Q: We've made a number of changes in our front yard over the past couple of summers — removed several large trees, rebuilt a stone wall, and added a bluestone walkway. The work was hard on our lawn and we've repaired the obvious damage, but with all the trucks and tractors driven around out there, I wonder if the soil has been packed down too much for the grass to grow. How can I tell?

— M. David, Southbury, Connecticut

A: Roger says: You're right — construction traffic is hard on lawns, even when it doesn't actually tear up the grass. You can pretty much assume that your lawn has been compacted, making it hard for roots to get air and water. (Heavy machinery isn't the only culprit: Constant foot traffic can turn soil as hard as pavement.) You can tell whether your lawn is compacted by pushing a long screwdriver into the turf. If it doesn't easily slide in more than 6 inches or so, the soil is too dense. Slow-draining soil is another sign of compaction. To correct this problem, you'll need to rent a core aerator. Be sure you get the kind that removes small plugs of earth from the lawn and scatters them over the grass. The holes it leaves behind provide a way for air and water to reach the root zone. Grass roots fill the holes in a few weeks; the plugs dissolve away after a month or so.

Core aerating is one of the best things you can do for a lawn, even if it hasn't been driven over. I run the aerator over the entire lawn in one direction, then make a second pass at 90 degrees to the first. Immediately afterward, I top-dress the lawn with 1/4 to 1/2 inch of aged leaf compost; if you don't have a compost pile, get it from a garden or local recycling center. I apply compost with a spreader if I can, but if it clogs up, I use my hand to flick the compost off the end of a flat shovel. Next, I scratch it around with a steel rake so it doesn't just sit on top of the grass. In areas where drainage is poor, I add sand.

Aerating is best done in the fall. (In the spring, it encourages crabgrass.) If you aerate and top-dress every fall for three years, you'll notice a substantial improvement in your lawn.
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