U.S.$2,750,000,000 REVOLVING TERM CREDIT FACILITY

Exhibit 4(a)

CREDIT AGREEMENT

BETWEEN

POTASH CORPORATION OF SASKATCHEWAN INC.

as Borrower

AND

THE BANK OF NOVA SCOTIA,

BANK OF MONTREAL,

ROYAL BANK OF CANADA,

EXPORT DEVELOPMENT CANADA,

CANADIAN IMPERIAL BANK OF COMMERCE,

RABOBANK NEDERLAND, CANADIAN BRANCH,

THE TORONTO-DOMINION BANK,

GOLDMAN SACHS LENDING PARTNERS LLC,

MORGAN STANLEY BANK, N.A.,

UBS AG CANADA BRANCH,

BANK OF AMERICA, N.A., CANADA BRANCH,

HSBC BANK CANADA,

BANK OF TOKYO-MITSUBISHI UFJ (CANADA),

SUMITOMO MITSUI BANKING CORPORATION OF CANADA,

COMERICA BANK

and such other persons as become parties hereto

as Lenders

AND

THE BANK OF NOVA SCOTIA

as Agent of the Lenders

FIRST AMENDING AGREEMENT MADE AS OF SEPTEMBER 23, 2011 TO THE CREDIT

AGREEMENT MADE AS OF DECEMBER 11, 2009

FIRST AMENDING AGREEMENT

THIS AGREEMENT is made as of September 23, 2011

BETWEEN:

POTASH CORPORATION OF SASKATCHEWAN INC., a corporation subsisting under the laws of Canada (hereinafter referred to as the “Borrower”),

OF THE FIRST PART,

- and -

THE FINANCIAL INSTITUTIONS SET FORTH ON THE SIGNATURE PAGES HEREOF UNDER THE HEADING “LENDERS:” (hereinafter referred to collectively as the “Lenders” and individually as a “Lender”),

OF THE SECOND PART,

- and -

THE BANK OF NOVA SCOTIA, a Canadian chartered bank, as agent of the Lenders (hereinafter referred to as the “Agent”),

OF THE THIRD PART.

WHEREAS each of Sumitomo Mitsui Banking Corporation of Canada and Comerica Bank (collectively, the “New Lenders”) has agreed to provide a Commitment and to become a Lender in accordance with the terms of the Credit Agreement;

AND WHEREAS certain of the existing Lenders have agreed to increase their existing Commitments;

AND WHEREAS the parties hereto have agreed to amend and supplement certain provisions of the Credit Agreement as hereinafter set forth;

NOW THEREFORE THIS AGREEMENT WITNESSES that in consideration of the covenants and agreements herein contained and other good and valuable consideration, the receipt and sufficiency of which are hereby conclusively acknowledged by each of the parties hereto, the parties hereto covenant and agree as follows:

1.1.            In this Agreement and the recitals hereto, unless something in the subject matter or context is inconsistent therewith:

“Agreement” means this agreement, as amended, modified, supplemented or restated from time to time.

“Credit Agreement” means the credit agreement made as of December 11, 2009 between the Borrower, certain of the Lenders and the Agent.

1.2.           Capitalized terms used herein without express definition shall have the same meanings herein as are ascribed thereto in the Credit Agreement.

1.3.           The division of this Agreement into Sections and the insertion of headings are for convenience of reference only and shall not affect the construction or interpretation of this Agreement. The terms “this Agreement”, “hereof”, “hereunder” and similar expressions refer to this Agreement and not to any particular Section or other portion hereof and include any agreements supplemental hereto.

1.4.           This Agreement shall be governed by and construed in accordance with the laws of the Province of Ontario and the federal laws of Canada applicable therein.

2.1.           Decrease in Margins.  The definition of “Applicable Pricing Rate” contained in Section 1.1 of the Credit Agreement is hereby amended to delete the chart contained therein in its entirety and to substitute the following chart therefor:

2.2.           Extension of Maturity Date.  The definition of “Maturity Date” contained in Section 1.1 of the Credit Agreement is hereby amended to delete “December 11, 2012” therefrom and to substitute the date “December 11, 2016” therefor. The parties hereto confirm and agree that the Maturity Date shall be and is hereby extended to December 11, 2016.

2.3.           Increase in Credit Facility.  The existing definition of “Credit Facility” contained in Section 1.1 of the Credit Agreement is hereby amended to delete “U.S.$2,500,000,000” where it

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appears in the first line thereof and to substitute therefor the amount of “U.S.$2,750,000,000”. The parties hereto hereby confirm and agree that the maximum principal amount of the Credit Facility is hereby increased to U.S.$2,750,000,000 from U.S.$2,500,000,000.

2.4.           Addition of New Lenders.

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Each New Lender and each Lender which has increased its Commitment pursuant hereto acknowledges to the Agent that a copy of the Credit Agreement (including a copy of the Schedules annexed thereto) has been made available to it for review and further acknowledges and agrees that it has received copies of such other Documents and such other information that it has requested for the purposes of its investigation and analysis of all matters related to this Agreement, the Credit Agreement, the other Documents and the transactions contemplated hereby and thereby. Each New Lender and each Lender which has increased its Commitment pursuant hereto acknowledges to the Agent that it is satisfied with the form and substance of the Credit Agreement (as amended and supplemented hereby) and the other Documents.

