $ 25,000,000
Comerica Bank...............................................  $ 25,000,000
Deutsche Bank A.G., New York and/or Cayman Island Branch....           Nil
</TABLE>

2.07.  Departing Lender.  The parties hereto confirm that Deutsche Bank A.G.,
New York and/or Cayman Island Branch (the "Departing Lender") 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 Lender is a
signatory to this agreement solely for the purpose of giving effect to this
Section 2.07.

2.08.  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.

2.09.  Extension Fee.  The Borrower hereby agrees to pay to each Lender (prior
to giving effect to this agreement and other than Deutsche Bank A.G., New York
and/or Cayman Island Branch) 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 Borrower
<PAGE>   4
          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.

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.
<PAGE>   5

     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/
                                           -------------------------------------

                                        BANQUE NATIONALE DE PARIS -- CHICAGO

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

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

                                        BANK OF MONTREAL

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

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

                                         TORONTO DOMINION (TEXAS), INC.

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

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

                                        COMERICA BANK

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

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

                                        DEUTSCHE BANK A.G., NEW YORK
                                        AND/OR CAYMAN ISLAND BRANCH

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

                                        By: /s/
                                           -------------------------------------
<PAGE>   8

     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 1st day of October, 1999.

                                        PCS NITROGEN, INC.

                                        By: /s/
                                           -------------------------------------
                                                                            c.s.

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