<PAGE>   1

                                                                   Exhibit 10(f)

                                                                  EXECUTION COPY

                 CANPOTEX PRODUCER AGREEMENT AMENDING AGREEMENT
                   (PRODUCTIVE CAPACITY/EXPANSIONS/DISASTERS)

THIS AMENDING AGREEMENT made as of the Effective Date.
BETWEEN:

             CANPOTEX LIMITED
             ("Canpotex")                                      OF THE FIRST PART

AND:

             AGRIUM INC.
             ("Agrium")                                       OF THE SECOND PART

AND:

             IMC KALIUM CANADA LTD.
             ("Kalium")                                        OF THE THIRD PART

AND:

             IMC CENTRAL CANADA POTASH INC.
             ("CCP")                                          OF THE FOURTH PART

AND:

             INTERNATIONAL MINERALS & CHEMICAL (CANADA)
             LIMITED PARTNERSHIP, A LIMITED PARTNERSHIP, BY ITS
             GENERAL PARTNER, INTERNATIONAL MINERALS &
             CHEMICAL (CANADA) GLOBAL LIMITED
             ("IMC LP")                                      OF THE FIFTH PART
             (Kalium, CCP and IMC LP being collectively
             referred to as the "IMC Group")

AND:

             POTASH CORPORATION OF SASKATCHEWAN INC.
             ("PCS")                                         OF THE SIXTH PART
             (Each of the above parties other than Canpotex and CCP being also
             hereinafter referred to individually as a "Producer" or
              collectively as the "Producers")

WHEREAS:

1.   Canpotex has entered into with each of the Producers, or their
     predecessors, a Producer Agreement, as amended from time to time
     (individually a "Producer Agreement" and collectively, the "Producer
     Agreements"); and

2.   The parties wish to further amend the Producer Agreements on the terms and
     subject to the conditions hereinafter contained.

NOW THEREFORE THIS AGREEMENT WITNESSES that in consideration of the premises,
the covenants and agreements hereinafter contained and other good and valuable
consideration paid by each of the parties to the other, the receipt and
sufficiency of which are hereby acknowledged, the parties hereby covenant and
agree as follows:
<PAGE>   2

1.   DEFINITIONS

1.01 Except as specifically defined in this Amending Agreement or as required by
     the context hereof, words and phrases capitalized herein and the terms
     "ton" and "potash" shall have the meanings given to them in the Producer
     Agreements.

1.02 In this Amending Agreement, including the premises and preamble hereof:

     (a)  "Additional Suspension Period" means a two year period immediately
          following the last day of a Suspension Period;

     (b)  "Affected Producer" means a Producer that has had one or more of its
          Mines affected by a Disaster or affected by an event or series of
          related events that any Producer reasonably believes has caused a
          Disaster at the first mentioned Producer's Mine;

     (c)  "Aggregate Productive Capacity" means the aggregate annual productive
          capacity from time to time of the Mine or Mines, as the case may be,
          of each of the Producers, taking into account any Productive Capacity
          Change at the applicable time provided for in section 4.07 or section
          5.04 hereof, expressed as a number of Product Tonnes per annum. As of
          the Effective Date, the Aggregate Productive Capacities of the
          Producers are as follows:

<TABLE>
                    <S>                    <C>
                    Agrium...............  1,741,000 Product Tonnes per annum
                    IMC Group............  6,492,600 Product Tonnes per annum
                    PCS..................  10,333,500 Product Tonnes per annum
</TABLE>

     (d)  "Amending Agreement" means this amending agreement, including the
          premises and preamble, and any schedules hereto;

     (e)  "Basic Entitlement" of a Producer means, subject to the terms and
          provisions, if applicable, of the Canpotex Chilean Entitlement
          Amending Agreement dated with effect January 1, 1996, and entered into
          by the parties hereto, other than CCP, or their predecessors, that
          percentage that each Producer's Aggregate Productive Capacity bears to
          the total Aggregate Productive Capacities of all Producers multiplied
          by the total forecast sales of potash, grade by grade, by Canpotex for
          each fiscal year;

     (f)  "Board of Directors" means the board of directors of Canpotex;

