<PAGE>   1

                                                                     EXHIBIT 4.2

THE SECURITIES REPRESENTED BY THIS CERTIFICATE AND SHARES ISSUABLE UPON EXERCISE
OF THIS WARRANT HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1993, AS
AMENDED, AND MAY NOT BE OFFERED FOR SALE, SOLD OR OTHERWISE DISPOSED OF EXCEPT
PURSUANT TO AN EFFECTIVE REGISTRATION STATEMENT FILED UNDER THE SECURITIES ACT
OF 1933, AS AMENDED, OR PURSUANT TO AN EXEMPTION FROM REGISTRATION UNDER SUCH
ACT. THE HOLDER OF THIS WARRANT AND ANY SHARES ISSUED PURSUANT TO THIS WARRANT
AGREES THAT THE COMPANY MAY REFUSE TO ISSUE SUCH SHARES OR TRANSFER THIS WARRANT
OR SUCH SHARES UNLESS THE COMPANY RECEIVES EVIDENCE SATISFACTORY TO IT AS TO
COMPLIANCE WITH SUCH EXEMPTIONS.

VOID AFTER 5:00 P.M., DENVER TIME, ON OCTOBER 12, 2004, OR IF NOT A BUSINESS
DAY, AS DEFINED HEREIN, AT 5:00 P.M., DENVER TIME, ON THE NEXT BUSINESS DAY.

                               WARRANT TO PURCHASE
                            _______________ SHARES OF
                                  RENTECH, INC.

         This certifies that, for good and valuable consideration, the
sufficiency of which is hereby acknowledged, _______________________________,
whose address is ____________________ ____________________________________, and
its registered, permitted assigns (collectively, the "Warrantholder"), is
entitled to purchase from Rentech, Inc., a corporation incorporated under the
laws of the state of Colorado (the Company), subject to the terms and conditions
hereof, at any time before 5:00 P.M., Denver time, on October 12, 2004, (or, if
such day is not a Business Day, at or before 5:00 P.M., Denver time, on the next
following Business Day), _______________ shares of fully paid and nonassessable
Common Stock, par value $.01 per share, at the Exercise Price. The Exercise
Price and the number of shares purchasable hereunder are subject to adjustment
from time to time as provided in Article III hereof.

                                    ARTICLE I

         Section 1.01: Definition of Terms. As used in this Warrant, the
following capitalized terms shall have the following respective meanings:

                  (a) Business Day: A day other than a Saturday, Sunday or other
         day on which banks in the State of Colorado are authorized by law to
         remain closed.

                  (b) Common Stock: Common Shares, $.01 par value, of the
         Company.

                  (c) Common Stock Equivalents: Securities that are convertible
         into or exercisable for shares of Common Stock.

                  (d) Exchange Act: The Securities Exchange Act of 1934, as
         amended.

                  (e) Exercise Price: $1.20 per Warrant Share, as such price may
         be adjusted from time to time pursuant to Article III hereof.


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                  (f) Expiration Date: 5:00 P.M., Denver time, on October 12,
         2004, or if such day is not a Business Day, the next succeeding day
         which is a Business Day.

                  (g) Holder: A Holder of Warrants.

                  (h) NASD: National Association of Securities Dealers, Inc.,
         and NASDAQ: NASD Automated Quotation System.

                  (i) Person: An individual, partnership, joint venture,
         corporation, trust, unincorporated organization or government or any
         department or agency thereof.

                  (j) SEC: The Securities and Exchange Commission or any other
         federal agency at the time administering the Securities Act or the
         Exchange Act.

                  (k) Securities Act: The Securities Act of 1933, as amended.

                  (l) Warrants: This Warrant and all other warrants that may be
         issued in its or their place (together evidencing the right to purchase
         an aggregate of 3,125,000 shares of Common Stock), originally issued to
         First Union Securities, Inc.

                  (m) Warrantholder: The Person to whom this Warrant is
         originally issued, or any successor in interest to whom the related
         Units of Rentech, Inc. have been transferred, each Unit consisting of
         four shares of Common Stock and one Warrant for the purchase of one
         share of Common Stock, in whose name this Warrant is registered upon
         the books to be maintained by the Company for that purpose. Only such
         successors in interest shall be deemed "permitted assigns" and only
         such assignments shall be deemed "permitted transfers" for purposes of
         this Warrant.

                  (n) Warrant Shares: Common Stock, Common Stock Equivalents and
         other securities purchased or purchasable upon exercise of the
         Warrants.

