Answer: When you seed a bare patch, loosen the soil down about six inches, work in some organic matter such as good quality compost, rake smooth, and seed. Keep the area evenly moist until the seed germinates and becomes established. Mow when the grass reaches about four inches high, and cut it high at three inches.
I would suggest you first run some basic soil tests and check the fertility and also the soil pH. This will tell you how much fertilizer to use and if you need to also add lime to adjust the soil pH. Top dressing with organic matter such as compost can also be helpful. If the soil is compacted, you could also consider doing a core aeration which pulls up plugs of soil. Your local county extension should be able to help you with the testing and interpreting the results, and may have suggestions about lawn care as well as hte best specific type of lawn grass to seed.
Correct mowing can also help the lawn stay thicker. Mow high at about three inches and mow often enough that you do not remove more than one third the grass height at a time. In spring, this can mean mowing more than once a week.
Keep in mind also that grass grows best in full sun all day or at least a half a day of sun. In shady areas or areas where there is competition from tree roots for example, you might want to consider using a ground cover instead of lawn grass.
I hope this helps!
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