Answer: There are several aspects to consider in answering your question. First off, lawn (and other plants) will not grow well in a thin or poor soil lacking humus regardless of pH, so you may want to consider improving the soil in terms of adding organic matter and actually building your own top soil in the process.
You will also certainly want to correct the pH to within an acceptable range, and this will be accomplished through adding organic matter which tends to buffer the pH, as well as through possibly adding an acidifier. The amount to use in total would be determined based on the type of soil (eg clay or loam or sandy) you have, how far you want to change the pH and which material you use; in general if there is already a lawn in place you need ot adjust it gradually to avoid shocking or even burning the plants. You may also find that the pH drifts lower on its own over time due to our acidic rainfall just as the excess lime may be leached out of it by rain.
I would suggest you consult with your county extension (677-8223) about the soil on your property and ask their suggestions about dealing with the pH as well as their suggestions on which varieties of grass seed to use in view of the soil conditions you have. The specific type of grass can make a big difference in the degree of lattitude you will have in adjusting the soil and in ongoing maintenance considerations.
You might also want to take a look at a basic lawn care book or two, one I particularly like for its no-nonsense approach is "Lawn Care for Dummies" by Lance Walheim, ISBN0-7645-5077-2, Dummy Press. Good luck with your new lawn!
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