     (g)  "Canpotex Auditors" means the external auditors of Canpotex from time
          to time which, as at the Effective Date, are Deloitte & Touche LLP;

     (h)  "Disaster" means any event or series of related events occurring with
          respect to a Mine that has or have the effect of reducing the
          productive capacity of such Mine by an amount equal to at least 50% of
          the Pre-Disaster Audit Amount of such Mine;

     (i)  "Effective Date" means 12:01 a.m. (Saskatchewan time) on January 1,
          1999;

     (j)  "Major Expansion" means any capital expansion undertaken by a Producer
          of an existing Mine or the construction of a new Mine, occurring after
          the Effective Date, which results in an increase of (or in the case of
          a new Mine the creation of) the productive capacity of such Mine of at
          least 200,000 Product Tonnes annually; for greater certainty, the
          expansions announced by Kalium and CCP on February 9, 1998 with
          respect to their Mines located near Belle Plaine, Saskatchewan and
          Colonsay, Saskatchewan, respectively, shall be governed by this
          Amending Agreement notwithstanding that they were announced prior to
          the Effective Date, and February 9, 1998 shall for all purposes hereof
          be considered the date of the Notice of Expansion for each of these
          expansions; such expansion at Kalium's Belle Plaine Mine shall be
          considered as a staged Major Expansion, as contemplated in section
          4.04 hereof;
<PAGE>   3

     (k)  "Mine" means a single potash mine and mill situated in Saskatchewan of
          a Producer and "Mines" means more than one Mine; for greater
          certainty, it is acknowledged that there are currently two Mines at
          IMC LP's facilities located near Esterhazy, Saskatchewan;

     (l)  "Notice of Expansion" means a notice provided for in section 4.01
          hereof, which shall be delivered in accordance with section 12.01
          hereof, and which shall be delivered not more than 180 days and not
          less than 60 days before the date of commencement of the Major
          Expansion;

     (m)  "Operating Days" with respect to a Mine means days that the Mine is
          operational and producing potash of any grade. If the production
          schedule at a Mine is 10 days of production followed by 4 days of
          non-production for maintenance and other purposes, then only the 10
          days of production would be considered as Operating Days for purposes
          hereof; accordingly, with such a production schedule, for purposes of
          this Agreement 90 Operating Days at a Mine would be achieved after 9
          consecutive 10 and 4 cycles, and 180 Operating Days would be achieved
          after 18 consecutive 10 and 4 cycles. Regardless of the production
          schedule used at a Mine, non-production days for maintenance or other
          purposes will not be considered "Operating Days" for purposes hereof,
          provided that in the case of a Mine, other than a Solution Mine,
          operating on a continuous production schedule, any 90 Operating Day
          period must be completed within 122 days from and including the first
          Operating Day of such period, and provided further that in the case of
          a Solution Mine operating on a continuous production schedule, any 180
          Operating Day period must be completed within 248 days from and
          including the first Operating Day of such period;

     (n)  "Operating Plan and Budget" means the annual operating plan and budget
          of Canpotex prepared by Canpotex management and approved by the Board
          of Directors;

     (o)  "Operating Results" means the operating results of a Producer's Mine
          as set out in its monthly production reports;

     (p)  "Plan" means a written plan submitted by or on behalf of an Affected
          Producer to Canpotex and the remaining Producers and certified by or
          on behalf of such Affected Producer as being a plan which it intends
          to begin to implement within 180 days and which when fully implemented
          will reasonably be likely to (i) restore to at least the Pre-Disaster
          Audit Amount the productive capacity of a Mine affected by a Disaster,
          or (ii) increase the productive capacity of another Mine of the
          Affected Producer by an amount equal to at least the difference
          between the Pre-Disaster Audit Amount and the Post-Disaster Audit
          Amount of the affected Mine;

     (q)  "Post-Disaster Audit Amount" means the productive capacity of a Mine
          as verified by the Canpotex Auditors following the occurrence of a
          Disaster affecting such Mine and before the implementation of any
          Plan, based on the Operating Results of such Mine for a demonstration
          period of 90 Operating Days, or 180 Operating Days in the case of a
          Solution Mine, chosen by the Affected Producer and based on 350
          Operating Days per year; provided that the production schedule
          employed during such demonstration period must be the same as that
          employed at such mine immediately prior to the occurrence of the
          Disaster;

