POTASH CORPORATION OF SASKATCHEWAN INC. 3.75% Notes due September 30, 2015

Exhibit 4(a)

      This Security is in global form within the meaning of the Indenture hereinafter referred to and is registered in the name of The Depository Trust Company, a New York corporation (“DTC”), or a nominee of DTC, which may be treated by the Company, the Trustee and any agent thereof as owner and holder of this Security for all purposes.

      Unless this certificate is presented by an authorized representative of DTC to the Company or its agent for registration of transfer, exchange, or payment, and any certificate issued is registered in the name of Cede & Co. or in such other name as is requested by an authorized representative of DTC (and any payment is made to Cede & Co. or to such other entity as is requested by an authorized representative of DTC), ANY TRANSFER, PLEDGE OR OTHER USE HEREOF FOR VALUE OR OTHERWISE BY OR TO ANY PERSON IS WRONGFUL inasmuch as the registered owner hereof, Cede & Co., has an interest herein.

      Unless and until it is exchanged in whole or in part for Securities in definitive form in the limited circumstances referred to in the Indenture, this global Security may not be transferred except as a whole by DTC to a nominee of DTC or by a nominee of DTC to DTC or another nominee of DTC or by DTC or any such nominee to a successor depositary or a nominee of such successor depositary.

POTASH CORPORATION OF SASKATCHEWAN INC.

3.75% Notes due September 30, 2015

     POTASH CORPORATION OF SASKATCHEWAN INC., a Canadian corporation (hereinafter called the “ Company ,” which term shall include any successor entity under the Indenture), for value received, hereby promises to pay to Cede & Co., as nominee for DTC, or registered assigns, upon presentation, the principal sum of FIVE HUNDRED MILLION DOLLARS ($500,000,000) on September 30, 2015 and to pay interest thereon from September 28, 2009 or from the most recent Interest Payment Date to which interest has been paid or duly provided for, semi-annually in arrears on March 30 and September 30 in each year, commencing March 30, 2010, at the rate of 3.75% per annum, until the entire principal amount hereof is paid or made available for payment.

     The interest so payable, and punctually paid or duly provided for on any Interest Payment Date will, as provided in the Indenture, be paid to the Person in whose name this Security (or one or more Predecessor Securities) is registered at the close of business on the Regular Record Date for such interest, which shall be March 15 or September 15 (whether or not a Business Day), as the case may be, next preceding such Interest Payment Date. Any such interest not so punctually paid or duly provided for shall forthwith cease to be payable to the Holder on such Regular Record Date, and may either be paid to the Person in whose name this Security (or one or more Predecessor Securities) is registered at the close of business on a Special Record Date for the

payment of Defaulted Interest to be fixed by the Trustee, notice whereof shall be given to Holders of Securities of this series not more than 15 days and not less than 10 days prior to such Special Record Date, or may be paid at any time in any other lawful manner not inconsistent with the requirements of any securities exchange on which the Securities may be listed, and upon such notice as may be required by such exchange, all as more fully provided in the Indenture.

     Payment of the principal of, interest on and Additional Amounts, if any, with respect to this global Security will be paid to DTC for the purpose of permitting DTC to credit the principal and interest received by it in respect of this global Security to the accounts of the beneficial owners thereof; provided , however , that if this Security is not a global Security, payment of the principal of, interest on and Additional Amounts, if any, with respect to this Security will be made at the office or agency of the Trustee in The City of New York, or elsewhere as provided in the Indenture, in such coin or currency of the United States of America as at the time of payment is legal tender for payment of public and private debts; and provided , further , that at the option of the Company payment of interest may be made by (a) check mailed to the address of the Person entitled thereto as such address shall appear in the Security Register or (b) transfer to an account of the Person entitled thereto located inside the United States.

     Additional provisions of this Security are set forth following the signature page hereof, which provisions shall for all purposes have the same effect as if set forth at this place.

     IN WITNESS WHEREOF, the Company has caused this instrument to be duly executed this 28 ^th day of September, 2009.

TRUSTEE’S CERTIFICATE OF AUTHENTICATION

     This is one or all of the Securities of the series designated “3.75% Notes due September 30, 2015” pursuant to the within-mentioned Indenture.