                  (o) $ or Dollars: United States dollars.

                                   ARTICLE II

                        DURATION AND EXERCISE OF WARRANT

         Section 2.01: Duration of Warrant. The Warrantholder may exercise this
Warrant at any time and from time to time before 5:00 P.M., Denver time, on the
Expiration Date. If this Warrant is not exercised on or before the Expiration
Date, it shall become void, and all rights hereunder shall thereupon cease. The
Warrants have been issued as part of a private placement by the Company of
3,125,000 Units, each consisting of four shares of Common Stock and one Warrant
for the purchase of one share of Common Stock. The Warrants may not be
transferred by a Warrantholder apart from the Units.

         Section 2.02: Exercise of Warrant.

                  (a) The Warrantholder may exercise this Warrant, in whole or
         in part, by presentation and surrender of this Warrant to the Company
         at its corporate office at 1331 17th

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         Street, Suite 720, Denver, Colorado 80202, or at the office of its
         stock transfer agent, if any, with the Subscription Form annexed hereto
         duly executed and accompanied by payment of the full Exercise Price for
         each Warrant Share to be purchased by means of a cashier's check.

                  (b) Upon receipt of this Warrant with the Subscription Form
         fully executed and accompanied by payment of the aggregate Exercise
         Price for the Warrant Shares for which this Warrant is then being
         exercised, the Company shall cause to be issued certificates for the
         total number of whole shares of Common Stock for which this Warrant is
         being exercised (adjusted to reflect the effect of the anti-dilution
         provisions contained in Article III hereof, if any, and as provided in
         Section 2.04 hereof) in such denominations as are requested for
         delivery to the Warrantholder, and the Company shall thereupon deliver
         such certificates to the Warrantholder. The Warrantholder shall be
         deemed to be the holder of record of the shares of Common Stock
         issuable upon such exercise, notwithstanding that the stock transfer
         books of the Company shall then be closed or that certificates
         representing such shares of Common Stock may not then be actually
         delivered to the Warrantholder. Notwithstanding any other provision of
         this Warrant, if at the time this Warrant is exercised and the Warrant
         Shares issuable upon exercise of this Warrant are not registered, the
         Company may require the Warrantholder to make such representations, may
         place such legends on certificates representing the Warrant Shares, and
         may place such stop transfer instructions and other transfer
         restrictions as may be reasonably required in the opinion of counsel to
         the Company to permit the Warrant Shares to be issued without such
         registration.

                  (c) In case the Warrantholder shall exercise this Warrant with
         respect to less than all of the Warrant Shares that may be purchased
         under this Warrant, the Company shall execute a new Warrant in the form
         of this Warrant for the balance of such Warrant Shares and deliver such
         new Warrant to the Warrantholder.

                  (d) The Company shall pay any and all stock transfer and
         similar taxes which may be payable in respect of the issue of this
         Warrant or in respect of the issue of any Warrant Shares.

         Section 2.03: Reservation of Shares. The Company hereby agrees that, at
all times following the execution of this Warrant, it will reserve for issuance
and delivery upon exercise of this Warrant such number of shares of Common Stock
or other shares of capital stock of the Company from time to time issuable upon
exercise of this Warrant. All such shares shall be duly authorized, and when
issued upon such exercise, shall be validly issued, fully paid and
non-assessable, free and clear of all liens, security interests, charges and
other encumbrances or restrictions on sale and free and clear of all preemptive
rights.

         Section 2.04: Fractional Shares. The Company shall not be required to
issue any fraction of a share of its capital stock in connection with the
exercise of this Warrant, and in any case where the Warrantholder would, except
for the provisions of this Section 2.04, be entitled under the terms of this
Warrant to receive a fraction of a share upon the exercise of this Warrant, the
Company shall, upon the exercise of this Warrant and receipt of the Exercise
Price, issue the greatest number of whole shares purchasable upon exercise of
this Warrant for which payment in full of the Exercise Price has been received.
The Company shall not be required to make any cash or other adjustment in
respect of such fraction of a share to which the Warrantholder would otherwise
be entitled.


