Document:

<PAGE>

                       ASSIGNMENT AND ASSUMPTION AGREEMENT

     This Assignment and Assumption Agreement (this "ASSIGNMENT") is entered
into as of June 6, 2000 by and among Forest Oil Corporation, a corporation
organized under the laws of the State of New York, with headquarters located at
1600 Broadway, Suite 2200, Denver, Colorado 80202 (the "ASSIGNOR"), Robert S.
Boswell, 320 High Street, Denver, Colorado 80218 (the "ASSIGNEE"), and Rentech,
Inc., a corporation organized under the laws of the State of Colorado, with
headquarters located at 1331 17th Street, Suite 720, Denver, Colorado 80202
("RENTECH").

                                    RECITALS

     A.   The Assignor entered into the following related agreements with
Rentech on March 18, 2000:

          (i)  That certain Stock Purchase Agreement (the "PURCHASE AGREEMENT")
     by which the Assignor purchased, (x) 1,000,000 shares (the "PURCHASED
     STOCK") of Rentech's common stock, par value $0.01 per share ("RENTECH
     COMMON STOCK"); (y) an option to purchase 2,000,000 shares of Rentech
     Common Stock at an exercise price of $1.25 per share and expiring December
     31, 2001 (the "2001 OPTION"), and (z) an option to purchase 1,000,000
     shares of Rentech Common Stock at an exercise price of $5.00 per share and
     expiring December 31, 2004 (the "2004 OPTION", and together with the 2001
     Option and the Purchased Stock, the "PURCHASED SECURITIES"). Capitalized
     terms not otherwise defined herein shall have the meaning ascribed to such
     terms in the Purchase Agreement.

          (ii) Those certain Option Agreements (the "OPTION AGREEMENTS") in
     respect of the 2001 Option and the 2004 Option (collectively, the
     "OPTIONS"), respectively.

          (iii) That certain Registration Rights Agreement in respect of the
     various transactions contemplated by the Purchase Agreement (the
     "REGISTRATION RIGHTS AGREEMENT").

     B.   The Assignor desires to sell, assign and transfer to the Assignee, and
the Assignee desires to accept, the following portions of the Assignor's
Purchased Securities (collectively, the "ASSIGNED SECURITIES"):

          (i)  from its Purchased Stock, 25,000 shares of Rentech Common Stock;

          (ii) from its 2001 Option, an option to purchase 50,000 shares of
     Rentech Common Stock on the same terms and conditions applicable thereto;
     and

          (iii) from its 2004 Option, an option to purchase 25,000 shares of
     Rentech Common Stock on the same terms and conditions applicable thereto.
<PAGE>

     C.   The Assignor also desires to sell, assign and transfer to the
Assignee, and the Assignee desires to accept, such of the Assignor's rights,
interests and obligations under the Registration Rights Agreement that relate to
the Assigned Securities (the "ASSIGNED REGISTRATION RIGHTS").

     D.   Rentech acknowledges the transactions contemplated by this Assignment.

                                   AGREEMENTS

     NOW, THEREFORE, in consideration of the covenants of the Assignee hereunder
and for other good and valuable consideration, the receipt and sufficiency of
which are hereby acknowledged, the parties hereto, intending to be legally
bound, hereby agree as follows:

     1.   ASSIGNMENT.

     (a)  The Assignor hereby grants, sells, assigns and transfers to the
Assignee the Assigned Securities and the Assigned Registration Rights.

     (b)  The Assignee hereby accepts the assignment described in Section 1(a)
hereof and assumes the responsibilities and obligations of the Assignor arising
from and after the date hereof, as holder of the Assigned Securities and
Assigned Registration Rights pursuant to the assignment described in Section
1(a) hereof.

     (c)  Rentech hereby acknowledges the transfer and assignment provided for
in this Section 1; for the avoidance of doubt, Rentech's consent to the
transactions contemplated by this assignment is not required under the relevant
provisions of the Purchase Agreement, the Option Agreements and the Registration
Rights Agreement. Rentech further agrees to record each such respective transfer
of the Assigned Securities in the books and records of Rentech and to issue new
stock certificates or other documents evidencing such transfers, as applicable.

