Answer: It sounds as though you did the right thing in adding topsoil and then laying new sod. The roots of the sod should readily grow into the top soil, as long as the sod was kept well irrigated. Browning grass might simply be that your sod contains both cool season and warm season grasses. As the weather changes from winter to spring the warm season grasses will come out of dormancy and the cool season grasses will go dormant. Or, the brown areas could indicate that the sod roots did not make good contact with the topsoil and the grass died out. You can tell by digging a chunk of sod out of one of the brown areas and inspecting the roots. If they never grew beyond the original root mass, it indicates the sod didn't receive enough water; if the roots grew into the top soil, then the browning could indicate winter damage or indicate poor drainage in those areas that have turned brown. It is important for you to know why the sod died so you can correct the underlying problem before reseeding or resodding. After you've determined the cause and taken corrective action you can rake and overseed the brown areas (again!) and then cover the seeds with a light layer of peat moss. This will keep the grass seeds moist and also hide them from the birds so they don't feast on the seeds before they have a chance to germinate.
Best wishes with your lawn!
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