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         Section 2.05: Listing. Prior to the issuance of any shares of Common
Stock upon exercise of this Warrant, the Company shall secure the listing of
such shares of Common Stock upon each national securities exchange or automated
quotation systems, if any, upon which shares of Common Stock are then listed
(subject to official notice of issuance upon exercise of this Warrant) and shall
maintain, so long as any other shares of Common Stock shall be so listed, such
listing of all shares of Common Stock from time to time issuable upon the
exercise of this Warrant; and the Company shall so list on each national
securities exchange or automated quotation system, and shall maintain such
listing of, any other shares of capital stock of the Company issuable upon the
exercise of this Warrant if and so long as any shares of the same class shall be
listed on such national securities exchange or automated quotation system.

                                   ARTICLE III

                      ADJUSTMENT OF SHARES OF COMMON STOCK
                         PURCHASABLE AND EXERCISE PRICE

         The Exercise Price and the number and kind of Warrant Shares shall be
subject to adjustment from time to time upon the happening of certain events as
provided in this Article III.

         Section 3.01: Mechanical Adjustments.

                  (a) If at any time prior to the exercise of this Warrant in
         full, the Company shall (I) declare a dividend or make a distribution
         on the Common Stock payable in shares of its Common Stock; (ii)
         subdivide, reclassify or recapitalize the outstanding Common Stock into
         a greater number of shares of Common Stock; (iii) combine, reclassify
         or recapitalize its outstanding shares of Common Stock into a smaller
         number of shares of Common Stock, or (iv) issue any shares of its
         capital stock by reclassification of its Common Stock (including any
         such reclassification in connection with a consolidation or a merger in
         which the Company is the continuing corporation), the Exercise Price in
         effect at the time of the record date of such dividend, distribution,
         subdivision, combination, reclassification or recapitalization shall be
         adjusted so that the Warrantholder shall be entitled to receive the
         aggregate number and kind of shares which, if this Warrant had been
         exercised in full immediately prior to such event, he would have owned
         upon such exercise and been entitled to receive by virtue of such
         dividend, distribution, subdivision, combination, reclassification or
         recapitalization. Any adjustment required by this paragraph 3.01(a)
         shall be made successively and become effective immediately after the
         record date, in the case of a dividend or distribution, or the
         effective date, in the case of a subdivision, combination,
         reclassification or recapitalization, to allow the purchase of such
         aggregate number and kind of shares.

                  (b) If at any time prior to the exercise of this Warrant in
         full, the Company shall fix a record date for the issuance or making a
         distribution to all holders of the Common Stock (including any such
         distribution to be made in connection with a consolidation or merger in
         which the Company is to be the continuing corporation) of evidences of
         its indebtedness, any other securities of the Company or any cash,
         property or other assets or securities, including without limitation
         shares of a subsidiary's capital stock (excluding a common stock
         dividend, combination, reclassification, recapitalization or issuance
         referred to in Section 3.01(a)), regular cash dividends or cash
         distributions in the ordinary course of business or subscription
         rights, options or warrants for Common Stock or Common Stock
         Equivalents (any such nonexcluded

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<PAGE>   5



         event being herein called a Special Dividend), then, except as provided
         in paragraph 3.01(c), the Exercise Price shall be decreased immediately
         after the record date for such Special Dividend to a price determined
         by multiplying the Exercise Price then in effect by a fraction, the
         numerator of which shall be the then current market price of the Common
         Stock (as defined in Section 3.01(e)) on such record date less the fair
         market value (as determined by the Company's Board of Directors) of the
         evidences of indebtedness, securities or property, or other assets
         issued or distributed in such Special Dividend applicable to one share
         of Common Stock or of such subscription rights or warrants applicable
         to one share of Common Stock, and the denominator of which shall be the
         then current market price per share of Common Stock (as so determined).
         Any adjustment required by this paragraph 3.01(b) shall be made
         successively effective on the record date for the Special Dividend and
         in the event that such distribution is not so made, the Exercise Price
         shall again be adjusted to be the Exercise Price that was in effect
         immediately prior to such record date.

                  (c) If at any time prior to the exercise of this Warrant in
         full, the Company shall make a distribution to all holders of the
         Common Stock of stock of a subsidiary or securities convertible into or
         exercisable for such stock, then in lieu of an adjustment in the
         Exercise Price or the number of Warrant Shares purchasable upon the
         exercise of this Warrant, each Warrantholder, upon the exercise hereof
         at any time after such distribution, shall be entitled to receive from
         the Company, such subsidiary or both, as the Company shall determine,
         the stock or other securities to which such Warrantholder would have
         been entitled if such Warrantholder had exercised this Warrant
         immediately prior thereto, all subject to further adjustment as
         provided in this Article III, and the Company shall reserve, for the
         life of the Warrant, such securities of such subsidiary or other
         corporation; provided, however, that no adjustment in respect of
         dividends or interest of such stock or other securities shall be made
         during the term of this Warrant or upon its exercise.

