Occasionally lawns will suffer from hard compacted soils or excessive thatch layers, but it is not very common. The process of dethatching and core aeration are both serious causes of stress to lawns. Therefore, unless these cultivation practices are necessary, its best to avoid them.
Thatch is the layer of dead and living shoots, stems, and roots that develop between the green vegetation of your lawn and the soil. It is not caused by grass clippings, which are primarily water and nutrients and are best left on the lawn after mowing to return to the soil. Instead, thatch builds up when organic matter is produced more quickly than it is decomposed. This is caused by a lack of earthworms which break down organic matter, using very high rates of nitrogen fertilizer annually or when growing Kentucky bluegrass which produces rhizomes that do not break down rapidly.
It's easy to tell if your lawn needs to be dethatched. Using a soil probe, bulb planter or shovel cut out a small core of soil and measure the layer on top of the soil surface. If the thatch is greater than half an inch, then proceed with dethatching; otherwise it's not necessary.
If, in fact, you do need to dethatch your lawn a vertical mower or dethatching implement, which features blades instead of spring tines, are available at rental businesses. Run the equipment across the lawn in several different directions and remove the organic material that is pulled up. You can prevent future thatch problems by reducing the amount of fertilizer you apply; most cool-season lawns only need 2 to 3 lbs of nitrogen per 1,000 square feet per year.
Also, maintain your soil pH in the proper range to increase microorganism activity which breaks down thatch. Start by taking a soil test.
Finally, don't apply insecticides to lawns to kill earthworms -- you really do need those.
Aerifying soil is all about relieving compaction. Soil compaction is possible when a force is applied to the soil surface and soil particles become compressed. This can happen when foot traffic from pets or kids or heavy equipment runs across the lawn (especially after a rain). Compaction is also more likely in newer neighborhoods where topsoil was removed during construction.
The easiest way to check for compaction is to insert a knife into a moist (not wet) soil. If you can insert the knife with just the force of your thumb, the soil is likely not compacted sufficiently to worry about. Dry soils (powdery when crushed in your hand) are always harder than moist soils, so do not perform this test without sufficient soil moisture. Proper aeration in the soil is necessary for root growth, gas exchange and support of soil microbes.
To relieve compaction, rent a core aerifyer which has hollow tines or spoons and removes soil cores from the top 2 to 3 inches of soil and drops those cores on the soil surface. This will help to decrease soil compaction and increase water infiltration, water percolation and soil aeration. The cores left on the soil surface will breakdown with rain. Run the equipment in several different directions and use only if the tines are reaching down to 2 to 3 inches. Shallow holes will do very little to relieve compaction.
For questions about either of these lawn care steps contact the Cooperative Extension Service at the Christian County office.
Kelly R. Jackson is the Christian County Extension Agent for horticulture. He can be reached at 270-886-6328 or visit Christian County Horticulture online at www.christiancountyextension.com.