How Roots Work
Roots are the bridge from soil to plant. They anchor the plant; they store food and water; and they take in and transport water and nutrients from the soil to the rest of the plant. They also improve soil conditions themselves. Roots are valuable even after the plant has died. Channels left by dead roots bring water and oxygen down to the microorganisms living in the soil. Roots also penetrate the subsoil, bringing up minerals that are not normally available in the topsoil.
Roots In Lousy Soil
Plants with poor root systems living in lousy soil are almost always in stress, full time. That means they are much, much more vulnerable to insect attack and the incidence of disease. If these problems surface you must spend time and money addressing them. Plants with healthy root systems seldom have insect or disease problems.
Roots In Ideal Soil
Ideal soil is loose, or friable. The looser the soil in the yard is, the better it drains and the more readily root systems can spread. A plant with an expansive root system has a greater ability to absorb water, oxygen, and nutrients from the soil. You can't see healthy roots, but their vigor is reflected in the hardiness and attractiveness of the plants in your landscape.
As a general rule, an ornamental plant's root system penetrates as deeply into the soil as its leaves rise above the soil. So a 12 inch tall hosta plant probably has roots that go down at least 12 inches. Large shrubs and trees can have roots that are two to three times wider and deeper than the height and breadth of the plant. However, it is very important to note that about 85 percent of a non-woody plant's roots are found in the first 6 inches of soil. A similar ratio is true for trees and shrubs. Plant roots that go deeper are mainly in search of water. This means that you don't have to work organic material into the soil very deeply to get it to the place where it is most needed.