     (r)  "Post-Expansion Audit Amount" in the context of a Major Expansion
          means the productive capacity of the expanded or new Mine, as the case
          may be, as verified by the Canpotex Auditors and based on the
          Operating Results of such Mine for a demonstration period of 90
          Operating Days, or 180 Operating Days in the case of a Solution Mine,
          chosen by the applicable Producer and based on 350 Operating Days per
          year;

     (s)  "Post-Plan Audit Amount" in the context of a Disaster means the
          productive capacity of the affected Mine following full implementation
          of the applicable Plan, as verified by the Canpotex Auditors and based
          on the Operating Results of such Mine for a demonstration period of 90
          Operating Days, or 180 Operating Days in the case of a Solution Mine,
          chosen by the Affected Producer and based on 350 Operating Days per
          year;
<PAGE>   4

     (t)  "Pre-Disaster Audit Amount" means, in respect of a Mine that has been
          affected by an event or series of related events that any Producer
          reasonably believes has caused a Disaster, the productive capacity of
          the Mine as verified by the Canpotex Auditors and based on the
          Operating Results of such Mine for the 90 Operating Day period, or 180
          Operating Day period in the case of a Solution Mine, with the highest
          cumulative production within the one year period immediately preceding
          the commencement of the Disaster, based on 350 Operating Days per
          year;

     (u)  "Pre-Expansion Audit Amount" in the context of a Major Expansion,
          means the following as applicable:

          (i)   in respect of a Major Expansion involving an existing Mine, the
                productive capacity of such Mine as verified by the Canpotex
                Auditors and based on the Operating Results of such Mine for the
                90 Operating Day period, or 180 Operating Day period in the case
                of a Solution Mine, with the highest cumulative production
                within the one year period immediately preceding the applicable
                Notice of Expansion, based on 350 Operating Days per year; or

          (ii)  in respect of a Major Expansion involving a new Mine, nil;

     (v)  "Product Tonne" means a ton of potash of any grade but expressed for
          purposes of this Amending Agreement as KCL; for greater certainty,
          "ton" for purposes hereof, as defined in the Producer Agreements,
          means a metric tonne which equals 1,000 kilograms;

     (w)  "Productive Capacity Change" means the difference between the
          Pre-Expansion Audit Amount and the Post-Expansion Audit Amount or the
          difference between the Pre-Disaster Audit Amount and the Post-Plan
          Audit Amount, for an existing Mine or a new Mine, as the case may be;

     (x)  "Solution Mine" means a Mine that utilizes a dissolution method to
          extract potash from an underground ore body and that uses one or more
          crystallization ponds as part of the potash production process; and

     (y)  "Suspension Period" means the three year period immediately following
          the occurrence of a Disaster.

1.03 For purposes of this Amending agreement, a Disaster will be deemed to:

     (a)  have commenced on the date of the occurrence of the event or the first
          of the series of related events that has or have caused the Disaster;
          and

     (b)  have occurred on the date that the productive capacity of the affected
          Mine is in fact reduced by an amount equal to at least 50% of its
          Pre-Disaster Audit Amount.

1.04 It is acknowledged that CCP is not a party to a Producer Agreement with
     Canpotex, and that potash produced by CCP at its Mine near Colonsay,
     Saskatchewan is sold by CCP to Canpotex pursuant to the Producer Agreement
     between Kalium and Canpotex. Accordingly, for purposes hereof Kalium and
     CCP are considered collectively as one entity, and "Producer" in reference
     to Kalium shall include both Kalium and CCP unless the context otherwise
     requires. CCP has entered into this Agreement in order to confirm these
     matters and to evidence its consent and agreement to be bound hereby.