                  (d) Whenever the Exercise Price payable upon exercise of each
         Warrant is adjusted pursuant to one or more of paragraphs (a), (b) and
         (c) of this Section 3.01, the number of Warrant Shares issuable upon
         payment of the Warrant Price shall simultaneously be adjusted by
         multiplying the number of Warrant Shares issuable upon exercise of each
         Warrant immediately prior to such adjustment by the Exercise Price in
         effect immediately prior to such adjustment and dividing the product so
         obtained by the Exercise Price, as adjusted.

                  (e) For the purpose of any computation under this Section
         3.01, the current market price per share of Common Stock at any date
         shall be deemed to be the average of the daily closing prices for 20
         consecutive trading days commencing 30 trading days before such date.
         The closing price for each day shall be the last sale price regular way
         or, in case no such reported sales take place on such day, the average
         of the last reported bid and asked prices regular way, in either case
         on the principal national securities exchange on which the Common Stock
         is admitted to trading or listed, or if not listed or admitted to
         trading on such exchange, the representative closing bid price as
         reported by NASDAQ, or other similar organization if NASDAQ is no
         longer reporting such information, or if not so available, the fair
         market price as determined in good faith by the Board of Directors of
         the Company.

                  (f) No adjustment in the Exercise Price shall be required
         unless such adjustment would require an increase or decrease of at
         least ten cents ($.10) in such price; provided, however,

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         that any adjustments which by reason of this paragraph (f) are not
         required to be made shall be carried forward and taken into account in
         any subsequent adjustment. All calculations under this Section 3.01
         shall be made to the nearest cent or to the nearest one-hundredth of a
         share, as the case may be. Notwithstanding anything in this Section
         3.01 to the contrary, the Exercise Price shall not be reduced to less
         than the then existing par value, if any, of the Common Stock as a
         result of any adjustment made hereunder.

                  (g) In the event that at any time, as a result of any
         adjustment made pursuant to Section 3.01(a), the Warrantholder
         thereafter shall become entitled to receive any shares of the Company
         other than Common Stock, thereafter the number of such other shares so
         receivable upon exercise of any Warrant shall be subject to adjustment
         from time to time in a manner and on terms as nearly equivalent as
         practicable to the provisions with respect to the Common Stock
         contained in Section 3.01(a).

                  (h) In the case of an issue of additional Common Stock or
         Common Stock Equivalents for cash, the consideration received by the
         Company therefor, without deducting therefrom any discount or
         commission or other expenses paid by the Company for any underwriting
         of, or otherwise in connection with, the issuance thereof, shall be
         deemed to be the amount received by the Company therefor. To the extent
         that such issuance shall be for a consideration other than cash, then,
         except as herein otherwise expressly provided, the amounts of such
         consideration shall be deemed to be the fair value of such
         consideration at the time of such issuance as reasonably determined in
         good faith by the Board of Directors of the Company. The term issue
         shall include the sale or other disposition of shares held by or on
         account of the Company or in the treasury of the Company, but until so
         sold or otherwise disposed of such shares shall not be deemed
         outstanding.

         Section 3.02: Notices of Adjustment. Whenever the number of Warrant
Shares or the Exercise Price is adjusted as herein provided, the Company shall
prepare and deliver forthwith to the Warrantholder a certificate signed by its
President, and by any Vice President, Treasurer or Secretary, setting forth the
adjusted number of shares purchasable upon the exercise of this Warrant and the
Exercise Price of such shares after such adjustment, setting forth a brief
statement of the facts requiring such adjustment and setting forth the
computation by which adjustment was made. The certificate of any independent
firm of public accountants of recognized standing selected by the Board of
Directors of the Company shall be conclusive evidence of the arithmetic
correctness of any computation made under this Section 3.

         Section 3.03: No Adjustment for Dividends. Except as provided in
Section 3.01 of this Agreement, no adjustment in respect of any cash dividends
shall be made during the term of this Warrant or upon the exercise of this
Warrant.

