Soil Science Basics

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Soil Science Basics For Evirothon John
Sloan Texas Agricultural Experiment Station Dallas
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Soils/Land Use Envirothon Key Points 1S Recognize
soil as an important and dynamic
resource. 2S Recognize and understand the
features of a soil profile. 3S Describe basic
soil properties and soil formation
factors. 4S Understand the origin of soil parent
materials. 5S Identify soil constituents (clay,
organic matter, sand and silt). 6S Identify and
list soil characteristics (e.g., texture,
structure, etc.) and their relation
properties. 7S Determine basic soil properties
and limitations (e.g., mottling and permeability)
by observing a soil pit or a soil profile. 8S
Understand the nature of plant nutrients, and
how they are held by soil. 9S Recognize the
characteristics of wetland (hydric)
soils. 10S Understand soil drainage classes and
know how wetlands are defined. 11S Understand
soil water, its movement, storage, and uptake by
plants. 12S Understand the effects of land use on
soils. 13S In land use planning discussions,
discuss how soil is a factor in or is impacted by
non-point source pollution. 14S Identify types of
soil erosion and discuss methods for reducing
erosion. 15S Utilize soil information, including
a soil survey.
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1S Recognize soil as an important and dynamic
resource.
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Physical
Chemical
Biological
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Five Crucial Ecological Roles of Soils
Recycling system for nutrients and organic wastes
Medium for plant growth
Habitat for soil organisms
System for water supply and purification
Engineering medium
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• A healthy soil has abundant biological activity.
• Biological activity contributes to beneficial
  chemical and physical soil properties.

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Living Organisms Microorganisms
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Living Organisms Soil Animals

• Nematodes
• Earthworms
• Springtails
• Mites
• Ants and termites
• Millipedes, centipedes

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2S Recognize and understand the features of a
soil profile.
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O
A
E
B
C
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Vegetation has an effect on the formation of the
soil profile
O
A
A1
E
A2
Bt
Bt
C1
C1
C2k
C2
C3
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3S Describe basic soil properties and soil
formation factors.
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Soil Forming Factors

• Parent material (unconsolidated material
• in which soil development occurs)
• Climate (temperature and precipitation)
• Biota (living organisms and
• organic residues)
• Topography (slope, aspect, elevation)
• Time

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4S Understand the origin of soil parent materials.
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Parent Materials Formation, Transportation and
Deposition
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5S Identify soil constituents (clay, organic
matter, sand and silt).
6S Identify and list soil characteristics (e.g.,
texture, structure, etc.) and their relation
properties.
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Whats in soil?
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Soil Color
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Soil Color

• A good indicator of organic matter content
• Light colorlow OM dark colorhigh OM
• Also influenced by parent material
• Compare the red soils of East Texas to the
  Blackland soils of Central Texas

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Soil Texture

• Sand
• Silt
• Clay

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Relative Sizes of Soil Particles
Course Sand
Clay
Silt
Very Fine Sand
Medium Sand
Fine Sand
1 mm
0 mm
2 mm
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Number of particles per soil volume
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Texture

• Texture can be estimated by the ribbon and feel
  method
• long ribbon high clay
• no ribbon sandy soil
• Major influence on water holding capacity
• Sand low water holding capacity
• Clay high water holding capacity
• Silt high available water capacity

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Soil Structure
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Soil structure

• Refers to the binding of soil particles into
  aggregates due to the presence of organic matter,
  clays, and oxides
• Good structure allows movement of water and air
  through the soil and into the rooting zone.

Range of soil structures
Single grained Aggregated
Massive (Sand)
(Silt loam) (brick)
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How to improve soil structure

• Increase soil organic matter with
• Compost
• Manures
• Mulches
• Cover crops (green manures)

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11S Understand soil water, its movement, storage,
and uptake by plants.
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Soil Water

• Mostly affected by soil texture
• Soil water terms
• Gravitational water
• Capillary water
• Hygroscopic water

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Saturation -all pores are completely filled with
water
Field Capacity -most pores are filled with water
except for the largest
Wilting Point -only small pores contain water
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0
Water-filled pore volume
100
Oven Air Wilting
Field
Saturated Dry Dry Point
Capacity
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Field Capacity
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Available water
Wilting Point
Percent Soil Water
10
Unavailable water
0
Sand
Sandy Loam
Loam
Silt Loam
Clay Loam
Clay
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Three-dimensional plot of infiltration rates for
sandy soils
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Three-dimensional plot of infiltration rates for
clay soils
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Sandy textured soils

• Water infiltrates and drains quickly, even when
  wet.
• Soil does not have a large reservoir of plant
  available water.
• Soils dry out quickly, leaving plants
  water-stressed.

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Clay textured soils

• Water infiltrates and drains slowly, especially
  when the soil is already wet.
• The surface can become saturated fairly quickly,
  making the soil susceptible to erosion.
• The surface can become crusted and seal off water
  infiltration.

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8S Understand the nature of plant nutrients, and
how they are held by soil.
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Cation Exchange Capacity(CEC)

• Negative charged sites in soils that attract and
  hold positive ions (cations)
• K, Na, Mg2, Ca2, NH4, Al3, H
• Two types
• Permanent (isomorphous substitution in clay
  minerals Al3 ? Si4 or Mg2 ? Al3)
• pH dependent (functional groups on organic matter
  or broken clay edges -O-H, -COO-H)

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Sources of Cation Exchange Sites
Organic matter (pH dependent)
Clay minerals (permanent)
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Soil pH-A master variable
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Soil pH

• A master variable because it affects a wide range
  of soil properties, including
• Chemical
• Biological
• Physical (indirectly)

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Normal pH range in soils
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Micronutrients
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Soil Acidity

• Soils tend to become more acidic with time and
  weathering.
• What causes soil acidity?

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GENERAL SOIL MAP OF TEXAS
Decreasing Rainfall
Increasing soil pH
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Alkaline SoilspH gt 7

• Calcareous soils
• High in calcium freqently contain calcite
• pH is controlled by calcium carbonate chemistry
• pH range of 7 to 8.4
• Saline and sodic soils
• High in sodium
• pH is controlled by sodium carbonate chemistry
• pH range of 8.4 to 10

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Houston Black Official State Soil of Texas Major
soil in the Blackland Prairie region of Texas.
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Nutrient Deficiencies in Alkaline Soils

• Phosphorus
• Forms insoluble calcium and magnesium phosphate
  minerals at high pH

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7S Determine basic soil properties and
limitations (e.g., mottling and permeability) by
observing a soil pit or a soil profile.
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9S Recognize the characteristics of wetland
(hydric) soils.
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• Wetland Soils
• Organic matter accumulation

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• Wetland Soils
• Oxydation-Reduction characteristics

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10S Understand soil drainage classes and know how
wetlands are defined.
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Relationship between landscape position and
drainage class
Well Drained
Moderately Well Drained
Somewhat Poorly Drained
Poorly Drained
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12S Understand the effects of land use on soils.
14S Identify types of soil erosion and discuss
methods for reducing erosion.
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13S In land use planning discussions, discuss how
soil is a factor in or is impacted by non-point
source pollution.
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15S Utilize soil information, including a soil
survey.