2.   PRIORITY OF THIS AGREEMENT

2.01 The provisions of this Amending Agreement shall amend the Producer
     Agreements and wherever there is a conflict between the provisions of this
     Amending Agreement and the provisions of the Producer Agreements, the
     provisions of this Amending Agreement shall apply. In all other respects,
     the provisions of the Producer Agreements are hereby confirmed and shall
     remain in full force and effect.
<PAGE>   5

3.   BASIC ENTITLEMENTS AND AGGREGATE PRODUCTIVE CAPACITIES

3.01 Prior to December 1st in each year, Canpotex management shall deliver to
     the Board of Directors the Operating Plan and Budget for the immediately
     following fiscal year which shall include a schedule setting out the Basic
     Entitlement of each Producer to supply potash, grade by grade, for export
     by Canpotex for that fiscal year to which such Operating Plan and Budget
     applies. Only one aggregate Basic Entitlement will be included in such
     schedule for the IMC Group.

3.02 For the fiscal year beginning January 1, 1999, and for each fiscal year
     thereafter, the Basic Entitlement of a Producer to supply potash to
     Canpotex for export, grade by grade, and for establishing the quantities of
     potash to be purchased by Canpotex from a Producer, including any Producer
     that becomes a party hereto after the Effective Date, shall be as
     determined in accordance with the applicable Producer Agreement as amended
     by this Amending Agreement.

3.03 For greater certainty, commencing with the 1999 fiscal year and continuing
     thereafter until changed pursuant to the terms and provisions hereof, the
     Aggregate Productive Capacities of the Producers shall be the Aggregate
     Productive Capacities specified in paragraph 1.02 (c) hereof.

3.04 It is acknowledged that the Aggregate Productive Capacity of the IMC Group
     is the aggregate of the productive capacities of Kalium, CCP and IMC LP. It
     is further acknowledged that Canpotex shall at all times be entitled to
     treat the collective productive capacities of Kalium, CCP and IMC LP as one
     aggregate Productive Capacity for purposes hereof and that it shall be the
     responsibility of the IMC Group to ensure that the obligations to deliver
     Product Tonnes to Canpotex consistent with such Aggregate Productive
     Capacity are satisfied by the IMC Group.

3.05 Notwithstanding that, in accordance with this Amending Agreement, the Basic
     Entitlement of the Producers will henceforth be calculated with reference
     to Aggregate Productive Capacities and Product Tonnes expressed as KCL, it
     is agreed that the purchase price per ton of potash finally payable by
     Canpotex to the Producers for each grade of potash purchased under the
     Producer Agreements shall continue to be calculated in tons of K(2)O
     equivalent.

4.   MAJOR EXPANSIONS

4.01 Any Producer which intends to undertake a Major Expansion shall give a
     Notice of Expansion to Canpotex and contemporaneously to the other
     Producers to that effect.

4.02 Following the delivery of a Notice of Expansion, the Producer undertaking
     such Major Expansion shall, as soon as reasonably practicable thereafter,
     supply the necessary Operating Results to the Canpotex Auditors to enable
     the Canpotex Auditors to verify the Pre-Expansion Audit Amount.

4.03 Following the delivery of a Notice of Expansion, the applicable Producer
     may, by notice to Canpotex and contemporaneously to the other Producers, at
     any time voluntarily revoke the Notice of Expansion, and shall be deemed to
     have revoked the Notice of Expansion if the audit to determine the Post-
     Expansion Audit Amount has not been completed within five years following
     the date that the Notice of Expansion was delivered, unless otherwise
     agreed by all of the Producers, In the case of a voluntary revocation, the
     applicable Producer shall provide notice thereof to Canpotex and each of
     the other Producers.

4.04 If a Producer provides a Notice of Expansion with respect to a proposed
     Major Expansion equal to or greater than 400,000 Product Tonnes increase in
     annual productive capacity, then for purposes of this Amending Agreement
     such Producer shall be entitled to treat the overall Major Expansion as a
     series of smaller Major Expansions. In such case the Producer shall be
     entitled to complete a series of demonstration periods, as contemplated in
     paragraph 1.02(r) hereof, in order to provide a series of Post-Expansion
     Audit Amounts and thereby have its Aggregate Productive capacity increased
     in stages during the period in which the overall Major Expansion is being
     completed; provided that each such staged increase must be at least 200,000
     Product Tonnes per annum.
<PAGE>   6

     If a Producer wishes to complete the overall Major Expansion in stages as
     contemplated above, it shall so indicate in its Notice of Expansion. The
     provisions of this Amending Agreement relating to Major Expansions shall
     apply to any such staged Major Expansions, mutatis mutandis.

