Fertilizer is a technology that can be researched at an Alchemy Lab. Every upgrade of Fertilizer increases your food production by 10%. Each level requires that the Alchemy Lab and Farm level is the same or higher than the level to be researched e.g. Level 9 Fertilizer requires at least a level 9 farm and level 9 alchemy lab. Level 10 needs 2 Divine insprations (One for the lab and one for the farm).