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smelly soil substrates

     I'd like to add a little to Karen's observations and precautions on 
     selecting soils for the aquarium. On the weekend I was looking for 
     some top soil to incorporate into my 75 gallon tank which I'm redoing. 
     At a local nursery/garden center I found a pile of nice black soil for 
     sale; U shovel, 3.50 / bag about 75 lbs (humpfffhh) and it was called 
     garden soil. Since it tends to rain a lot, the stuff was pretty water 
     logged. Took it home and opened the bag, -yuck-! a powerful H2S 
     stench! I took it back for a refund. Yesterday I cruised another place 
     and found the same smelly stuff also labelled "garden soil" but 
     another pile nearby was labelled top-soil and it did not have the 
     nasty smell. I also found out after talking with one of the nursery 
     specialists that this soil works fine in gardens and is dug up by a 
     local company from a bog (first clue). What makes this soil go "sour"
     (get H2S)? it is a heavy clay soil, it has a lot of organic material 
     in it and it is submerged. It also probably doesn't have the benefit 
     of aquatic or semi-aquatic plant roots growing through it to oxygenate 
     the soil. I'll also bet that it has a fair amount of sulfates and not 
     enough iron in it. My feeling is that our local BC soils are very low 
     in iron content AND they are also very heavy clays. Iron compounds 
     will react with H2S and form FeS.
     Moral: use your nose when getting soil. Avoid a sour smelling soil. It 
     already has high H2S content and that's definitely going to inhibit 
     the growth of your aquatic plants.
     A strong smell could also be ammonia and this is not really a problem 
     (although any soil with enough manure to smell of ammonia is probably 
     too rich to be used without mixing with sand, gravel or vermiculite)
     The ammonia smell is easy to learn; it's the smell of manure or just 
     take a whiff from the ammonia cleaner bottle.
     H2S smell is the smell of natural gas. It is a petro chemical smell. 
     Your nose can detect H2S in concentrations in parts per billion.
     When you take up an aquarium pot or soil, I wouldn't be overly 
     concerned with a mild ammonia smell but an H2S smell means you have 
     too much organic material, probably too much sulfate (possibly from 
     fertilizers made from sulfates) and not enough vigorous plant growth 
     to keep the redox potential of the substrate at a safe level. (recall 
     the plants provide oxygen in the substrate through their roots)
     As for the use of chemical fertilizers like osmocote: short term 
     evaluations probably won't show problems like sulfer accumulation. The 
     less sulfer in the fertilizer the better IMHO if you intend to put it 
     in the substrate.
     Steve in Vancouver where the sun has finally put in an appearance! :-)