     Any voluntary or deemed revocation of such a Notice of Expansion shall
     result in a revocation of the Notice of Expansion only as it relates to the
     stages of the Major Expansion for which a Post-Expansion Audit Amount has
     not been verified as at the date of such revocation.

4.05 Following completion of a Major Expansion, the applicable Producer shall,
     as soon as reasonably practicable thereafter, supply the necessary
     Operating Results to the Canpotex Auditors to enable the Canpotex Auditors
     to verify the Post-Expansion Audit Amount.

4.06 The Pre-Expansion Audit Amount verification and the Post-Expansion Audit
     Amount verification by the Canpotex Auditors shall be delivered to Canpotex
     and the Producers as soon as reasonably practicable after completion of
     each.

4.07 If a Major Expansion has been fully completed and verified in compliance
     with this Article 4, the Aggregate Productive Capacity of the applicable
     Producer shall thereafter be increased by the amount of the Productive
     Capacity Change effective from and after:

     (a)  January 1 in the immediately following fiscal year, provided such
          Major Expansion has been fully completed and verified on or before
          December 31 in any year; or

     (c)  July 1 in any fiscal year, provided such Major Expansion has been
          fully completed and verified on or before June 30 in such fiscal year.

4.08 Any Producer who operates a Solution Mine will not accumulate crystal
     potash product in a crystallization pond or ponds that would unduly
     influence a Post-Expansion Audit Amount.

5.   DISASTERS

5.01 (a)  If after the Effective Date an event or series of related events
          occurs that the Affected Producer reasonably believes has or have
          caused a Disaster at a Mine, the Affected Producer shall, as soon as
          practicable, give notice of such event(s) (an "Owner Disaster Notice")
          to the other Producers and contemporaneously to Canpotex. If such an
          event or series of related events occurs but no Owner Disaster Notice
          is given, any of the other Producers that reasonably believes that
          such event(s) has or have caused a Disaster shall be entitled to give
          notice of such event(s) (a "Non-Owner Disaster Notice") simultaneously
          to all of the other parties hereto.

     (b)  If either an Owner Disaster Notice or a Non-Owner Disaster Notice is
          given, an audit shall be completed to determine the Pre-Disaster Audit
          Amount, and the Affected Producer shall, as soon as reasonably
          practicable, and in any event within six months following the date any
          such Notice is given, supply the necessary Operating Results to the
          Canpotex Auditors to enable the Canpotex Auditors to verify the
          Pre-Disaster Audit Amount. The Pre-Disaster Audit Amount verification
          shall be delivered to Canpotex and the Producers as soon as reasonably
          practicable thereafter.

     (c)  If either an Owner Disaster Notice or a Non-Owner Disaster Notice is
          given the Affected Producer shall, as soon as reasonably practicable
          and in any event within one year following the date that any such
          Notice is given, supply the necessary Operating Results to the
          Canpotex Auditors to enable the Canpotex Auditors to verify the
          Post-Disaster Audit Amount. The Post-Disaster Audit Amount
          verification shall be delivered to Canpotex and the Producers as soon
          as reasonably practicable thereafter.

     (d)  The Pre-Disaster Audit Amount and the Post-Disaster Audit Amount will
          be used to confirm whether or not a Disaster has in fact occurred at
          the Mine.

     (e)  Each party hereto represents to the others that as of the Effective
          Date it was not aware of, and on the date of signing this Amending
          Agreement is not aware of, the occurrence of any event or series of
          related events at any of its Mines that has or have caused a Disaster
          at any such Mines,
<PAGE>   7

          provided that with respect to PCS's Patience Lake Mine near Saskatoon,
          Saskatchewan, PCS's representation as aforesaid relates only to the
          time period since PCS acquired, directly or indirectly, such Mine in
          1993.