     2.   REPRESENTATIONS OF ASSIGNEE. The Assignee hereby understands, agrees
with, and represents and warrants to the Assignor, with respect to the
transactions contemplated by this Assignment, that:

     (a)  EMPLOYMENT STATUS WITH ASSIGNOR. The Assignee is a director and an
officer of the Assignor; and

     (b)  ACCREDITED INVESTOR STATUS. The Assignee is an "accredited investor"
as that term is defined in Rule 501(a) of Regulation D. The Assignee has such
knowledge and experience in financial and business matters that he is capable of
evaluating the merits and risks of the transactions contemplated by this
Assignment. The Assignee is aware that he may be required to bear the economic
risk of the transactions contemplated by this Assignment for an indefinite
period, and is able to bear such risk for an indefinite period.
<PAGE>

     (c)  RELIANCE. The Assignee understands the Assigned Securities and the
Assigned Registration Rights are being offered to him in reliance on specific
exemptions from the registration requirements of the applicable United States
federal and state securities laws and in compliance with the conditions and
terms of the Purchase Agreement, the Option Agreements and the Registration
Rights Agreement regarding the assignment and transfer of the Purchased
Securities, and that the Assignor is relying upon the truth and accuracy of, and
the Assignee's compliance with, the representations, warranties,
acknowledgements, understandings, agreements and covenants of the Assignee set
forth herein in order to determine the availability of such exemptions and the
eligibility of the Assignee to acquire the Assigned Securities and the Assigned
Registration Rights.

     3.   ASSUMPTION.

     (a)  The Assignee hereby agrees to assume and timely perform all contract
obligations of the Assignor with respect to the Assigned Securities and the
Assigned Registration Rights.

     (b)  The Assignee further agrees with Rentech to be bound by the terms and
conditions of the Purchase Agreement, the Option Agreements and Registration
Rights Agreement and any other related documents to which the Assignor is a
party.

     (c)  Except as expressly set forth in this Assignment, the Assignee does
not hereby assume or agree to pay any other liabilities or obligations of the
Assignor of any nature whatsoever.

     4.   FURTHER ASSURANCES. The Assignor, for itself and its successors and
assigns, hereby covenant and agree with the Assignee that it will do, execute,
acknowledge and deliver, or will cause to be done, executed, acknowledged and
delivered, all such further acts, transfers, assignments, powers of attorney and
assurances as may be required to better assure, convey, transfer, confirm and
vest unto the Assignee the Assigned Securities and the Assigned Registration
Rights and to aid and assist the Assignee in collecting and reducing the same to
possession.

     5.   GOVERNING LAW. THIS ASSIGNMENT SHALL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE WITH THE LAWS OF THE STATE OF COLORADO.

     6.   COUNTERPARTS. This Assignment may be executed in two or more identical
counterparts, all of which shall be considered one and the same agreement and
shall become effective when counterparts have been signed by each party and
signature pages from such counterparts have been delivered to the other party.

                           [ SIGNATURE PAGE FOLLOWS ]

<PAGE>

     IN WITNESS WHEREOF, the undersigned have caused this Agreement to be
signed, all as of the date first written above.

                                  ASSIGNOR:

                                  FOREST OIL CORPORATION, a Colorado corporation

                                       By: /s/ Joan C. Sonnen
                                          ------------------------------
                                          Name:  Joan C. Sonnen
                                          Title: Vice President

                                  ASSIGNEE:

                                  ROBERT S. BOSWELL,

                                       /s/ Robert S. Boswell
                                       ---------------------------------

                                  RENTECH, INC.,

                                       By: /s/ Ronald C. Butz
                                          ------------------------------
                                          Name:  Ronald C. Butz
                                          Title: Vice President, Chief
                                                 Operating Officer<PAGE>

