Soil Science
Soil Categories
• Topsoil
• The upper most layer found in the soil profile.
• Contains living and non-living material.
• Animals, bacteria, plants, minerals, soil particles and organic matter.
• Subsoil
• Second layer found in the soil profile.
• Contains much of the same material as topsoil but lacks the
organic matter found in the topsoil.
• Parent Material
• Last layer found in the soil profile.
• Made of different rock or plant formations
• Limestone, Sandstone, Garnet, and Peat
Soil Formation
• Soils are formed from the weathering of the parent materials.
• There are two types of weathering that affects soils.
• Physical Weathering
• Parent material is directly exposed to the weathering affects of the
atmosphere.
• Water, Ice, Snow, Wind, Waves and Gravity
• Chemical Weathering
• Effects of atmospheric chemicals or biologically produced chemicals
on the parent material.
Soil Makeup
• Soil is made up of four major components.
•
•
•
•
Sand
Silt
Clay
Minerals
Soil Makeup
• Sand
•
•
•
•
Largest of the soil components.
Range in size of 2.0 to .05 mm.
Drains rapidly.
Course in texture.
Soil Makeup
• Silt
• Second smallest of the soil components.
• Range in size from .063 to .002 mm.
• Can be a soil, a suspended sediment in surface water, or found on
the bottom of a body of water.
• Moderate drainage.
Soil Makeup
• Clay
• Smallest of the soil components.
• Formed from chemical weathering of other soil components.
• Drains very slowly.
Soil Makeup
• Minerals
• There are 16 nutrients that are essential for plant growth and
reproduction that are found in soil.
• Carbon, Hydrogen, Oxygen, Nitrogen, Phosphorus, Potassium, Sulfur,
Calcium, Iron, Magnesium, Boron, Manganese, Copper, Zinc,
Molybdenum, and Chlorine.
Soil Ecosystem
• Relationship
between the living
and nonliving
material found in the
soil.
• An example of this
can be found in the
Nitrogen cycle
Land Use Classification
• Agricultural land can be classified as eight different land
capability classes, which determines the final three land use
classes.
• The three land use classes are:
• Row Crop – used to produce many different agriculture products
• Pasture – used for the production of grazing animals and forage
production
• Forest – used for the production of trees and wildlife habitat.
Land Capability Classes
• The eight land capability classes allow for the classification of
the land and their end use.
• Row Crops
•
•
•
•
Class I – used for row crops with few limitations
Class II – used for row crops with some limitations
Class III – used for row crops with many limitations
Class IV – used for row crops with major limitations
Land Capability Classes
• The eight land capability classes allow for the classification of
the land and their end use.
• Pasture
• Class V – areas that are level, but subject to flooding, Used for
pasture, grazing
• Class VI – Limitations prevent row crops, used for pasture
Land Capability Classes
• The eight land capability classes allow for the classification of
the land and their end use.
• Forests/Wildlife
• Class VII – these soils have severe limitations, used for forests and
wildlife.
• Class VIII – these soils have severe limitations used only for forests
and wildlife.
Factors that affect Land
Capability
• There are seven factors that affect the land capability class of
a given area.
• Each of these factors places limitations on the areas for it’s
end use
•
•
•
•
•
•
•
Slope
Topsoil Thickness
Erosion
Topsoil Texture
Permeability of the Subsoil
Drainage
Effective Depth
Factors that affect Land
Capability
• Slope
• The measured amount of rise or fall on a piece of land over a
given distance.
• Slope is measured over a distance of 25’, 50’ or 100’ and is
expressed in percent.
• Categories of slope
Land Use Category
•
•
•
•
•
•
Nearly Level
Very Gentle
Gentle
Sloping
Strongly Sloping
Steep
0-2%
2-5%
5-8%
8-12%
12-17%
Over 17%
Class I
Class II
Class III
Class IV
Class VI
Class VII
Factors that affect Land
Capability
• Topsoil Thickness
• is the measured amount of topsoil found on the site. It can be
found by measuring down to the color or texture change.
• Categories of Topsoil Thickness
Land Use
•
•
•
•
•
Extremely Thick
Very Thick
Thick
Moderate
Thin
Over 40”
Class III
20 to 40”
10 to 20”
5 to 10”
Less than 5”
Class I
Class I
Class I
Class II
Factors that affect Land
Capability
• Erosion
• The removal of soil by natural or man made means
• Erosion is expressed as a percent of the original topsoil which
has eroded.
Original topsoil thickness
Measured topsoil thickness
Eroded topsoil = 3”
12”
9”
3” of eroded topsoil ) 12” of original topsoil = 25%
erosion
Factors that affect Land
Capability
• Erosion
• Categories of Erosion
• None to Slight – Less than 25% eroded
• Moderate – 25 to 75% eroded
• Severe – Over 75% eroded
Land Use
Class I
Class II
Class III
Factors that affect Land
Capability
• Topsoil Texture
• Determined by the size of the soil particles in the topsoil.
• Fine - smooth and sticky when wet, particles feel fine as flour. When
balled in the palm of your hand, it holds the shape and shows finger
marks, a long ribbon of soil can be formed by rubbing the soil
between the thumb and fore finger.
• Medium – medium texture soil is a mixture of sand, silt, and clay
particles. It is between fine and coarse. The ball will show some
finger marks and hold its shape. A short thick ribbon can be formed.
• Coarse – coarse textured soil are made up of mostly sand particles.
Sand feels gritty and particles are large enough to be easily seen.
The ball breaks in your hand and almost no ribbon can be formed.
• All Topsoil Texture class are land use class I.
Factors that affect Land
Capability
• Drainage
• Refers to the surface ad internal drainage. The best clue to soil
drainage is color.
• The color of most subsoils is determined by the amount of iron
compound.
• When the soil is aerated, the iron reacts with the oxygen giving
the soil a yellow or red color. In soils that are not well aerated
the soils will have a gray color or mottled with gray.
• The more gray mottles found in the soil profile the slower the
drainage.
Factors that affect Land
Capability
• Drainage
• Categories of Drainage
Land Use
• Excessively Drained – Course texture, sandy material
continues for a depth of 40”
Class III
• Well-drained – No gray mottles in top 30”
Class I
• Moderately well-drained – No gray mottles in top 20”
Class II
• Somewhat poorly drained – No gray mottles in top 10” Class III
• Poorly Drained – Gray matrix or gray mottles found in
top 10”. Soil may be gray completely to the surface.
Class IV
• Very wet – Surface water remains for extended
periods of time.
Class V
Factors that affect Land
Capability
• Effective Depth
• The depth that plant roots can easily penetrate. Usually the
combined thickness of the topsoil and subsoil, measured to the
parent material.
• Categories of Effective Depth
Land Use
•
•
•
•
Deep - Over 40”
Moderate – 20 to 40”
Shallow – 10 to 20”
Very Shallow Less than 10”
Class I
Class II
Class III
Class IV