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Soil | Supply & Demand | Jobs on a Farm | In the Genes | What's the System?


One of the first things you probably learned about plants was that they needed certain things to live and grow — water, sunlight, and soil. Plants use these to live and grow and each has its own job in the larger system. This section takes a closer look at soil.

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.

Photo by Jeff Vangua,
USDA Natural Resources
Conservation Service

What is soil?

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

How is soil formed?

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.

Factors of Soil Production
with Examples and Influences:

Parent Material:

  • organic material
  • partially weathered rock
  • volcanic ash
  • deposited sediment
  • rock moved by glaciers


  • climate affects the number of plants and animals in an area
  • decomposition is slower in cold and dry climates
  • rainfall and water amounts affect the speed and amount of weathering


  • the slope of the land — steep or flat
  • steep slopes can have high rates of erosion and loose new soil quickly

Biological Factors:

  • more organisms causes new soils to have a greater amount of organic material
  • human activity can aid or deter new soil production
  • animals mix and aerate soils
  • tree and plant roots can grow deep causing changes in the lower horizons (layers) of the soil


  • new soil is always being formed
  • the more time each factor is given, the greater the influence on the soil

The Survey Says!

To understand the soil of an area, soil scientists take samples from many locations. These samples are part of a larger study called a soil survey. A soil survey is a carefully planned study of the soils in an area. The scientists examine, describe, classify and map the soils.

A soil profile is one part of a formal soil survey. A soil profile shows and describes the layers of the soil just below the surface of the earth. This profile is one tool used to classify the soils of the area. REAL TREE growers are able to use this classification when deciding what trees to purchase and plant for their crops.

To create a soil profile, pedologists look at a cross-section of the land. In this cross-section, they can observe and describe each layer or horizon they see. There are about five or six identified horizons that the soil scientists look for.

Important Note: The horizons in each area will be different in type and amount. Below shows one example and gives information about five general horizon layers.

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.


Photo by Erwin Cole,
USDA Natural Resources
Conservation Service

The earth has a limited amount of land that is usable for growing crops. Understanding the composition of the soil allows REAL TREE growers to plant trees that will grow well in their area.

Each grower is dependent on the amount and quality of the soil on their operation. The information in this section is just a start to the knowledge that experienced growers use to make good decisions about this very complex natural resource.

a person who studies the science of land cultivation and management

natural processes which cause material to be worn away from the earth's surface

dark organic material in soils produced by decomposition of plant and animal matter

an inorganic element, such as calcium, magnesium, and potassium, that is essential to the nutrition of humans, animals, and plants

of, or made from, living organisms

parent material:
the material in which soils are formed

the top layer of the earth's surface consisting of rock and mineral particles mixed with organic matter

soil profile:
shows and describes the layers of the soil just below the surface of the earth

soil survey:
a carefully planned study of the soils in an area

an assemblage or combination of things or parts forming a complex or unitary whole

any of the chemical or mechanical processes by which rocks exposed to the weather undergo changes and break down

To learn more about the science of soil, download this PDF file from the University of Missouri-Columbia!

Soil Science - Student Guide

Examine the soil in your area! These investigations will help get you started!

A Soil Profile

The Color of Soil

The study of soil science is called Pedology.

There are many career opportunities for soil scientists. Learn more!

Careers in Soil Science

It takes 500 years or more to make one inch of soil.

You can find out more about the soil in your state by visiting this link!

State Soils - USDA

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