Hello Yeol, I'm enjoying your great mod, and I come up with an idea for making this mod more interesting: add a product chain of fertilizer.

For the use of fertilizer: The fertilizer can increase the production of farms. I think it could be like: 400 units of initial production and 400 units (or maybe higher) production depending on the supply of fertilizer. However, I'm not sure if a setting like this is possible in TPF. If not, I would suggest adding a "natural farm" using no fertilizer for the maps with no fertilizer industry. Since we are using both chemical and organic fertilizers, I suggest assigning the organic one to the rural farm (and extend the end year to 1950), and assigning the chemical one to the crop farm.

For the production of fertilizer: The chemical fertilizer is made from coal (or oil) and mineral product containing specific chemical elements. To reduce complexity, the mineral ore can be replaced by stone or limestone in the game (some types of fertilizer really need the limestone in production). A fertilizer factory is necessary in this case. The organic fertilizer is made from the waste of people and animals, so the raw material can come from cities or from livestock farms. Theoretically, a compost yard is needed, but it can be also neglected and the farm uses waste directly. I think the icon of "Guano" in campaign era 1 mission 4 would be helpful.