Russ Hatz

A close relationship exists between trees and the soils they grow in. Soil is the storage place for water, nitrogen and minerals dissolved in water, all of which are needed by trees to live and grow. The roots, reaching out for water between the soil particles, serve to anchor the tree as well.

Soil is a naturally occurring mixture of mineral and organic ingredients with definite form, structure and composition. The exact composition of soil changes from one location to another. Following is the average composition of the major soil ingredients by volume: o 45 percent minerals (clay, silt, sand, gravel, stones); o 25 percent water (the amount varies depending on upon precipitation and the water-holding capacity of the soil); o 25 percent air (an essential ingredient for living organisms); and o 5 percent organic matter or humus (both living and dead organisms).

As you can see, soil is composed primarily of minerals, which are produced from "parent materials" that are weathered or broken into small pieces. Beyond occasional stones, gravel and other rock debris, most of the mineral particles are called sand, silt or clay. These mineral particles give soil its texture. Sand particles, the largest of the three, feel gritty and can be easily seen with the naked eye. Silt particles feel like flour and are difficult to see with the naked eye. Individual clay particles usually feel sticky when wet and cannot be seen with the naked eye.

Water and air, which together make up about 50 percent of the soil, occupy "pore spaces," the area between the mineral particles. In these small spaces, water and air are available for use by plants. Pore spaces are essential for soil organisms' growth, soil productivity and plant growth.

The final ingredient of soil is organic matter. It is comprised of dead plant and animal material and the billions of organisms that inhabit the soil. Although the smallest component by volume, organic matter is critical to soil health. It is the main source of nitrogen for growing plants.

Forest soils support a great variety of plants and animals, including Douglas-fir, big game species and microorganisms. The type and quantity of such life depends on the soil, climate and rainfall. All these elements are interrelated, and together they make up what is called the ecosystem.

Russ Hatz, a former state staff forester for the Natural Resources Conservation Service in Portland, Ore., is now a cultural resources specialist for the agency. He can be reached via email at russ.hatz@or.usda.gov.

This article appeared in the Northwest Woodlands Magazine, Summer 2001- Published quarterly by the World Forestry Center as a benefit of membership in the Oregon Small Woodlands Association, Washington Farm Forestry Association, Idaho Forest Owners Association and Montana Forest Owners Association.