         Section 3.04: Preservation of Purchase Rights in Certain Transactions.
In case of any consolidation or merger of the Company with or into another
Person (other than a merger with a subsidiary in which the Company is the
continuing corporation and that does not result in any reclassification, capital
reorganization or other change of outstanding shares of Common Stock) or in case
of any sale, lease, transfer or conveyance to another corporation of the
property and assets of the Company as an entirety or substantially as an
entirety, the Company shall, as a condition to such transaction cause such
successor or purchasing corporation, as the case may be, to execute with the
Warrantholder an agreement granting the Warrantholder the right thereafter, upon
payment of the Exercise Price in effect

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immediately prior to such transaction, to receive upon exercise of this Warrant
the kind and amount of shares and other securities and property which he would
have owned or have been entitled to receive as a result of such transaction had
this Warrant been exercised immediately prior to such action. Such agreement
shall provide for adjustments in respect of such shares of stock and other
securities and property, which shall be as nearly equivalent as may be
practicable to the adjustments provided for in this Article III and shall
provide that such rights may be exercised at any time prior to the Expiration
Date. In the event that in connection with any such transaction, additional
shares of Common Stock shall be issued in exchange, conversion, substitution or
payment, in whole or in part, for, or of, a security of the Company other than
Common Stock, any such issue shall be treated as an issue of Common Stock
covered by the provisions of Article III. The provisions of this Section 3.04
shall similarly apply to successive reclassifications, capital reorganizations,
consolidations, mergers, sales or conveyance.

         Section 3.05: Form of Warrant After Adjustments. The form of this
Warrant need not be changed because of any adjustments in the Exercise Price or
the number or kind of the Warrant Shares, and Warrants theretofore or thereafter
issued may continue to express the same price and number and kind of shares as
are stated in the Warrant, as initially issued.

         Section 3.06: Treatment of Warrantholder. Prior to due presentment for
registration of transfer of this Warrant, the Company may deem and treat the
Warrantholder as the absolute owner of this Warrant (notwithstanding any
notation of ownership or other writing hereon) for all purposes and shall not be
affected by any notice to the contrary.

                                   ARTICLE IV

                          OTHER PROVISIONS RELATING TO
                             RIGHTS OF WARRANTHOLDER

         Section 4.01: No Rights as Shareholders; Notice to Warrantholders.
Nothing contained in this Warrant shall be construed as conferring upon the
Warrantholder or his or its transferees the right to vote or to receive
dividends or to consent or to receive notice as a shareholder in respect of any
meeting of shareholders for the election of directors of the Company or of any
other matter, or any rights whatsoever as shareholders of the Company. The
Company shall give notice to the Warrantholder by registered mail if at any time
prior to the expiration or exercise in full of the Warrants, any of the
following events shall occur:

                  (a) the Company shall authorize the payment of any dividend to
         all holders of Common Stock;

                  (b) the Company shall authorize the issuance to all holders of
         Common Stock of any additional shares of Common Stock or Common Stock
         Equivalents or of rights, options or warrants to subscribe for or
         purchase Common Stock or Common Stock Equivalents or of any other
         subscription rights, options or warrant;

                  (c) a dissolution, liquidation or winding up of the Company;
         or

                  (d) a capital reorganization or reclassification of the Common
         Stock (other than a subdivision or combination of the outstanding
         Common Stock) or any consolidation or merger

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         of the Company with or into another corporation (other than a
         consolidation or merger in which the Company is the continuing
         corporation and that does not result in any reclassification or change
         of Common Stock outstanding) or in the case of any sale or conveyance
         to another corporation of the property of the Company as an entirety or
         substantially as an entirety.

Such giving of notice shall be completed at least 15 Business Days prior to the
date fixed as a record date or effective date or the date of closing of the
Company's stock transfer books for the determination of the shareholders
entitled to such dividend, distribution, or subscription rights, or for the
determination of the shareholders entitled to vote on such proposed merger,
consolidation, sale, conveyance, dissolution, liquidation or winding up. Such
notice shall specify such record date or the date of closing the stock transfer
books, as the case may be. Failure to provide such notice shall not affect the
validity of any action taken in connection with such dividend, distribution or
subscription rights, or proposed merger, consolidation, sale, conveyance,
dissolution, liquidation or winding up.

         Section 4.02: Lost, Stolen, Mutilated or Destroyed Warrants. If this
Warrant is lost, stolen, mutilated or destroyed, the Company may, upon receipt
by the Company of evidence of ownership reasonably satisfactory to it and on
such terms as to indemnity or otherwise as it may in its discretion impose
(which shall, in the case of a mutilated Warrant, include the surrender
thereof), issue a new Warrant of like denomination and tenor as, and in
substitution for, this Warrant.

