Substrate questions

Hello everybody,

I've been following this list for quite sometimes now and found it very 
useful.  I think it has more info than all the books I've read about 
maintaining a planted tank. 

I've read many posts about the different substrate.  However, I 
think I'm planning to go about using topsoil mixing w/ vermiculite, a 
layer of sand and finally covered w/ gravel.  I have this one thing about 
topsoil and the possibility of contanminations and toxin in the 
topsoil.  I've seen a bag of topsoil at my local Eagle store but it 
doesn't say anything about content of the soil.  Is there a way to 
sterilze the topsoil???

Would somebody please enlighten me about this question and maybe 
suggest other possible substrate material?