<SEC-DOCUMENT>0001243786-16-000253.txt : 20160310
<SEC-HEADER>0001243786-16-000253.hdr.sgml : 20160310
<ACCEPTANCE-DATETIME>20160310160045
ACCESSION NUMBER:		0001243786-16-000253
CONFORMED SUBMISSION TYPE:	4/A
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20160303
FILED AS OF DATE:		20160310
DATE AS OF CHANGE:		20160310

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			MOSAIC CO
		CENTRAL INDEX KEY:			0001285785
		STANDARD INDUSTRIAL CLASSIFICATION:	AGRICULTURE CHEMICALS [2870]
		IRS NUMBER:				201026454
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		3033 CAMPUS DRIVE, SUITE E490
		CITY:			PLYMOUTH
		STATE:			MN
		ZIP:			55441
		BUSINESS PHONE:		7635772700

	MAIL ADDRESS:	
		STREET 1:		3033 CAMPUS DRIVE, SUITE E490
		CITY:			PLYMOUTH
		STATE:			MN
		ZIP:			55441

	FORMER COMPANY:	
		FORMER CONFORMED NAME:	GLOBAL NUTRITION SOLUTIONS INC
		DATE OF NAME CHANGE:	20040401

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			O'Rourke James Calvin
		CENTRAL INDEX KEY:			0001453221

	FILING VALUES:
		FORM TYPE:		4/A
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-32327
		FILM NUMBER:		161497738

	MAIL ADDRESS:	
		STREET 1:		ATRIA CORPORATE CENTER, SUITE E490
		STREET 2:		3033 CAMPUS DRIVE
		CITY:			PLYMOUTH
		STATE:			MN
		ZIP:			55441
</SEC-HEADER>
<DOCUMENT>
<TYPE>4/A
<SEQUENCE>1
<FILENAME>wf-form4a_145764363382079.xml
<DESCRIPTION>FORM 4/A
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4/A</documentType>

    <periodOfReport>2016-03-03</periodOfReport>

    <dateOfOriginalSubmission>2016-03-07</dateOfOriginalSubmission>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001285785</issuerCik>
        <issuerName>MOSAIC CO</issuerName>
        <issuerTradingSymbol>MOS</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001453221</rptOwnerCik>
            <rptOwnerName>O'Rourke James Calvin</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O THE MOSAIC COMPANY</rptOwnerStreet1>
            <rptOwnerStreet2>3033 CAMPUS DRIVE, SUITE E490</rptOwnerStreet2>
            <rptOwnerCity>PLYMOUTH</rptOwnerCity>
            <rptOwnerState>MN</rptOwnerState>
            <rptOwnerZipCode>55441</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>President &amp; CEO</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable></nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Stock Option (Right to Buy)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>28.49</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2016-03-03</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>179211</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <footnoteId id="F2"/>
            </exerciseDate>
            <expirationDate>
                <value>2026-03-03</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Common Stock</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>179211.0</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>179211</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">At the time the original Form 4 was filed, the exact number of shares underlying the stock option was not known, as the final Black Scholes calculation with respect to the stock option had not been completed by a third-party provider. The reporting person is hereby amending this line item to reflect the final calculation of the number shares underlying the stock option.</footnote>
        <footnote id="F2">Grant Date 03/03/2016; option becomes exercisable in three equal installments commencing on the first anniversary of the grant date and on each anniversary thereafter.</footnote>
    </footnotes>

    <remarks></remarks>

    <ownerSignature>
        <signatureName>/s/ Mark J. Isaacson, Attorney-in-Fact for James C. O'Rourke</signatureName>
        <signatureDate>2016-03-10</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