         Section 4.03: Split-Up, Combination, Exchange and Transfer of Warrants.
This Warrant may be split up, combined or exchanged for another Warrant or
Warrants containing the same terms to purchase a like aggregate number of
Warrant Shares. If the Warrantholder desires to split up, combine or exchange
this Warrant, he or it shall make such request in writing delivered to the
Company and shall surrender to the Company this Warrant and any other Warrants
to be so split-up, combined or exchanged. Upon any such surrender for a
split-up, combination or exchange, the Company shall execute and deliver to the
person entitled thereto a Warrant or Warrants, as the case may be, as so
requested. The Company shall not be required to effect any split-up, combination
or exchange which will result in the issuance of a Warrant entitling the
Warrantholder to purchase upon exercise a fraction of a share of Common Stock or
a fractional Warrant.

         Section 4.04: Registration Rights. The Warrant Shares, once issued, are
subject to the rights and benefits of the Registration Rights Agreement
pertaining to the Warrant Shares, between the Company, First Union Securities,
Inc., and others; provided, however, that notwithstanding anything in such
Registration Rights Agreement to the contrary, the Company hereby consents to
the transfer of the registration rights contained therein with respect to the
Warrant Shares to the extent that transfers of the Warrant and the Warrant
Shares are permitted hereunder.

         Section 4.05: Redemption. This Warrant may be redeemed by the Company
at any time after sixty (60) days from the date of its original issuance and
prior to the Expiration Date upon at least thirty (30) days' written notice (the
"Notice of Redemption") to the holder of this Warrant for $.05 per Warrant Share
if the closing high bid quotation of the Company's common stock exceeds one
hundred twenty percent (120%) of the Exercise Price for a period of twenty (20)
consecutive trading days immediately preceding the mailing by the Company of the
Notice of Redemption. The Notice of Redemption shall be sent to the last known
address of the Holder of this Warrant as maintained on the books and records of
the Company. The redemption date shall be fixed in the Notice of Redemption and
shall be not earlier than thirty (30) days after the date of mailing of the
Notice of Redemption. Upon receipt of a Notice of

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<PAGE>   9



Redemption, the Holder of this Warrant may exercise this Warrant to the extent
of the number of Warrant Shares called for redemption until the last business
day prior to the redemption date. A "business day" shall mean any day other than
Saturday, Sunday or other day on which banks are required or authorized to be
closed in the City of Denver or the State of Colorado. If on the redemption date
the Company shall have paid the redemption price or shall have reserved and set
apart an amount sufficient to pay the redemption price, then this Warrant shall
expire to he extent of the number of Warrant Shares called for redemption
pursuant to the Notice of Redemption. Warrants called for redemption in whole or
in part shall be tendered to the Company on or before the redemption date. The
redemption price shall be paid to the Warrantholder within three (3) business
days after the redemption date, and shall be delivered by check sent to the last
known address of the warrant holder maintained on the books and records of the
Company.

                                    ARTICLE V

                                  OTHER MATTERS

         Section 5.01: Amendments and Waivers. The provisions of this Warrant,
including the provisions of this sentence, may not be amended, modified or
supplemented, and waiver or consents to departures from the provisions hereof
may not be given unless the Company has obtained the written consent of Holders
of at least a majority of the outstanding Warrants. Holders shall be bound by
any consent authorized by this section whether or not certificates representing
such Warrants have been marked to indicate such consent.

         Section 5.02: Governing Law. This Warrant shall be governed by and
construed in accordance with the laws of the state of Colorado.

         Section 5.03: Severability. In the event that any one or more of the
provisions contained herein, or the application thereof in any circumstances, is
held invalid, illegal or unenforceable, the validity, legality and
enforceability of any such provisions in every other respect and of the
remaining provisions contained herein shall not be affected or impaired thereby.

         Section 5.04: Computations of Consent. Whenever the consent or approval
of Holders of a specified percentage of Warrants is required hereunder, Warrants
held by the Company or its affiliates (other than the Warrantholder or
subsequent Holders if they are deemed to be such affiliates solely by reason of
their holdings of such Warrants) shall not be counted in determining whether
such consent or approval was given by the Holders of such required percentage.