5.02 If after the Effective Date a Disaster occurs, the Aggregate Productive
     Capacity and Basic Entitlement of the Affected Producer shall not be
     reduced as provided for in section 5.05 hereof provided that each of the
     following events occur:

     (a)  on or prior to the 30th day preceding the end of the Suspension
          Period, the Affected Producer submits a Plan to Canpotex and the other
          Producers that is reasonably likely to achieve the results specified
          therein; and

     (b)  the Affected Producer fully implements the Plan before the expiration
          of the Additional Suspension Period and such implementation achieves
          the results specified in the Plan.

5.03 Following full implementation of the Plan, the Affected Producer shall, as
     soon as reasonably practicable thereafter, supply the necessary Operating
     Results to the Canpotex Auditors to enable the Canpotex Auditors to verify
     the Post-Plan Audit Amount. The Post-Plan Audit Amount verification shall
     be delivered to Canpotex and the Producers as soon as reasonably
     practicable thereafter.

5.04 If a Plan has been fully implemented and verified in compliance with this
     Article 5, the Aggregate Productive Capacity of the Affected Producer
     shall, subject to the terms hereof, thereafter be the Aggregate Productive
     Capacity of the Affected Producer immediately before the Disaster:

     (a)  minus the pre-Disaster Audit Amount; and

     (b)  plus the Post-Plan Audit Amount;

     provided that the Aggregate Productive Capacity calculated as aforesaid
     shall not be increased above the Aggregate Productive Capacity of the
     Affected Producer immediately before the Disaster, unless such increase
     will be at least 200,000 Product Tonnes.

5.05 If a Disaster occurs and any of the events specified in section 5.02 hereof
     does not occur within the time frame set forth therein, then unless
     otherwise agreed by all Producers, the Aggregate Productive Capacity of the
     Affected Producer shall be reduced by an amount equal to the difference
     between the Pre-Disaster Audit Amount and the Post-Disaster Audit amount.

5.06 Any Producer who operates a Solution Mine will not accumulative crystal
     potash product in a crystallization pond or ponds that would unduly
     influence a Post-Plan Audit Amount or Post-Disaster Audit Amount.

6.   FURTHER ASSURANCES

6.01 Each of the parties hereto hereby covenants and agrees to be bound by,
     observe, perform and do all things and take all action, steps, proceedings
     and execute such further and other assurances, documents and agreements
     whether under corporate seal or otherwise as are reasonably necessary or
     required to fully implement and give effect to all of the terms and
     provisions of this Amending Agreement.

7.   GOVERNING LAW

7.01 This Amending Agreement shall be construed and interpreted in accordance
     with the laws of the Province of Saskatchewan and the laws of Canada
     applicable therein.

8.   ENTIRE AGREEMENT AND CONFIRMATION

8.01 This Amending Agreement contains the whole of the agreement between and
     among the parties hereto relating to the subject matter hereof and there
     are no collateral or precedent representations, agreements, or conditions
     not specifically set forth herein, and no modification amendment or
     variation shall be effective or binding upon the Parties hereto unless
     agreed to in writing beforehand.
<PAGE>   8

      Notwithstanding the above, it is acknowledged that the Producer
      Agreements shall remain in force, as amended hereby.

9.    ENUREMENT

9.01  The parties hereto covenant and agree that this Amending Agreement shall
      be binding upon and enure to the benefit of the parties hereto, their
      successors and permitted assigns.

10.   ASSIGNMENT

10.01 The parties hereto covenant and agree that this Amending Agreement may not
      be assigned in whole or in part by any of the parties hereto except in
      accordance with the terms and provisions of the Producer Agreement
      applicable to such assignment.

11.   SEVERABILITY

11.01 It is hereby agreed that in the event any clause, provision, paragraph,
      subparagraph or section of this Amending Agreement is held invalid as
      contrary to any statute or regulation or law in that regard by a court of
      competent jurisdiction, the invalidity of such shall in no way affect the
      validity of any other clause provision, paragraph, subparagraph or section
      of this Amending Agreement and each and every such clause, provision,
      paragraph, subparagraph or section of this Amending Agreement shall be
      severable from each and every other.