                       ASSIGNMENT AND ASSUMPTION AGREEMENT

     This Assignment and Assumption Agreement (this "ASSIGNMENT") is entered
into as of June 6, 2000 by and among Forest Oil Corporation, a corporation
organized under the laws of the State of New York, with headquarters located at
1600 Broadway, Suite 2200, Denver, Colorado 80202 (the "ASSIGNOR"), David H.
Keyte, 2181 S. Clayton Street, Denver, Colorado 80210 (the "ASSIGNEE"), and
Rentech, Inc., a corporation organized under the laws of the State of Colorado,
with headquarters located at 1331 17th Street, Suite 720, Denver, Colorado 80202
("RENTECH").

                                    RECITALS

     A.   The Assignor entered into the following related agreements with
Rentech on March 18, 2000:

          (i)  That certain Stock Purchase Agreement (the "PURCHASE AGREEMENT")
     by which the Assignor purchased, (x) 1,000,000 shares (the "PURCHASED
     STOCK") of Rentech's common stock, par value $0.01 per share ("RENTECH
     COMMON STOCK"); (y) an option to purchase 2,000,000 shares of Rentech
     Common Stock at an exercise price of $1.25 per share and expiring December
     31, 2001 (the "2001 OPTION"), and (z) an option to purchase 1,000,000
     shares of Rentech Common Stock at an exercise price of $5.00 per share and
     expiring December 31, 2004 (the "2004 OPTION", and together with the 2001
     Option and the Purchased Stock, the "PURCHASED SECURITIES"). Capitalized
     terms not otherwise defined herein shall have the meaning ascribed to such
     terms in the Purchase Agreement.

          (ii) Those certain Option Agreements (the "OPTION AGREEMENTS") in
     respect of the 2001 Option and the 2004 Option (collectively, the
     "OPTIONS"), respectively.

          (iii) That certain Registration Rights Agreement in respect of the
     various transactions contemplated by the Purchase Agreement (the
     "REGISTRATION RIGHTS AGREEMENT").

     B.   The Assignor desires to sell, assign and transfer to the Assignee, and
the Assignee desires to accept, the following portions of the Assignor's
Purchased Securities (collectively, the "ASSIGNED SECURITIES"):

          (i)  from its Purchased Stock, 25,000 shares of Rentech Common Stock;

          (ii) from its 2001 Option, an option to purchase 50,000 shares of
     Rentech Common Stock on the same terms and conditions applicable thereto;
     and

          (iii) from its 2004 Option, an option to purchase 25,000 shares of
     Rentech Common Stock on the same terms and conditions applicable thereto.
<PAGE>

     C.   The Assignor also desires to sell, assign and transfer to the
Assignee, and the Assignee desires to accept, such of the Assignor's rights,
interests and obligations under the Registration Rights Agreement that relate to
the Assigned Securities (the "ASSIGNED REGISTRATION RIGHTS").

     D.   Rentech acknowledges the transactions contemplated by this Assignment.

                                   AGREEMENTS

     NOW, THEREFORE, in consideration of the covenants of the Assignee hereunder
and for other good and valuable consideration, the receipt and sufficiency of
which are hereby acknowledged, the parties hereto, intending to be legally
bound, hereby agree as follows:

     1.   ASSIGNMENT.

     (a)  The Assignor hereby grants, sells, assigns and transfers to the
Assignee the Assigned Securities and the Assigned Registration Rights.

     (b)  The Assignee hereby accepts the assignment described in Section 1(a)
hereof and assumes the responsibilities and obligations of the Assignor arising
from and after the date hereof, as holder of the Assigned Securities and
Assigned Registration Rights pursuant to the assignment described in Section
1(a) hereof.

     (c)  Rentech hereby acknowledges the transfer and assignment provided for
in this Section 1; for the avoidance of doubt, Rentech's consent to the
transactions contemplated by this assignment is not required under the relevant
provisions of the Purchase Agreement, the Option Agreements and the Registration
Rights Agreement. Rentech further agrees to record each such respective transfer
of the Assigned Securities in the books and records of Rentech and to issue new
stock certificates or other documents evidencing such transfers, as applicable.