         Section 5.05: Notice. Any notices or certificates by the Company to the
Holder and by the Holder to the Company shall be deemed delivered if in writing
and delivered in person or by registered mail (return receipt requested) to the
Holder addressed to him in care of First Union Securities, Inc. at the following
address:

                           1001 Fannin, 22nd Floor
                           Houston, TX  77002-6760
                           Attention: John Bishop, Managing Director


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<PAGE>   10



or, if the Holder has designated by notice in writing to the Company any other
address, to such other address. If notice is to the Company, such notice is to
be addressed to Rentech, Inc. at:

                           1331 17th Street, Suite 720
                           Denver, CO 80202
                           Attention:  Chief Operating Officer

         The Company may change its address by written notice to the Holder and
the Holder may change its address by written notice to the Company.

         Section 5.06: Investment Representation of the Holder. By accepting
delivery of this Warrant, the Holder represents to the Company that it has
acquired this Warrant solely for its own account for investment and not with a
view to distribution or sale in violation of the Securities Act, but subject,
nevertheless, to any requirement of law that the disposition of the Holder's
property be at all times within its control. The Holder represents that it
understands that this Warrant has been issued in a transaction that is exempt
from the registration requirements of the Securities Act and that this Warrant
must be held by the Holder and may not be resold unless subsequently registered
under the Securities Act or an exemption from such registration is available,
and if so resold, may only be resold and assigned with the shares of Common
Stock of the Company acquired by the Warrantholder concurrently with this
Consent through a private placement transaction with the Company. The Holder
also represents that it understands and acknowledges that this Warrant may only
be exercised by the Warrantholder.

         IN WITNESS WHEREOF, this Warrant has been duly executed by the Company
as of the ______ day of _______________, 1999.

                                             RENTECH, INC.


                                             By:
                                                --------------------------------
                                             Name:
                                                  ------------------------------
                                             Title:
                                                   -----------------------------

Attest:
       --------------------------------


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<PAGE>   11



                                   ASSIGNMENT

    (To be executed only upon a permitted assignment of Warrant Certificate)

         For value received, the undersigned __________________________________
having transferred the Units of Rentech, Inc. that are registered in the records
of Rentech, Inc. as owned by the undersigned, each Unit consisting of four
shares of the Common Stock of Rentech, Inc. and one warrant for the purchase of
one share of Common Stock of Rentech, Inc., to the assigns identified below,
hereby sells, assigns and transfers unto _________________________
______________________ the within Warrant Certificate, together with all right,
title and interest therein, and does hereby irrevocably constitute and appoint
___________________________ ____________________, as attorney to transfer said
Warrant Certificate on the books of the within-named Company with respect to the
number of Warrants set forth below, with full power of substitution in the
premises:

<TABLE>
<CAPTION>
         Name(s) of
         Assignee(s)              Address              No. of Warrants
         --------------------------------------------------------------
<S>                               <C>                  <C>

</TABLE>








And if said number of Warrants shall not be all the Warrants represented by the
Warrant Certificate, a new Warrant Certificate is to be issued in the name of
said undersigned for the balance remaining of the Warrants registered by said
Warrant Certificate.

Dated:                  Signed:
      ----------------         -------------------------------------------------

                        NOTE: The above signature should correspond exactly with
                        the name on the face of this Warrant Certificate

                                       11

<PAGE>   12



                                SUBSCRIPTION FORM

                    (To be executed upon exercise of Warrant)


To: Rentech, Inc.
1331 17th Street, Suite 720
Denver, CO 80202

         The undersigned permitted Warrantholder hereby irrevocably elects to
exercise the right of purchaser represented by the within Warrant Certificate
for, and to purchase thereunder, ____________________ shares of Common Stock, as
provided for therein, and tenders herewith payment of the purchase price in full
in the form of cash or a certified or cashier's check for the amount of
$______________________.

         Please issue a certificate or certificates for such Common Stock in the
name of, and pay any cash for any fractional share to:

                            Name
                                 -----------------------------------------------

                            (Please print Name, Address and Social Security No.)

                            Signature
                                      ------------------------------------------

                            NOTE: The above signature should correspond exactly
                            with the name on the first page of this Warrant
                            Certificate or with the name of the permitted
                            assignee appearing in the assignment form below.

         And if said number of shares shall not be all the shares purchasable
under the within Warrant Certificate, a new Warrant Certificate is to be issued
in the name of said undersigned for the balance remaining of the shares
purchasable thereunder rounded up to the next higher number of shares.



                                       12