Mulm and Humus/Substrate Additives

     According to an article by Jim Kelly (who may still be lurking on this 
     list even now) humus should make a an excellent substrate additive.  
     Humus is the end product of the decomposition of biological materials. 
     According to Kelly it has a high cation exchange coefficient (CEC), 
     meaning the ability to hold onto nutrient cations and give them up to 
     plant roots.
     When I do water changes I siphon mulm and other muck from the bottom, 
     then just pitch the whole mess out into the garden.  This stuff should 
     be equivalent to humus, or nearly so.  Has anyone thought of saving 
     this stuff and accumulating enough of it to add to the substrate of a 
     new tank?  If I save it into a bucket and let it accumulate and        
     settle, the decomposition process should go to completion after a 
     while.  Then it could be mixed into the lower layer of a new substrate 
     What do y'all think?
     Bob_Hoesch at fws_gov