<PAGE>
 
                                                                    EXHIBIT 10.8

                         AMENDMENT NO. 4 OF AGREEMENT


    This Amendment No. 4 of Agreement ("Amendment No. 4") is effective as
of January 1, 1997, between Office Cherifien des Phosphates ("OCP") and
Mississippi Phosphates Corporation ("MPC").

    WHEREAS, MPC and OCP are parties to that certain Agreement with an effective
date of September 15, 1991, for the sale and purchase of all MPC's requirements
of phosphate rock at its Pascagoula, plant ("Agreement"); and

    WHEREAS, the Agreement has been amended by Amendment No. 1 effective as of
July 1, 1992, Amendment No. 2 effective as of July 1, 1993, and Amendment No. 3
effective as of January 1, 1995; and

    WHEREAS, MPC and OCP desire to further amend the Agreement as hereinafter
set forth; and

    WHEREAS, MISSISSIPPI CHEMICAL CORPORATION, the parent company of MPC, agrees
to be substituted to MPC in taking responsibility of the latest with respect to
the environmental problems linked to the gypsum stacks; and

    NOW, THEREFORE, MPC and OCP hereby agree as follows:

    1.   Article I of the Agreement is hereby amended by changing the second
sentence of the first paragraph thereof to read in its entirety as follows:

         The term of this Agreement shall commence on the Effective Date and
         shall continue until June 30, 2016.

    2.   The Agreement is hereby amended by adding thereto a new Article XIII,
which shall read in its entirety as follows:

                                 ARTICLE XIII

         Subject to its timely receipt of all required governmental permits,
         authorizations and approvals in acceptable form, MPC intends to
         construct a new phosphogypsum disposal facility (the "New Gypsum
         Facility"), at an estimated cost of $16.2 million, which will
         ultimately replace the phosphogypsum disposal facility currently being
         utilized by MPC (the "Old Gypsum Facility").  With respect to the New
         Gypsum Facility and the Old Gypsum Facility, MPC and OCP agree as
         follows:

             (a)  Cost of Products Sold, as defined in Article IV hereof, shall
         include depreciation expense related to the actual cost of the New
         Gypsum Facility. As each ton of phosphogypsum is placed in the New
         Gypsum Facility, Cost of Products Sold shall be increased by a "units-
         of-production" depreciation charge equal to the quotient of (i) the
         undepreciated cost of the New Gypsum Facility divided by (ii) the
         estimated number of tons of unused capacity 
<PAGE>
 
         remaining in the New Gypsum Facility. It is expected that this capacity
         will be utilized at reasonably uniform rates over its estimated useful
         life of 18 years.

             (b)  Cost of Products Sold, as defined in Article IV hereof, shall
         include charges related to the estimated cost of closing the New Gypsum
         Facility. As each ton of phosphogypsum is placed in the New Gypsum
         Facility, Cost of Products Sold shall be increased by a closure charge
         equal to the quotient of (i) the unamortized estimated cost of closing
         the New Gypsum Facility divided by (ii) the estimated number of tons of
         unused capacity remaining in the New Gypsum Facility. The estimated
         cost of closing the New Gypsum Facility is $13.5 million. Closure
         charges related to the Old Gypsum Facility will continue to be
         recognized as its remaining capacity is utilized. The estimated closure
         cost of the Old Gypsum Facility is $9.4 million, out of which an amount
         of $2.24 million has to be amortized as from July 1, 1996, till
         saturation of the Old Gypsum storage.

             (c)  If, prior to the actual closure of either the New Gypsum
         Facility or the Old Gypsum Facility, a third party assumes
         responsibility for the cost of closing either the New Gypsum Facility
         or the Old Gypsum Facility, then MPC agrees to pay to OCP an amount
         equal to fifty percent (50%) of the unspent portion of all closure
         charges which have been recognized and reflected in all previous
         calculations of Additional Price pursuant to Article IV of this
         Agreement and which relate to the facility for which the third party
         has assumed responsibility to close. MPC further agrees that OCP shall
         have no responsibility for effecting the actual closure of either the
         New Gypsum Facility or the Old Gypsum Facility.

    3.   The Agreement is hereby amended by adding thereto a new Article XIV,
which shall read in its entirety as follows:

                                  ARTICLE XIV

         Subject to its timely receipt of all required governmental permits,
         authorizations and approvals in acceptable form, MPC intends to
         construct an expansion of the Pascagoula Plant (the "Pascagoula Plant
         Expansion"), which is expected to increase the Pascagoula Plant's
         annual production rate to approximately 900,000 short tons, at an
         estimated cost of $10.45 million. Cost of Products Sold, as defined in
         Article IV hereof, shall include depreciation expense related to the
         actual cost of the assets comprising the Pascagoula Plant Expansion.
         The assets comprising the Pascagoula Plant Expansion will be
         depreciated on a "straight-line" basis over their estimated useful
         lives, which range from five (5) to eighteen (18) years and set forth
         in the table hereto as Schedule 1 to this Amendment No. 4.


