Soil doesn't come "one-size-fits-all." Each species of plant needs the soil it lives in to meet its specific needs in order to grow successfully. This is true for all plants — from the tallest trees to the tiniest sprouts. Understanding this is important for all of us, but especially those who cultivate plant crops. Finding a good combination of soil and tree species can mean the difference between a healthy tree crop or a poor one.

REAL TREE growers need to learn all they can about the soils available on their operations. They must become agronomists. Growers use the work of pedologists (those who study the soil) and their own knowledge of how trees and soils work together to make good decisions about what species of conifers to plant and how to cultivate what they have planted.

Soil is the mineral and organic material on the surface of the earth. It is a valuable natural resource that has formed over hundreds of years and continues to form each and every day.

Soil is a complex mixture of sand, silt, clay, and bits of decaying animal and plant tissue. The ingredients in a soil define what type of soil it is and what is able to grow in it.

Photo by Lynn Betts,
USDA Natural Resources
Conservation Service

Soil production takes a long time. Over hundreds and hundreds of years, forces and factors work together to create the soil that we use for agriculture (and most other activities on earth). It's the organization of those factors and forces that creates different types of soils.

New soils begin with some type of parent material. Most often, this parent material is made up of materials (sediment) that has been moved by wind, rain, glaciers, and other forces.

In some soils, parent material is created as natural forces begin to break down rock and materials on and near the surface of the earth. This weathering process is caused by many things — wind, rain, and freezing temperatures or plant, animal, chemical, and human activity.

Over hundreds of years, weathering and erosion continues to slowly change parent materials into smaller and smaller particles. These particles sift and filter to create layers on the Earth's surface. Pedologists call these layers horizons.

The type and number of plants and animals living in an area plays a big part in the formation of new soils. As they go through their life cycles, organisms add organic materials to the make up of the soil.

Plant roots move through soil layers opening places for water and air to collect. These spaces affect the quality of the soil created. Other organisms digest the food they find there and mix the soil with their movements. Both plants and animals then become part of the soil as they decompose after dying.

Humans also affect soil production. Building, agriculture, and other human activities cause changes to the soils by adding or changing chemicals, changing parent materials, and changing the rate of erosion.

The physical features (or topography) of the land also play a role in the type of soil that is created an area. The topography of an area affects how much moisture is held in the soil and the erosion patterns of an area. The quality and development of soil is very much effected by these factors.

Look at the profile to the right. Four horizons are shown.

The O horizon is the topmost horizon. It contains quite a bit of living material and humus — plants, decaying leaves, needles, moss, and such. This is a thin horizon and is often very dark in color.

The A horizon is below the O horizon. It is made mostly of minerals and is the location of quite a few plant roots. This layer is also dark in color due to the amount of humus located there.

The B horizon is below the A horizon. It's also known as the subsoil layer. This horizon usually is lighter in color and has less organic material than the layers above it (O and A). The B horizon is described by the kind and amount of minerals found within it.

The C horizon is below the B horizon. It contains some parent material that has been slightly weathered. Because this layer is usually deeper than the layers above it, the material in this layer is less weathered.

The R horizon is the lowest horizon. It is the layer of bedrock — a solid rock layer.

Those working on a soil profile look at each visible layer. They observe, study, test, measure and describe the thickness, texture, and make-up in each layer present. This information is included as part of the soil survey of the area.