CULT: Raised Bed Soil Too Light

Greetings.
 
I'd like to make the soil, so called, in the raised bed where I grow  my 
bearded irises more heavy. It is already mostly organic matter, and I have an  
idea it is too light. There is no natural soil in it, and that was the idea  
since my garden soil has a lot of southern blight in it.
 
One end of the bed has more sand and that remains more moist,  with the usual 
problems, and when it is dry it is really light, so I  know sand is not the 
solution. I always add alfalfa when I turn the bed  over, but that does not 
heavy things up much long term. 
 
I was considering pea gravel. I just want the plants to have something to  
get their roots down into that gives more support. Fertilizer and so forth  I 
can handle from above but some of the plants are not as  well anchored as I 
would like. 
 
I was thinking about some fresh clay cat litter. I use the regular clay ph  
5.5 stuff --non-clumping--as one ingredient in my mix for rooting cuttings  of 
this and that--one third each litter, vermiculite, potting soil--a formula I  
got from Art Tucker--- but this is one of those situations where I'd hate  to 
be wrong. Cutting mix is supposed to hold water, of course. 
 
What is the deal on mushroom compost? Did not Bill Smoot say something here  
about ten years ago about mushroom compost being too rich for the bearded 
stuff?  I suppose I should go to the archives and putter about, search term  
"mushroom." 
 
Anyone got any bright ideas? 
 
Thanks.
 
Cordially,
 
Anner Whitehead
Richmond VA USA USDA Zone 7 

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