Answer: You are correct in that the bagged topsoil can have varying qualities, although a peat-based mix is likely a potting soil or perhaps a "peat humus" product. Since it is often difficult to match the existing topsoil with new purchased soil, it is probably better to improve the soil you have and raise it that way. Generally improving the soil means adding organic matter such as compost, rotted leaves or similar materials.
Patching a lawn area requires several steps, incorporating any needed amendments into the existing soil and then reseeding. The specific amendments would be determined on the basis of soil tests. Your County Extension (765-3500) should be able to help you with the tests and interpreting the results.
However, even before you patch those areas you might need to investigate and fix the drainage problem that caused the erosion in the first place. If you don't, the new soil and grass seed will erode, too. Drainage can be a very tricky thing, so if the problem is at all severe you should consult with a professional drainage and grading expert. Sooner is better than later because this type of probelm tends to get worse over time.
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