As long as it is plain ole "organic potting soil", not organic planting mix. Mix has all those additives you don't want. Many people use Miracle Grow Organic Potting Soil. I have used the Just Natural Organic Potting Soil from Lowes for years in my garden and in the potted planted (lilies) that go in our ponds. I knew it was safe because it's just plain organic soil. So if that is what you have it should be fine. Two things you need to know. One most, not all, organic soils are going to leech heavy in the beginning. What I did was added my soil (1-1/2"), smashed it down as tight as possible, then added barely enough water to make it wet, not soggy or swimming, just squishy wet, smash it down some more, cap with whatever you want (1-1/2"), then place a big plate on top of cap and very slowly fill with water. I ran my filter as soon as filled, use carbon or purgen to help with tannin removal. Then each day for 7 days I drained and filled the tank 100%, by day 4 the tannins were getting less (at least I could see the tank back in the mornings), so keep up the WC's and keep running the filter. On day 8 I drained the tank, filled 1/2 full, planted heavily (from front to back solid), finished filling, turned on lights, ran filter. I did about a 1/3 WC's every couple days for about 2 weeks again to help with tannins and to help remove excess nutrients. Then that was it. Another thing, point two, is you will get gas build up in the soil for about a month, alot for the first 2-3 weeks, so use a BBQ meat fork or something similar that you can go around poking the soil a few times a day. This releases any gas build up from the organic soil breaking down. Eventually this will stop. Something else is you will get diatoms really quickly with all the nutrients the soil releases so don't panic. My tank was fully cycled by day 10 and I put one of my bristlenose pleco's in to take care of things. Plus if you can get some water sprite to plant and leave in for a couple months, this plant will grow like a weed and will also help remove excess nutrients from the water. You can take this plant out in about 3 months and plant something else if you want to. Your swords will love the excess nutrients in the beginning also. I had two 12" amazon swords that I planted and they grew out of the top of my tank within 2-3 weeks. Caution, put the swords where you want them, in soil with their vast root systems you will trash a dirt tank trying to remove them! Crypts will also grow great and usually without melt in dirt. Both swords and crypts can do lower light, which you can upgrade later if you want. Water sprite will also be fine in lower light. Hope this helps.