2.5.           New Schedule A; Revised Commitments.  Schedule A to the Credit Agreement is hereby deleted in its entirety and replaced with Schedule A attached hereto, inter alia, to increase the Commitment of certain Lenders to the amount set forth opposite its name on such new Schedule A, to set out the Commitment of each New Lender and to set out the new Swingline Sub-Commitment of each Swingline Lender.

2.6.           New Permitted Lien.  The definition of “Permitted Liens” contained in Section 1.1 of the Credit Agreement is hereby amended to add the following new subparagraph (r.1) immediately after the existing subparagraph (r):

“(r.1) Liens which are not otherwise Permitted Liens; provided that (i) the aggregate amount of obligations secured thereby does not at any time exceed U.S.$300,000,000 (or the Equivalent Amount thereof in any other currency) and (ii) such Liens do not attach generally to all or substantially all of the undertaking, assets and property of the Borrower (such as a Lien in the nature of a floating charge on all or substantially all of the undertaking, assets and property of a person);”.

2.7.           Increase in Accordion. Section 2.18(c) of the Credit Agreement is hereby amended to delete “U.S.$3,000,000,000” where it appears in the second line thereof and to substitute therefor the amount of “U.S.$3,250,000,000”.

2.8.           Deletion of Tangible Net Worth Covenant.  Section 9.1(g) of the Credit Agreement is hereby amended to delete the heading and the text therefrom in their entirety and to substitute the phrase “intentionally deleted.” therefor.

2.9.           Increase in Subsidiary Debt Limit.  Section 9.2(e) of the Credit Agreement is hereby amended to delete the amount “U.S.$650,000,000” therefrom and to substitute the amount “U.S.$1,000,000,000” therefor.

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2.10.           Change in Law.  Section 11.3(1) of the Credit Agreement is hereby amended to delete the word “hereof” contained at the end of the first paragraph thereof and to substitute the words “of the first amending agreement hereto made as of September 23, 2011” therefor.

2.11.           Event of Default Thresholds.  Sections 10.1(j), 10.1(k) and 10.1(m) of the Credit Agreement are each hereby amended to delete the amount “Cdn.$40,000,000” each time it appears in such Sections and to substitute the amount “Cdn.$100,000,000” therefor.

3.1.           Fees.  The Borrower hereby agrees to pay to the Agent, for each Lender, a fee in United States Dollars in an amount equal to 0.25% of the Commitment of each such Lender set out in Schedule A hereto.

The Borrower hereby represents and warrants as follows to each Lender and the Agent and acknowledges and confirms that each Lender and the Agent is relying upon such representations and warranties:

It is a corporation duly incorporated and organized and validly subsisting in good standing under the laws of Canada. It is duly qualified, registered or licensed in all jurisdictions where such qualification, registration or licensing is required, except where the failure to be so qualified would not have and would not reasonably be expected to have a Material Adverse Effect. It has all requisite capacity, power and authority to own, hold under licence or lease its properties necessary for the conduct of its business and to carry on its business as currently conducted. It has all requisite corporate capacity, power and authority to enter into and carry out the transactions contemplated by this Agreement.

All necessary action, corporate or otherwise, has been taken to authorize the execution, delivery and performance by the Borrower of this Agreement. It 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 contained in the opinion of the Borrower’s counsel delivered pursuant to Section 5(c).

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

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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 Governmental Authority or any other party of which the failure to have received or obtained would have or would reasonably be expected to have a Material Adverse Effect.

The representations and warranties set out in this Agreement shall survive the execution and delivery of this Agreement and the making of each Drawdown, notwithstanding any investigations or examinations which may be made by or on behalf of the Agent, the Lenders or Lenders’ Counsel. Such representations and warranties shall survive until the Credit Agreement has been terminated.

The amendments and supplements to the Credit Agreement contained in herein shall be effective upon, and shall be subject to, the following conditions precedent:

The foregoing conditions precedent are inserted for the sole benefit of the Lenders and the Agent and may be waived in writing by the Lenders, in whole or in part (with or without terms and conditions).

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The Credit Agreement and the other Documents to which the Borrower is a party and all covenants, terms and provisions thereof, except as expressly amended and supplemented by this Agreement, shall be and continue to be in full force and effect and the Credit Agreement as amended and supplemented by this Agreement and each of the other Documents to which the Borrower is a party is hereby ratified and confirmed and shall from and after the date hereof continue in full force and effect as herein amended and supplemented, with such amendments and supplements in Section 2 hereof being effective from and as of the date hereof upon satisfaction of the conditions precedent set forth in Section 5 hereof.

The parties hereto shall from time to time do all such further acts and things and execute and deliver all such documents as are required in order to effect the full intent of and fully perform and carry out the terms of this Agreement.

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

This Agreement may be executed in any number of 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, and it shall not be necessary in making proof of this Agreement to produce or account for more than one such counterpart. Such executed counterparts may be delivered by facsimile or other electronic transmission and, when so delivered, shall constitute a binding agreement of the parties hereto.

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IN WITNESS WHEREOF the parties hereto have executed this Agreement.

SCHEDULE A

LENDERS AND COMMITMENTS