     2.   REPRESENTATIONS OF ASSIGNEE. The Assignee hereby understands, agrees
with, and represents and warrants to the Assignor, with respect to the
transactions contemplated by this Assignment, that:

     (a)  EMPLOYMENT STATUS WITH ASSIGNOR. The Assignee is an officer of the
Assignor; and

     (b)  ACCREDITED INVESTOR STATUS. The Assignee is an "accredited investor"
as that term is defined in Rule 501(a) of Regulation D. The Assignee has such
knowledge and experience in financial and business matters that he is capable of
evaluating the merits and risks of the transactions contemplated by this
Assignment. The Assignee is aware that he may be required to
<PAGE>

bear the economic risk of the transactions contemplated by this Assignment for
an indefinite period, and is able to bear such risk for an indefinite period.

     (c)  RELIANCE. The Assignee understands the Assigned Securities and the
Assigned Registration Rights are being offered to him in reliance on specific
exemptions from the registration requirements of the applicable United States
federal and state securities laws and in compliance with the conditions and
terms of the Purchase Agreement, the Option Agreements and the Registration
Rights Agreement regarding the assignment and transfer of the Purchased
Securities, and that the Assignor is relying upon the truth and accuracy of, and
the Assignee's compliance with, the representations, warranties,
acknowledgements, understandings, agreements and covenants of the Assignee set
forth herein in order to determine the availability of such exemptions and the
eligibility of the Assignee to acquire the Assigned Securities and the Assigned
Registration Rights.

     3.   ASSUMPTION.

     (a)  The Assignee hereby agrees to assume and timely perform all contract
obligations of the Assignor with respect to the Assigned Securities and the
Assigned Registration Rights.

     (b)  The Assignee further agrees with Rentech to be bound by the terms and
conditions of the Purchase Agreement, the Option Agreements and Registration
Rights Agreement and any other related documents to which the Assignor is a
party.

     (c)  Except as expressly set forth in this Assignment, the Assignee does
not hereby assume or agree to pay any other liabilities or obligations of the
Assignor of any nature whatsoever.

     4.   FURTHER ASSURANCES. The Assignor, for itself and its successors and
assigns, hereby covenant and agree with the Assignee that it will do, execute,
acknowledge and deliver, or will cause to be done, executed, acknowledged and
delivered, all such further acts, transfers, assignments, powers of attorney and
assurances as may be required to better assure, convey, transfer, confirm and
vest unto the Assignee the Assigned Securities and the Assigned Registration
Rights and to aid and assist the Assignee in collecting and reducing the same to
possession.

     5.   GOVERNING LAW. THIS ASSIGNMENT SHALL BE GOVERNED BY AND CONSTRUED IN
ACCORDANCE WITH THE LAWS OF THE STATE OF COLORADO.

     6.   COUNTERPARTS. This Assignment may be executed in two or more identical
counterparts, all of which shall be considered one and the same agreement and
shall become effective when counterparts have been signed by each party and
signature pages from such counterparts have been delivered to the other party.

                           [ SIGNATURE PAGE FOLLOWS ]

<PAGE>

     IN WITNESS WHEREOF, the undersigned have caused this Agreement to be
signed, all as of the date first written above.

                                  ASSIGNOR:

                                  FOREST OIL CORPORATION, a Colorado corporation

                                       By: /s/ Joan C. Sonnen
                                           ---------------------------------
                                           Name:  Joan C. Sonnen
                                           Title: Vice President

                                  ASSIGNEE:

                                  DAVID H. KEYTE,

                                        /s/ David H. Keyte
                                        ------------------------------------

                                  RENTECH, INC.,

                                       By: /s/ Ronald C. Butz
                                           ---------------------------------
                                           Name:  Ronald C. Butz
                                           Title: Vice President, Chief
                                                  Operating Officer

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