But another major issue has to do with the soil. Most urban soils are no longer suitable for adequate root growth. As larger buildings, wider roads and larger and more powerful construction equipment press down on soil surfaces, the overall result is massive soil compaction. Soil Scientist Jim Patterson cites some startling facts. In Washington, DC, he has found that the soils on the Mall stretching between the Capital building and the Washington Monument are so compacted that the average soil density is comparable to a cinder block.
Pore space is the ratio of soil particles to voids where air and water can be found.
While the ideal pore space for growing trees is about 50 percent, the average pore space on the Mall is often less than 35 percent and sometimes drops as low as 13 percent. The effect, Patterson says, is extreme stress on tree roots when soil is either completely wet or dry. Soil oxygen decreases and plant roots congregate in cracks or grow right at the surface rather than penetrating the rock-hard soils. Clearly, we can no longer dig a hole in the pavement and expect a tree to grow. Without attention to rooting needs, the average tree is planted and begins to decline as soon as it outgrows the soil in which it was planted.