Answer: Over time, and especially if there is a lot of foot traffic, lawn areas can become compacted. Roots have difficulty penetrating compacted soils and water tends to run off rather than being readily absorbed. If your lawn is more than 5 years old, if there's a lot of foot traffic, if water runs off rather than soaking in, and if the lawn is thin and struggling regardless of the amount of water and fertilizer you apply, it may be time to aerate. I recommend using a core aerator (rather than a slicer), and leaving the plugs or cores on the lawn to dissolve with rainwater or water from the hose. After aerating, sprinkle some sand or compost over the lawn. Either material will works its way into the holes the core aerator made, which will help loosen the soil and absorb water. Hope this information helps you decide if, and when your lawn might need aerating.
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