Arborists have known for more than a century that trees and shrubs grow poorly in compacted soil. This situation has been exacerbated with the advent of big, efficient, heavy construction machinery.

One step that has been taken to alleviate the soil problem on both residential and commercial properties has been the practice of stripping off the top foot or so of topsoil and stockpiling it. The original topsoil is then redistributed over the site when construction activity is finished. This is certainly a good thing. It works well for turf, flowers and most small plants. However, topsoil redistribution really hasn't solved the soil compaction problem completely, particularly for large shrubs and many species of trees. Researchers in Denmark think they know why. Using a sophisticated new gamma-ray probe, they determined that on every one of the residential and commercial sites in the study there was serious compaction of the underlying subsoil, starting about a foot below the soil surface. It didn't make much difference whether construction had been done years ago or just recently. Although the study is ongoing, these researchers concluded that, after the topsoil was stripped away, heavy construction equipment and other forms of traffic roamed freely on this subsoil base. After all, they had already taken the precaution of preserving the good topsoil, so what harm could come of making heavy use of what was left?

In almost all of the sites used, the condition of the topsoil was good to excellent. Although the roots of most trees and shrubs exist in only the top foot or so of soil, apparently the compacted substratum made planting in such places very much like planting in thin soil over a rock ledge, a condition that limits the successful growth of many landscape trees and shrubs. Gas exchange (including oxygen), percolation, drainage and level of the water table are just some of the conditions that can get adversely affected. Water tends to stand in concave (saucer-like) places on the property because it can't percolate readily in the compacted sub-stratum. This will cause the roots of even smaller plants to drown. Once compacted, it is difficult and expensive to loosen subsoils. Other means must be taken to assure good landscape results. Here are some ideas:

Any soil subjected to construction traffic or activity will be compacted in the process;

Prepare your final landscape plan prior to the start of construction traffic to keep it outside future landscape areas;

Don't depend on topsoil stockpiling as the complete solution to soil preservation.

Be sure to consult your landscape architect and arborist before construction begins. Sometimes, it may be prudent to move valuable specimen plants to another location on site, so both the present and the future planting areas will have to be protected.