According to the Department of Horticulture and Crop Science at Ohio State University, thatch is "a layer of organic debris that develops between the soil surface and the grass; it is fibrous in nature, and somewhat spongy. It is made up of old rhizomes, stems, crowns, and roots of the grass plants. If the thatch layer is deep, the lawn will feel spongy underfoot when walking across it. The grasses that form thatch layers include Kentucky bluegrass and some of the red fescues. Take a spade and cut into the turf to determine the depth of thatch. A little thatch is okay, but it becomes a problem as it approaches a half-inch in depth. If thatch is under or nearing a half inch in depth, it can be managed by core aerating. This method pulls up cores of soil that are deposited on the lawn surface. They break down over time, and microbes in the soil sift down through the thatch, degrading it."

Thatch problems

Thatch problems are due to a combination of biological, cultural, and environmental factors. Cultural practices can have a big impact on thatch. For example, heavy nitrogen fertilizer applications or overwatering frequently contribute to thatch, because they cause the lawn to grow excessively fast. Avoid overfertilizing and overwatering.

Despite popular belief, short clippings dropped on the lawn after mowing are not the cause of thatch buildup. Clippings are very high in water content and breakdown rapidly when returned to lawns after mowing, assuming lawns are mowed on a regular basis (not removing more than one-third of the leaf blade). A thatch depth of ¼ inch or less is acceptable. Your clippings are collecting and decomposing at a good rate.

 If you’ve got a ¼ inch or more it’s time to think about thatching or aerating. With a thatch that's too thick you’re inviting unwanted guest and preventing your soil from getting much needed moisture as well as oxygen to your root systems. Thatching is done in many ways. You can do it manually with a rake.

A good wire rake, or a heavier thatch rake will do wonders. 

 You can use a power de-thatcher. Many local gardening suppliers will lease equipment. Keep in mind; when you rent this type of machinery, it will take two or more to transport. If you’re not familiar with operating one you may want to hire some one. There are thatching mower blades you can use in place of your cutting blade. And again, if your not familiar with changing blades on a lawnmower, I highly recommend you get some one else to do it. There’s a lot more involved than what meets the eye. Aeration holes can be very beneficial, but unless your soil is completely compacted where rain water runs off, it’s not necessary to tackle thick thatch. It is, however, great for your soil and your grass roots.

Or you can use the simplest method of all and aerate as you stroll through your yard. Landscape USA makes this possible with a pair of aerator shoes.

There are also decomposition products available at garden supply stores and nurseries that you can spread on your lawn to help decompose thatch by means of microorganisms. Ringer's Lawn Rx ™ is a product that will do this and you won't have to break your back. Thus far, organic thatch control through biological means has proven inconsistent.