MULTIPLE WAYS: Will Need - Stim Berries, a Phiomia, a compost Bin and Thatch or a Dung Beetle. Process - Put Stim Berries into a Phiomia and have it consume them. Take the Poop created by this process and put 3 of these medium sized poops into a Compost Bin with 50 Thatch and wait for it to produce Fertilizer. Dung Beetles will produce Fertilizer by putting Poop in their inventory.
Just use the toilet glitch. Build a toilet and supply it with a water source. Use it when your constipation is full (your hunger bar changes color). After you’ve done your business, hop off the toilet, flush and you’ll get a fertilizer. But your constipation won’t go away. Hop back on the toilet and repeat. Unlimited fertilizer.