Grass does not grow well in compacted soil. The spaces between particles of soil that normally store air and water are so compressed in compacted soil that little air or water is present. Microorganisms cannot reproduce and generate the nutrients needed by grass plants. Grass roots, seriously deprived of these vital elements (air, water, and nutrients), are badly stunted; the grass is stressed. There are a number of ways to determine if the soil under your turf is so seriously compacted that it needs to be repaired.
Screwdriver Test for Compaction
Test for compaction by inserting a large screwdriver into the turf when the soil is dry. If it is hard to push the screwdriver down into the soil, that suggests a compaction problem. It is relatively easy to push a screwdriver into uncompacted soil that is dry.
Look For Tree Surface Roots
Perhaps one of the most obvious indicators of serious soil compaction is tree roots that are exposed in the lawn. Normally a tree's feeder roots range through the top 4 to 6 inches of soil seeking air, water and nutrients. This is, of course, exactly the same area where the grass roots are looking for nutrients, air and water. The limited amount of air in compacted soil not only stunts grass roots, but it forces tree roots to come toward the surface of the soil in search of oxygen. So the presence of tree roots bulging above the soil surface signals compaction of the soil. See the file “Dealing With Surface Tree Roots” for help in solving that particular problem.
How Much Organic Material In Your Soil?
After looking at your compaction situation, the next issue is whether your soil has sufficient organic matter to be healthy and fertile. Healthy grass needs a soil that provides sufficient nutrition for those plants. Healthy soil teams with microbial and earthworm activity which generates nutrients in the form that plants can best utilize. It is organic matter that feed the bacteria and earthworms. No organic matter, no soil microbial life.
Organic matter is that stuff that will decompose. It includes old dead roots, decomposing leaves, decomposing grass clippings, dead worms, dead microbes, and anything else that was alive and then died and decomposed.
If you have not left your grass clippings on the lawn with every mowing and if you have not added any amendments such as peat moss or compost to your lawn in the past five years, you can assume that your soil is quite devoid of organic matter and that you definately need to add some this season.
You can eyeball your soil for a rough indication of the organic content. Dig a hole in your lawn down about 12 inches. Most organic matter is very dark in color so the darker the upper layer of topsoil (with some exceptions we won't worry about at this moment) the more likely you have a generous percentage of organic matter. If the top soil is the same light color as the subsoil, the dirt down at the bottom of a one foot deep hole, then you can suspect that you've used up most of the organic materials or worse, the topsoil was removed when the house was built and never replaced by the contractor.
Ideally the 3 to 5% organic matter, or humus, is distributed throughout the top 12 inches of soil. So if all the topsoil is in the top two inches and all the rest of that 12 inch layer is subsoil, you will have a problem growing healthy grass. Remember, those roots need to go down at least 4 to 6 inches, and 8 to 12 inches is better. If you determine that you are growing grass in subsoil, then we suggest you consider a major renovation of the soil.