change 

<vehicle type="cultivator"> 

to 

<vehicle type="fertilizingCultivator">

---------------------------------------------------------------------------

ADD BELOW TO XML

<fillUnit>
        <fillUnitConfigurations>
            <fillUnitConfiguration>
                <fillUnits>
                    <fillUnit unit="$l10n_unit_literShort" fillTypes="liquidfertilizer" capacity="0"/>
                </fillUnits>
            </fillUnitConfiguration>
        </fillUnitConfigurations>
</fillUnit>


<sprayer fillUnitIndex="1" unloadInfoIndex="1" loadInfoIndex="1">
        <usageScales workingWidth="SET WIDTH OF IMPLEMENT HERE" scale="1"/>
    </sprayer>

<fertilizingCultivator needsSetIsTurnedOn="true"/>