The number of fertilities is fixed on every island, but sometimes you can choose one of the fertilities by planting seeds, that you can buy from NPC. This is indicated by a "question mark - fertility"
The resources or also known as raw materials can be refilled using appropriate warehouse Items a.k.a. refillers. See Laboratory Modules. In Deep Ocean Expansion You can make even more powerful refillers. A type of resource however cannot be added or removed from an island so they are fixed.
There is also the another "fertility" of the farms (productivity) which can be affected via ecobalance status.
I understand your question about the resources and seeds, I just began to dig around and altering the game code. If I stumble upon the answer to your question I will post a reply ASAP. There is just a butt load of coding, and the <Tags> anno decides to use is anyone's good quess.
Okay, I went through and did a scan throughout the XML files of each Data, Patches, etc.
So far I found one set of values I can edit.
I found them in Patch6.rda
I didn't have the seed list so I went with the seed sowing option.
This is the code I added in
<FertilitySets><Name>Main Plenty River</Name> (I believe this is the option you choose in game settings in the actual game)
(this is the question mark option letting you research other seeds from techs.)
You will see the other fertilies listed, it was a pain but i added like 4-6 to each FertilitySet Group
I'm sure you can replace the Wildcard with the seed name, I just used techs to research the seeds, I verfied that only one question mark will show on the interface but if you move your mouse over the island in the mini map you will see 4-6 question marks.
Also I'm not 100% but for good reason I backed up the orginal XML and copied the modified one into
the file i edited was randommapgenerator.xml this file is under addon0.rda , you need RDA explorer to open this file then find randommapgenerator.xml ,export it to desktop , make the change then import it back and over write the change
unfortunatelly im unable to upload randommapgenerator.xml it wont let me ,
I can't be 100 % sure, but i think i did accidently try it, and it worked. , i edited randommapgenerator.xml file and load save game . Again i can't be sure about that, please dont quot me on this...., you need to backup the randommapgenerator.xml before you try it ,just in case the game crashes .