My friend gave me some red bat guano and said to use it for my plants, but being as I have a hydro system, is there a way I could use this at all? Maybe sprinkle some on the rock wool or stir in the rez?

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You can make a tea out of the guano by letting it sit in water overnight and filtering out the particulates. I would do a 10 percent by volume guano to water ratio. After this you will want to PH balance the solution to 5.5 and test to make sure the PPM isnt too high. If its too high you can always add water. Go for a ppm of 500. The guano is going to only supplement your solution- its mainly nitrates so your going to still need other micro and macro nutes, which is why i say keep the ppm low at 500. You can make tea out of worm castings as well- some organic farmers will spray their crops and water with the same ratio, using the same method described above in much larger tanks.