Okay. So out of curiousity, I've been doing some poking around, as well as my own experiment, and have found that some potting mixes and dirt can leech ammonia. NeonFlux is totally right that this can cause a mini-cycle if it's not watched carefully.
I just got finished with my experiment to see which of my substrates was the source of the ammonia. I put one part Organic Miracle Gro Potting Mix and one part water into a glass cup, and then one part Eco Complete Black substrate and one part water into another glass cup. I then filtered the water repeatedly (and patiently) until the water ran clear. I then tested using my ATI test kit. Surprise! The ammonia was coming from the potting mix. The test was off that charts at 8.0+ ppm.
So what did I do wrong?
I never mineralized my soil, or even soaked and drained it like some would suggest before I dirted and capped my tank. I slapped it in my tank haphazardly. This worked for my tank only because I'm okay with the ammonia presence since I'm doing a fishless cycle. However, since you have fish, I'd be weary of using soil that hasn't been rinsed, drained, and processed.
TL : DR (too long, didn't read): If you're using processed/mineralized soil/dirt then you'll be okay. If you're using newly bought soil/dirt out of a bag, then be very wary and diligent with water changes in case of mini-cycle even after seeding your tank.
Last edited by rachleedy; 05-22-2013 at 06:32 PM.