12.   NOTICES

12.01 Any notice, demand, request, declaration or communication required or
      permitted to be made or given hereunder or under the Producer Agreements
      shall be in writing and shall be given by personal service upon an officer
      of the party to whom it is intended or mailed by prepaid registered mail
      or transmitted by telecopier, at the following addresses:

      (a)  CANPOTEX LIMITED
           111 - 2nd Avenue South
           Suite 400
           Saskatoon, Saskatchewan, Canada S7K 1K6
           Attention:    President
           Facsimile:    (306) 653-5505

      (b)  AGRIUM INC.
           13131 Lake Fraser Drive, S.E.
           Calgary, Alberta, Canada T2J 7E8
           Attention:    President
           Facsimile:    (403) 225-7600

      (c)  IMC KALIUM CANADA LTD.
           Plant Site
           Belle Plaine, Saskatchewan, Canada S0G 0G0
           Attention:    President
           Facsimile:    (847) 607-3101

      (d)  IMC CENTRAL CANADA POTASH INC.
           PO Box 1500
           Colonsay, Saskatchewan, Canada S0K 0X0
           Attention:    President
           Facsimile:    (847) 607-3101
<PAGE>   9

      (e)  INTERNATIONAL MINERALS & CHEMICAL (CANADA)
           LIMITED PARTNERSHIP
           c/o INTERNATIONAL MINERALS & CHEMICAL (CANADA)
           GLOBAL LIMITED
           General Delivery, Highway 80 East
           Esterhazy, Saskatchewan, Canada S0A 0X0
           Attention:    President
           Facsimile:    (847) 607-3101

      (f)  POTASH CORPORATION OF SASKATCHEWAN INC.
           PCS Tower
           Suite 500, 122 - 1st Avenue South
           Saskatoon, Saskatchewan, Canada S7K 7G3
           Attention:    President
           Facsimile:    (306) 933-8888

For these purposes, such notice shall be mailed by prepaid registered mail
addressed to the recipient at the address above set forth or at such other
address and/or to the attention of any such other person or officer as either of
the parties may from time to time or at any time advise by notice in writing to
the other parties hereto. The date of receipt of any notice shall be deemed to
be the tenth business day next following the date of such mailing, provided that
if at the date of such mailing, interruption in the operation of the postal
service in Canada or in the United States has or is likely to delay the mailing
and receipt of such notice, the same shall be served personally on an officer of
the other parties hereto or served by telecopy transmission. Notice shall be
effective upon the deemed date of receipt in the case of delivery by mail; upon
the actual date of receipt, in the case of delivery by personal service; and on
the next day following the date of transmission, in the case of delivery by
telecopier. Any of the parties hereto may, in the manner provided for the giving
of notice as aforesaid, change its address for service.

13.   EXECUTION

13.01 The parties hereto agree that this Agreement may be executed
      simultaneously in two or more counterparts, each of which shall be deemed
      to be an original, but all of which when taken together shall constitute
      one and the same instrument.

IN WITNESS WHEREOF the parties hereto have hereunto affixed their corporate
seals attested by their proper officers in that behalf effective as and from the
Effective Date, it being acknowledged that actual execution did not occur until
after the Effective Date.
                                         CANPOTEX LIMITED

                                         Per: /s/
                                         ---------------------------------------
                                         Per: /s/
                                         ---------------------------------------


                                         AGRIUM INC.

                                         Per: /s/
                                         ---------------------------------------
                                         Per: /s/
                                         ---------------------------------------
<PAGE>   10

                                         IMC KALIUM CANADA LTD.

                                         Per: /s/
                                         ---------------------------------------
                                         Per: /s/
                                         ---------------------------------------


                                         IMC CENTRAL CANADA POTASH INC.

                                         Per: /s/
                                         ---------------------------------------
                                         Per: /s/
                                         ---------------------------------------


                                         INTERNATIONAL MINERALS & CHEMICAL
                                         (CANADA) LIMITED PARTNERSHIP, a limited
                                         partnership, by its General Partner,
                                         INTERNATIONAL MINERALS & CHEMICAL
                                         (CANADA) GLOBAL LIMITED

                                         Per: /s/
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                                         Per: /s/
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                                         POTASH CORPORATION OF SASKATCHEWAN INC.

                                         Per: /s/
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                                         Per: /s/
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