Core Aeration technique involves "pulling cores" of soil and turf out of the lawn and laying them on the surface of the lawn. The resulting holes can be left alone to fill back in over time (usually very quickly), or they can be filled with amendments such as compost, sand, fertilizer, stone dust, gypsum or whatever is called for in the soil test. You should aerate prior to overseeding or top-dressing of any material if possible. 

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