                                       2
<PAGE>
 
    4.   The Agreement is hereby amended by adding thereto a new Article XV,
which shall read in its entirety as follows:

                                  ARTICLE XV

         MPC agrees to indemnify, hold harmless and defend OCP from and against
         any and all losses, costs, damages, injuries, liabilities, claims,
         demands, penalties, or causes of action arising out of or in connection
         with the environmental conditions at the Old Gypsum Facility and the
         New Gypsum Facility.

         In any case including MPC failure, MISSISSIPPI CHEMICAL CORPORATION,
         the parent company of MPC, will be sole responsible and will pay for
         all losses, costs, damages, injuries, liabilities, claims, demands,
         penalties, or causes of action arising out of or in connection with the
         environmental conditions at the Old Gypsum Facility and the New Gypsum
         Facility.

    5.   The Agreement is amended by attaching thereto Exhibit F which is
attached as Schedule 1 to this Amendment No. 4.

    6.   Except as specifically set forth in this Amendment No. 4, all of the
terms and conditions of the Agreement, as heretofore amended, shall continue in
full force and effect.

    7.   All capitalized terms used in this Amendment No. 4 and not otherwise
defined herein shall have the meanings set forth in the Agreement.

    IN WITNESS WHEREOF, MPC and OCP have caused this Amendment No. 4 to be duly
executed as of the 1st day of January, 1997.

    MADE OUT IN DUPLICATE ON FEBRUARY 20, 1997.


MISSISSIPPI PHOSPHATES CORPORATION        OFFICE CHERIFIEN DES PHOSPHATES
AND MISSISSIPPI CHEMICAL CORPORATION

By: /s/ Charles O. Dunn                  By: /s/ Mourad Cherif
    -------------------                      -----------------
    Charles O. Dunn                          Mourad Cherif
    President                                General Manager





                                       3
<PAGE>
 
                  SCHEDULE 1 TO AMENDMENT NO. 4 OF AGREEMENT
                                        
                                   EXHIBIT F

                        THE PASCAGOULA PLANT EXPANSION



    The Pascagoula Plant Expansion shall consist of all the new equipments and
facilities and all improvements and modifications to the existing production
units and storage facilities at the Pascagoula Plant which are assembled,
constructed and installed in order to increase the Pascagoula Plant's annual
production rate to approximately 900,000 short tons.  The major components of
the Pascagoula Plant Expansion are set forth hereinafter together with their
estimated useful lives and straight-line depreciation in US$/year.
<PAGE>
 
                  SCHEDULE 1 TO AMENDMENT NO. 4 OF AGREEMENT
                                        
                                   EXHIBIT F

                        THE PASCAGOULA PLANT EXPANSION



<TABLE>
<CAPTION>
              CAPITAL COST AND PRELIMINARY DEPRECIATION SCHEDULE
                       INCREASE DAP PRODUCTION CAPACITY
 
                                                                                 ST. LINE
                                                              USEFUL LIFE      DEPRECIATION
                                                COST        (YRS) ESTIMATED       $/Year
                                             -----------    ---------------    ------------
<S>                                          <C>                <C>              <C>
I.   PHOSPHORIC ACID
     PLANT EXPANSION
 
     A.  Additional Digestor & Cooling       $ 1,251,000        10 Years         $125,100
     B.  Filte and Evaporator Upgrades       $ 1,765,000        10 Years         $176,500 
     C.  Recycle & Sulfuric Acid                                                          
         System Modification in Phos Acid    $   214,000        10 Years         $ 21,400 
     D.  Sulfuric Acid Transfer Piping       $   513,000        10 Years         $ 51,300 
                                             -----------
                                             $ 3,743,000
 
II.  DAP STORAGE                             $ 3,499,000        18 Years         $194,389
 
III. DAP PLANT EXPANSION
 
     A.  Neutralizer Addition                $   586,000        15 Years         $ 39,067
     B.  Weak Acid Scrubber                  $   231,000        15 Years         $ 15,400
         Modification
     C.  Product Cooler Upgrade              $   475,000        15 Years         $ 31,667
     D.  Sulfuric Acid Catalyst              $   430,000         5 Years         $ 86,000
         Replacement
     E.  Granulator Upgrade                  $   373,000        15 Years         $ 24,867
     F.  Upgrade Recycle Circuit             $   477,000        15 Years         $ 31,800
     G.  Reslope Dryer                       $   141,000        15 Years         $  9,400
                                             -----------
                                             $ 2,713,000
 
IV.  ENGINEERING AND SUPERVISION             $   498,000        13.75 Years      $ 36,218
                                             -----------
 
TOTAL I, II, III & IV                        $10,453,000
                                             -----------
</TABLE>