After looking at your compaction situation, the next issue is whether your soil has sufficient organic matter to be healthy and fertile. Healthy landscape plants need 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 roots that have died, decomposing leaves, decomposing grass clippings, dead worms, dead microbes, and anything else that was alive and then died and can be decomposed.
Soil Under The Lawn - 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 definitely need to add some this season.
Soil Under Landscape Plants
If you rake up all the leaves each fall and bag them for the trash and if you have little or no mulch around your trees, shrubs, and garden plants, you can be sure you have lousy soil because there is little or no organic material there to feed the earthworms and soil microbs.
Testing For Organic Content
It is very difficult to get an accurate laboratory reading of the percentage of organic matter that exists in any home landscape. First of all, the organic matter will vary in volume all over the property. There can be variations in a bed every other square foot.
A poor man’s test is fairly simple. If you have sufficient organic matter in the soil you will have a good population of earthworms and you can count earthworms as we describe in the wildlife section of this file.
You can eyeball your soil for a rough indication of the organic content. Dig a hole in your lawn or garden 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, healthy trees and shrubs or healthy anything. Remember, roots of lawns, trees and shrubs need to go down at least 4 to 6 inches, and 8 to 12 inches is better. If you determine that you are growing your landscape plants in subsoil, then we suggest you consider a major renovation of the soil.