3.75% Notes due September 30, 2015

     This Security is one or all of a duly authorized issue of securities of the Company (herein called the “ Securities ”) issued and to be issued in one or more series under an Indenture, dated as of February 27, 2003 (herein called the “ Indenture ”), between the Company and The Bank of Nova Scotia Trust Company of New York, as trustee (herein called the “ Trustee ”, which term includes any successor trustee under the Indenture), to which Indenture and all indentures supplemental thereto reference is hereby made for a statement of the respective rights, limitation of rights, duties and immunities thereunder of the Company, the Trustee and the Holders of the Securities and of the terms upon which the Securities are, and are to be, authenticated and delivered. This Security is one or all of the series designated as the “3.75% Notes due September 30, 2015.”

     The Securities in this series are redeemable, in whole or in part, at the Company’s option at any time and from time to time at a Redemption Price equal to the greater of (i) 100% of the principal amount of the Securities to be redeemed and (ii) the sum of the present values of the Remaining Scheduled Payments discounted to the relevant Redemption Date on a semi-annual basis (assuming a 360-day year consisting of twelve 30-day months) at the Adjusted Treasury Rate plus 25 basis points, together with, in each case, accrued interest on the principal amount of the Securities to be redeemed to the Redemption Date.

     In connection with such optional redemption, the following defined terms apply:

     “ Adjusted Treasury Rate ” means, with respect to any Redemption Date, the rate per annum equal to the semi-annual equivalent yield to maturity (computed as of the third Business Day immediately preceding that Redemption Date) or interpolated (on a day count basis) of the Comparable Treasury Issue, assuming a price for the Comparable Treasury Issue (expressed as a percentage of its principal amount) equal to the Comparable Treasury Price for that Redemption Date.

     “ Comparable Treasury Issue ” means the United States Treasury security or securities selected by the Independent Investment Banker that would be utilized, at the time of selection and in accordance with customary financial practice, in pricing new issues of corporate debt securities of comparable maturity to the remaining term of the Securities of this series.

     “ Comparable Treasury Price ” means, with respect to any Redemption Date, (a) the average of the Reference Treasury Dealer Quotations for that Redemption Date, after excluding the highest and lowest of such Reference Treasury Dealer Quotations, or (b) if the Independent Investment Banker for the Securities obtains fewer than four such Reference Treasury Dealer Quotations, the average of all such quotations.

     “ Independent Investment Banker ” means one of the Reference Treasury Dealers appointed by the Company to act as the “Independent Investment Banker.”

     “ Reference Treasury Dealer ” means Banc of America Securities LLC and HSBC Securities (USA) Inc. and their respective successors and two other nationally recognized investment banking firms, each of which is a primary U.S. Government securities dealer in New York City (herein called a “ Primary Treasury Dealer ”) specified from time to time by the

Company; provided , however , that if any of the foregoing shall cease to be a Primary Treasury Dealer, the Company shall substitute therefor another nationally recognized investment banking firm that is a Primary Treasury Dealer.

     “ Reference Treasury Dealer Quotations ” means, with respect to each Reference Treasury Dealer and any Redemption Date, the average, as determined by the Independent Investment Banker, of the bid and asked prices for the Comparable Treasury Issue (expressed in each case as a percentage of its principal amount) quoted in writing to the Independent Investment Banker by such Reference Treasury Dealer at 3:30 p.m., New York City time, on the third Business Day preceding that Redemption Date.

     “ Remaining Scheduled Payments ” means, with respect to each Security to be redeemed, the remaining scheduled payments of the principal thereof and interest thereon that would be due after the related Redemption Date but for such redemption; provided , however , that, if that Redemption Date is not an Interest Payment Date with respect to such Security, the amount of the next succeeding scheduled interest payment thereon will be reduced by the amount of interest accrued thereon to that Redemption Date.

     Notice of any redemption will be mailed at least 30 days but not more than 60 days before the Redemption Date to each Holder of the Securities in this series to be redeemed. On and after any Redemption Date, interest will cease to accrue on the Securities in this series or any portion thereof called for redemption. On or before any Redemption Date, the Company shall deposit with the Trustee or