As I mentioned on that thread an ImprovementModifier is needed.
I don't happen to have an example on hand, though, of how the modifier would need to be structured. Well, I do now because I Frankensteined some code together from modifiers I had in some of my personal mods.
The ModifierType would need to be MODIFIER_SINGLE_PLOT_ADJUST_PLOT_YIELDS I would think.
Untested but I would think this should do:
Code:
<GameData>
<Modifiers>
<Row>
<ModifierId>FARM_WHEAT_PRODUCTION_EXAMPLE</ModifierId>
<ModifierType>MODIFIER_SINGLE_PLOT_ADJUST_PLOT_YIELDS</ModifierType>
<SubjectRequirementSetId>RESOURCE_IS_WHEAT_EXAMPLE</SubjectRequirementSetId>
</Row>
</Modifiers>
<ModifierArguments>
<Row>
<ModifierId>FARM_WHEAT_PRODUCTION_EXAMPLE</ModifierId>
<Name>YieldType</Name>
<Value>YIELD_PRODUCTION</Value>
</Row>
<Row>
<ModifierId>FARM_WHEAT_PRODUCTION_EXAMPLE</ModifierId>
<Name>Amount</Name>
<Value>1</Value>
</Row>
</ModifierArguments>
<Requirements>
<Row>
<RequirementId>REQUIRES_WHEAT_IN_PLOT_EXAMPLE</RequirementId>
<RequirementType>REQUIREMENT_PLOT_RESOURCE_TYPE_MATCHES</RequirementType>
</Row>
</Requirements>
<RequirementArguments>
<Row>
<RequirementId>REQUIRES_WHEAT_IN_PLOT_EXAMPLE</RequirementId>
<Name>ResourceType</Name>
<Value>RESOURCE_WHEAT</Value>
</Row>
</RequirementArguments>
<RequirementSets>
<Row>
<RequirementSetId>RESOURCE_IS_WHEAT_EXAMPLE</RequirementSetId>
<RequirementSetType>REQUIREMENTSET_TEST_ALL</RequirementSetType>
</Row>
</RequirementSets>
<RequirementSetRequirements>
<Row>
<RequirementSetId>RESOURCE_IS_WHEAT_EXAMPLE</RequirementSetId>
<RequirementId>REQUIRES_WHEAT_IN_PLOT_EXAMPLE</RequirementId>
</Row>
</RequirementSetRequirements>
<ImprovementModifiers>
<Row>
<ImprovementType>IMPROVEMENT_FARM</ImprovementType>
<ModifierId>FARM_WHEAT_PRODUCTION_EXAMPLE</ModifierId>
</Row>
</ImprovementModifiers>
</GameData>
So if you wanted a Farm to add +1 Production to both Wheat and Rice you'd need one modifier for each resource, or you'd have to use the "REQUIREMENTSET_TEST_ANY" and add an additional RequirementId for Rice to the "RESOURCE_IS_WHEAT_EXAMPLE" RequirementSetId. There is probably a simpler way to specify "any" resource for a specified Improvement-Type instead of listing each valid resource for a given improvement, but I haven't looked into that.