I actually just wrote an entire article about it that you can read here if you want more info on the process:
KNF Fermented Plant Juice
But in a nutshell, I use almost anything that grows in my yard. It depends on what usage I want for it.
For figs specifically, I use fig fruits at all stages of growth, espcially if I have some I don’t enjoy the taste of for whatever reason. I’ve used an FPJ adjacent type ferment of about finger thick fig roots that made an excellent cutting soak for rooting.
(This one requires an adjustment to the usual method though.) You can use fresh fig leaves as well, but that’s harder if you don’t want to sacrifice them.
I use any weed that grows in my yard, except for poisonous ones or nightshade type. This can be used on any plant, including figs. Mugwort is a great one! Nettle, dandelion, comfrey, so many others are good for it. The one I just made is from fresh tumbleweed. Generally you want the newest growth and cut the tips for FPJ, but you can use the whole plant as well.
For my vegetable garden, I use tomatoes in FFJ to make a seed soak and to fertilize tomato seed starts during the winter. Same with melons, peppers, squash, greens, berries, whatever you grow that is fresh. Typically, you want to use tomatoes and pepper FFJ on their own kind, same with citrus and grapes. But the others can be used on most plants.
You can do it with fresh from your garden root vegetables as well, such as beets or carrots, but you need to chop them up very small, almost pulverize them. Some feel that ferments made from roots benefit the roots on your plant.
You can make FPJ with seaweed or fresh water plants/weeds as well.
*Never use anything store bought for FPJ/FFJ.*
You can also do it by plant quality that you want to confer over to the plants you are growing. For example, I live in a dry, drought area. So I look for plants that thrive naturally in that situation and make FPJ (or JLF) out of them for my plants that aren’t adapting as well to the dryness here. Same with winter hardy plants. It’s not that they magically can then tolerate those same circumstances in the same way. But they can share the native microbes, enzymes, and nutrients that help with those characteristics.
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I wasn’t quite sure what type of info you were wanting. The other ingredient is brown sugar.
Edited to add: I forgot aloe! I’ll be making an FPJ out of aloe as soon as the rainy weather stops here so I can gather it. I’ll use this on rooting cuttings as well.