Question
I came across your response on the internet. I live in So.Cal and have just installed in ground sprinklers. My question is; I have fairly compact decomposed granite where I live. I have been told to water daily, apply 2 inches or so of compost and seed over that. Do I need to rototill the compost into the soil for a strong/healthy lawn. I appreciate your feedback.

Answer
Bonsai sometimes uses Decomposed Granite as a potting medium.  But Bonsai growers don't use Soil, and they don't grow normal plants.  DG is rarely (or never) the best Soil for a landscape.  The California Dept of Transportation pursued answers to this same question last year in a study they arranged along California State Highway 299.  Seems this volcanic Decomposed Granite gets its own shorthand, 'DG', to ease indepth discussion of ways to deal with the problems it causes, including rainwater runoff and erosion.

Cal DOT named this report 'Effect of Compost Incorporation on Infiltration Capacity and Erosion From a Decomposed Granite Road Cut'.  And they learned that 'incorporation of unscreened yard waste compost (24 percent volume/volume) into a DG road cut increased its infiltration capacity':

redorbit.com/news/science/1113349/effect_of_compost_incorporation_on_infiltration_capacity_and_erosion_from/index.html?source=r_science

Which is of course what you want to do.  In spades.

In 2006, one of the winners at a posted California State Science Fair, Benjamin P. Wagner, did research on the effects of Soil on an Earthworms population.  'Decomposed Granite Soil' was tested along with 'Garden Soil', 'Peat Moss' and 'Mulch'.  After 6 weeks, the researchers noticed a loss of 3 Earthworms from the plot in the DG Soil.  However, it was the Peat Moss, NOT the DG, that fared WORST.  

www.usc.edu/CSSF/History/2005/Projects/J1922.pdf

I think this is going to be a difficult -- but very do-able -- project.  After all, the Sierra Foothills are world class winemakers, and the region's signature Soil is DG.

Seasoned Manure and Compost and Peatmoss are typically incorporated.  However, I have a feeling it may not be as simple as spreading a layer of Compost or anything else on top of the DG, or even simply tilling it in, because DG is a challenging material to work with.

There may be additional attention you'll wish you'd given it after a few seasons of less than satisfactory results.  And when you're finished, you may find that even under the best of circumstances you must adjust or alter standard irrigation practices or some other routine; one page posted by University of California at Davis tells gardeners, 'Established Lawns and landscapes do not require daily Summer irrigation except in a few extremely hot inland and desert areas that also have Sandy or Decomposed Granite Soils.'

My recommendation: Bring this problem to your highly educated, very specialized authorities out there, specifically the University of California at Davis, which has all sorts of agricultural and related data at its fingertips and knows how DG responds to X Y and Z.  Maybe start with their Small Farms department:

www.sfc.ucdavis.edu/docs/about.html

Or perhaps you've done this already, and just want me to sign off on their advice.  Sign my name.  They are up close and personal with DG, a rare and challenging substrate as far as gardening goes.  I could make an answer up, but you won't be happy, and I won't sleep at night.  Keep me posted and thanks for writing,

THE LONG ISLAND GARDENER