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CHARACTERIZING SOILAg 1 Unit 1: Soil Science
Objectives
Outline the processes involved in soil formation
Describe a mature soil profile Discuss the eight land capability
classes Define soil series and explain
how those differ from land capability classes
What Is Soil?
Defining soil is not as simple as it seems.
How do you define soil? We will consider soil to be the
layer of organic material on earth’s surface containing both organic and inorganic matter that is capable of supporting plant life.
What Is Soil?
Supports our buildings, catches and stores our water and provides us with food, fiber and forest products.
So durable it can last as long as our species survives.
So fragile it can be destroyed over night
Complex. Once destroyed it can’t be replaced.
Soil Components
Inorganic material- 45-48%
Organic matter- 2-5%
Water- 25% Air-25%
Air
Water
InorganicMatter
Oraganic
Ideal Soil
½ air, ½ water ½ organic, ½ inorganic
Soil Formation
Slow Results from natural forces
acting on the mineral and rock portions of the earth’s surface
Parent Material
Materials that soil is formed from Five general categories
Minerals and rocks Glacial deposits Loess deposits Alluvial and marine deposits Organic deposits
Minerals
Solid InorganicChemically uniformCommon minerals that soils form from: Feldspars, micas, silica, iron oxides, calcium carbonates
Mica
Rocks
Not chemically uniform Made up of minerals Classified into three groups
Igneous Sedimentary
Almost ¾ of the earths surface Metamorphic
Glacial Deposits
During the Ice Ages, glaciers moved taking surface material with them.
Much of Midwestern US is covered by glacial deposits
Loess Deposits
Generally thought of as wind blown silt
Much of the soil in the eastern Mississippi Valley are loess soils
South Dakota- 1934
Alluvial & Marine Deposits
Alluvial deposits-left by fresh water Occur when water fans out after
traveling down hill and then slowing down on more level areas
Often in the form of floodplains and deltas
Marine deposits- formed on an ancient ocean floors
Organic Deposits
Occur in marshy or swampy areas where plant life is lush
When plants die they fall in the water and decay slowly
Eventually builds up and forms muck or peat soil
Peat & Muck Soils
Peat soils are made up of recognizable plant materials
Muck soils are more completely decayed so that plant parts are no longer recognizable
In the case of lakes the deposits may become thick
Weathering
Major types Temperature change Water action Plant roots Ice expansion Mechanical grinding
Soil Organic Matter
Two basic Original tissue: that portion of the
organic matter that remains recognizable
Humus: organic matter that is decomposed to the point that it is not longer recognizable
Soil Organic Matter Functions Acts as a cementing agent for soil
particles Returns plant nutrients to the soil,
most notably phosphorous, sulfur and nitrogen
Helps store soil moisture Makes soil more tillable Provides food for soil
microorganisms, which make the soil capable of plant production
SOIL PROFILE
Soil Profile
Horizons- visibly distinct layers in the soil A Horizon: Top and surface soil
B Horizon: Subsoil C Horizon: parent material
Soil Profile
C Horizon
Except for peat and muck soils consists of weathered rock and minerals
Looks like the rock it came from May be soft enough to dig in or to
crumble. Very few plant roots
B Horizon
More thoroughly weathered Often a different color Contains little organic matter May be much finer and harder packed Plant roots begin to appear
A Horizon
SurfaceHigh organic matter contentMay be grayish, brownish or blackish in color as a result of humus content
Most productiveMost plant nutrients are available here
PHYSICAL PROPERTIES
Slope
Single most important factor in determining the productive potential of the soil
Affects productive potential in many ways Runoff rates Relates to the danger of soil erosion Affects the use of farm machinery Size and shapes of fields
Texture
The proportions of sand, silt and clay
Course soils have more sand Fine soils have more clay.
Soil Drainage
How well water flows through the soil naturally
Well drained soils-excess water moves out quickly
Poorly drained soils- hold excess water
Flood Hazard
How likely a field will receive flood damage
Frequent flood plains do not have good productive potential
Example: James River Bottom
Erosion
Refers to the degree that the soil has already been damaged.
May range from none to severe
Top Soil & Subsoil Thickness
Refers to the depth of each layer that is available for plant root growth.
Very thin top and subsoil’s limit crop production
LAND CAPABILITY CLASSES
Land Capability Classes
Categorize the productive potential of the soil
System begins with class 1 (most useful) to class 8 (least useful)
Class 1
Very good, productive land Nearly level Well drained Medium textured Not subject to erosion or flooding Easily cultivated Very little land is class 1; even in the
most productive farming areas!
Class 2
Good for all types of farming Has some limitations
Gentle slope Suffers from the effects of past erosion Less than ideal soil depth May also have drainage problems that
can be corrected with the use of drainage systems
Requires careful soil management to prevent erosion damage
Class 3
Moderately good land May be cultivated and farmed regularly but it has
some important limitations Moderate slope; making it high susceptible to
erosion damage May already have severe erosion damage Drainage problems Shallow rooting zone
Some shallow wetlands are Class 3 if they can be improved with drainage systems
Should be farmed with care to prevent erosion damage
Class 4
Severe limitations Can be cultivated with careful soil
management practices May have strong slope and be subject to
severe erosion May be very shallow May be very dry or very wet
If wet even drainage systems may still allow occasional water logging
Class 5
Nearly level Little erosion hazard but may be in a
frequent floodplain May be very wet, very dry or very rocky Swampy areas are generally class 5 Quite suitable for pasture, wildlife habitat
or forest production
Class 6
Serious limitations May have very rocky or very shallow
unusable surface soil. Can be used for tree production,
permanent pasture or wildlife habitat
Class 7
Severely limiting properties May be very steep May be severely eroded with deep gullies Very strong slope with course soils may
cause land to be rated at a class 7 May be impractical for establishing
pasture but native grasses may be utilized with controlled grazing
Land may be used for forest production, wildlife and recreation
Class 8
Has one or more extreme limitations Examples of Class 8 land
Sand dunes River washes Stream beds Lake bottoms Other nearly barren areas
Soil Classification
NRCS and USDA use a soil taxonomy
They are broken down into:OrderSuborderGreat GroupSubgroupFamilySeries
SOUTH DAKOTA’S STATE SOIL
Houdek
Deep, well drained, loamy soil, Represents many soils formed in South Dakota
under the influence of prairie grass The dark color of the surface layer is a result of
decayedplants and other materials that have been
deposited overthousands of years. Of major economic importance to SD because
it is used as crop or rangeland.
South Dakotas State Soil
Designated as the State Soil in 1990 Has been mapped on about 600,000
acres. Total Land Area of SD is 75,885 sq mi. or
48,566,400 acres Average annual precipitation is 22 inches, Average annual air temperature is about
48 F.
South Dakota’s State Soil
Houdek Soil Profile
Houdek Soil ProfileSurface layer: dark grayish brown loamSubsoil - upper: dark grayish brown clay loamSubsoil - middle: grayish brown clay loamSubsoil - lower: light olive brown clay loamSubstratum: light yellowish brown clay loam
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