<PAGE>   1
 
   
                                                                    EXHIBIT 4(d)
    
 
                            THIRD AMENDING AGREEMENT
 
     THIS AGREEMENT made as of the 2nd day of October, 1998.
 
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., MORGAN GUARANTY TRUST COMPANY OF
                       NEW YORK, UNION BANK OF SWITZERLAND, NEW YORK BRANCH,
                       BANK OF AMERICA NT&SA, 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 -
 
                       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 and December 15, 1997 (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.
    
 
                                   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.
    
 
                                       28
<PAGE>   2
 
   
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 opposite the applicable S & P rating:
 
<TABLE>
<S>                                         <C>
--------------------------------------------------------------------------------------
       S & P'S CORPORATE CREDIT OR
         DEBT RATING OF BORROWER                        APPLICABLE MARGIN
--------------------------------------------------------------------------------------
 A- or above                                              .30% per annum
--------------------------------------------------------------------------------------
 BBB+                                                     .35% per annum
--------------------------------------------------------------------------------------
 BBB                                                      .40% per annum
--------------------------------------------------------------------------------------
 BBB-                                                     .50% per annum
--------------------------------------------------------------------------------------
 BB+ or below                                             .75% per annum
--------------------------------------------------------------------------------------
</TABLE>
 
     (ii)  the definition of "CONVERSION DATE" is deleted and replaced by the
        following:
 
        "CONVERSION DATE" means October 1, 1999, 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>
<S>                                         <C>
--------------------------------------------------------------------------------------
       S & P'S CORPORATE CREDIT OR
         DEBT RATING OF BORROWER                         STANDBY FEE RATE
--------------------------------------------------------------------------------------
 A- or above                                              .06% per annum
--------------------------------------------------------------------------------------
 BBB+                                                     .08% per annum
--------------------------------------------------------------------------------------
 BBB                                                      .10% per annum
--------------------------------------------------------------------------------------
 BBB-                                                     .12% per annum
--------------------------------------------------------------------------------------
 BB+ or below                                             .20% per annum
--------------------------------------------------------------------------------------
</TABLE>
 
   
2.03. ESTABLISHMENT OF CREDIT FACILITY.  Section 2.01 of the Credit Agreement is
hereby amended by replacing "U.S. $1,285,000,000" in the last line thereof with
"U.S. $925,000,000".
    
 
   
2.04. 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........................................    $165,000,000
Banque Nationale de Paris -- Chicago........................    $135,000,000
Bank of Montreal............................................    $ 75,000,000
Canadian Imperial Bank of Commerce..........................    $ 75,000,000
Bank of America NT&SA.......................................    $ 60,000,000
Deutsche Bank A.G., New York and/or Cayman Island Branch....    $ 60,000,000
Toronto Dominion (Texas), Inc...............................    $ 50,000,000
Citibank N.A., New York.....................................    $ 50,000,000
Credit Suisse First Boston..................................    $ 40,000,000
Rabobank Nederland, New York Branch.........................    $ 25,000,000
Morgan Guaranty Trust Company of New York...................             Nil
Union Bank of Switzerland, New York Branch..................             Nil
</TABLE>
    
 
   
2.05. DEPARTING LENDERS.  The parties hereto confirm that each of Morgan
Guaranty Trust Company of New York and Union Bank of Switzerland, New York
Branch (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 each of the Departing Lenders is a signatory to this agreement
solely for the purpose of giving effect to this Section 2.05.
    
 
                                       29
<PAGE>   3
 
   
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.
    
 
                                  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) -- (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
        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.
    
 
                                       30
<PAGE>   4
 
   
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/
 
                                            ------------------------------------
 
                                       31
<PAGE>   5
 
                                          ROYAL BANK OF CANADA
 
                                          By: /s/
 
                                            ------------------------------------
 
                                          By: /s/
 
                                            ------------------------------------
 
                                          BANK OF MONTREAL
 
                                          By: /s/
 
                                            ------------------------------------
 
                                          By: /s/
 
                                            ------------------------------------
 
                                          BANQUE NATIONALE DE PARIS -- CHICAGO
 
                                          By: /s/
 
                                            ------------------------------------
 
                                          By: /s/
 
                                            ------------------------------------
 
                                          TORONTO DOMINION (TEXAS), INC.
 
                                          By: /s/
 
                                            ------------------------------------
 
                                          By: /s/
 
                                            ------------------------------------
 
                                       32
<PAGE>   6
 
                                          MORGAN GUARANTY TRUST
                                          COMPANY OF NEW YORK
 
                                          By: /s/
 
                                            ------------------------------------
 
                                          By: /s/
 
                                            ------------------------------------
 
   
                                          UNITED BANK OF SWITZERLAND,
    
                                          NEW YORK BRANCH
 
                                          By: /s/
 
                                            ------------------------------------
 
                                          By: /s/
 
                                            ------------------------------------
 
                                          BANK OF AMERICA NT&SA
 
                                          By: /s/
 
                                            ------------------------------------
 
                                          By: /s/
 
                                            ------------------------------------
 
                                          CANADIAN IMPERIAL BANK OF COMMERCE
 
                                          By: /s/
 
                                            ------------------------------------
 
                                          By: /s/
 
                                            ------------------------------------
 
                                       33
<PAGE>   7
 
                                          CITIBANK N.A., NEW YORK
 
                                          By: /s/
                                            ------------------------------------
 
                                          By: /s/
                                            ------------------------------------
 
                                          CREDIT SUISSE FIRST BOSTON
 
                                          By: /s/
                                            ------------------------------------
 
                                          By: /s/
                                            ------------------------------------
 
                                          DEUTSCHE BANK A.G., NEW YORK AND/OR
                                          CAYMAN ISLAND BRANCH
 
                                          By: /s/
                                            ------------------------------------
 
                                          By: /s/
                                            ------------------------------------
 
                                          RABOBANK NEDERLAND,
                                          NEW YORK BRANCH
 
                                          By: /s/
                                            ------------------------------------
 
                                          By: /s/
                                            ------------------------------------
 
                                       34
<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 2nd day of October, 1998.
 
                                          PCS NITROGEN, INC.
 
                                          By: /s/
 
                                            ------------------------------------
                                                                            c.s.
 
                                          By: /s/
 
                                            ------------------------------------
 
                                       35