Question
HI there!

Thanks for taking my question.

We recently had some trees removed from our yard and plan to lay sod.  Prior to the trees being removed, there was a lot of moss and muddy areas that didn't seem to have good drainage (even on the areas that had grass), and we want to take the necessary steps to improve the drainage and amend the soil prior to installing the sod.

I've heard conflicting ideas on the amount of topsoil needed - we're planning to care for the lawn with only organic fertilizers, etc, so it's important to us that the soil is as good as possible.  I've heard that for an organic lawn, about 10 inches of topsoil is needed, however, our landscaper thinks I'm nuts for suggesting that!  He said that he thinks we've got about 3 inches or so already, and that it's already rich in organic matter due to it previously being wooded.  So the sod will add another inch or so, to make 4 inches.  there is also a layer of wood chips from the trees being removed and the chips being spread out.  I assume this will be tilled into the existing soil.

He is going to do some grading, will add lime, and then lay the sod.  I think I've convinced him to at least add a truckload of compost, but wanted to get your opinion about the topsoil.  Is it worth pushing for this and paying the additional cost to get a couple more inches of soil, or do you think it sounds like what we have should be sufficient?  I guess I'm most concerned about the drainage and the soil's ability to support an organic lawn.

I *think* we have a somewhat clay soil - we're in the northeast - zone 6.

Thanks for your help!

Answer
Hi Maggie, I would not add any soil or compost.  I would till the entire area, do whatever grading may be necessary to fix your drainage problem and then lay the sod.  I would not spread the sawdust and chips from the cut tree.  As they decompose they will rob the nitrogen and lower your pH.  You landscaper should check you pH and adjust it to 6.5 if necessary.
The sod you get is probably growing in clay, which is good.  Clay holds nutrients and water well.  By tilling the area, you blend your top soil into the clay for a uniform consistency so the sod has a similar soil to root into. You do not want a loose layer of soil trapped between layers of clay.  It will be difficult to water correctly and prone to climatic stress.
Plants cannot absorb organic nitrogen, the molecule is much too large.  Microbes must reduce it to inorganic nitrogen for the plant to absorb it.  By tilling the existing soil, you loosen the soil for root development and it encourages microbial growth.  Jim