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PHYSICAL PROPERTIES OF SOIL

LEARNING PLAN 4

INTRO TO SOIL SCIENCE

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PHYSICAL PROPERTIES OF SOIL

Soil characteristics: grower can see or feel

Neither chemical nor biological, but both affect them
Greatly affect how soils are used to grow plants or other activities

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EFFECTS OF PARTICLE SIZE

Affects two important features

Specific surface area
Soil pores: number and size

Macropores (aeration pores): large
Micropores: small

Specific Surface Area

Important because reactions occur on surface of soil particles and water is held as film around particles
Soils with greater surface area have more total water films
Clay and silt have largest surface area so hold the most water

Soil pores

Soils high in clay have many small pores, soils high in sand have fewer but larger pores
Macropores create easiest pathways so largely determine the movement of air and water through the soil

As water drains down through the soil, it pulls air in behind it, which occupies the macropores

Micropores retain some water as it drains down through the soil

Water films surrounding adjacent particles merge, holding water in place but excluding oxygen
Generally occupied by water so micropores block movement of air
Sometime larger micropores are distinguished as mesopores (medium-sized pores that hold readily plant-available water)

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SOIL TEXTURE

Most fundamental soil property

Soil is composed of small particles

Result of massive rocks of different mineralogy that have weathered
Particles vary in size, shape and chemical composition

Affects physical and chemical properties that influence crop growth

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SOIL TEXTURE

Determined by the proportion of soil particles

Sand (large)
Silt (medium)
Clay (small)

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SOIL SEPARATES

Division of mineral particles

Used by soil scientists

Consists of three broad classes

Sand (divided into four subcategories)
Silt
Clay

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SAND

Characteristics

Largest of the soil separates
Composed mainly of weathered grains of quartz or other minerals
Particles range in size
Enough sand in a soil creates large pores, so sand improves water infiltration (rate at which water enters the soil) and aeration
Large amounts of sand lower the ability of the soil to retain water and nutrients

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SILT

Characteristics

Medium-sized soil separate
Silt particles are silky or powdery to the touch, like talc
Best ability to hold large amounts of water in a form plants can use
Erodes readily in moving water and wind

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CLAY

Characteristics

Smallest of the soil separates
Consists of tiny, sheet-like crystals
Results from chemical reactions between weathered minerals to form tiny particles of new minerals

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USDA SYSTEM OF SOIL SEPARATES

Separate	Diameter (mm)	Comparison	Feel
Very coarse sand	2.00-1.00	36”	Grains easily see, sharp, gritty
Coarse sand	1.00-0.50	18”
Medium sand	0.50-0.25	9”
Fine sand	0.25-0.10	4 ½“	Gritty, each grain barely visible
Very fine sand	0.10-0.05	1 ¾“
Silt	0.05-0.002	7/16”	Grains visible to eye, silky to touch
Clay	<0.002	1/32”	Sticky when wet, dry pellets hard, harsh

Comparison shows the difference by setting a very coarse sand grain equal to 3 feet in diameter

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TEXTURAL CLASSIFICATION

All three separates are found in most soils
Actual percentages are called soil texture
12 textural classes are shown in the soil triangle

Soil usually consists of more than one soil separate

Amount of sand, silt, and clay in a soil can be measured by mechanical analysis

Determining soil texture

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HOW TO TEST THE TEXTURE

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CHARACTERISTICS OF TEXTURAL CLASSES

Soils can generally be classified as fine, medium, or coarse

Indicative of a number of soil properties

Infiltration –water enters
Percolation –drains through
Water-holding capacity

Modifying soil texture

Impractical, except in small areas

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SOIL TEXTURE AS DEFINED BY SOIL TEXTURAL CLASS

Soil Textural Group	Soil Textural Class	Feel by Hand Texturing
Coarse to very coarse	Sand, loamy sand	Gritty - does not ribbon or leave a stained smear on hand
Moderately coarse	Sandy loam	Gritty – leaves smear on hand, does not ribbon – breaks into small pieces
Medium	Loam, silt loam, silt	Smooth and flour-like, does not ribbon, breaks into pieces about ½ in long or less
Moderately fine	Sandy clay, sandy clay loam, clay loam, silty sandy clay loam, silty clay, clay	Forms ribbon: clays form longer ribbons than clay loams. Clay loam feels gritty

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SOIL STRUCTURE

The arrangement of soil separates into units call soil aggregates

Peds: naturally occurring aggregates
Clods: clumps of soil caused by tillage

Classification traits

Type: shape
Class: ped size
Grade: ped distinctiveness and strength

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SOIL STRUCTURE

Arrangement of soil aggregates into different forms gives structure
Natural processes that aid in forming aggregates:

Wetting and drying
Freezing and thawing
Microbial activity that aids in the decay of organic matter
Activity of roots and soil animals
Adsorbed cations

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FORMATION OF SOIL STRUCTURE

Soil creates a loose ped and the second cements it
Weak aggregates are cemented to make them distinct and strong

Two-step process

Can be degraded by tillage, falling raindrops, and compaction

Structure is not a permanent soil feature

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STRUCTURELESS SOIL

No aggregation of fine particles when dry
Single-grain

Sandy soils and soils without structure

Massive soils

Fine textured soils that lack structure or function as a soil mass
Lack permeability
Destroyed or puddled soils
Example: Rut of tractor that has been stuck in mud

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TYPES OF SOIL STRUCTURE

Commonly found in A horizons
Rounded surfaces

Granular structure

Usually found in E horizons
Rectangular with long vertical dimension

Platy structure

Typical of many B horizons
Cube-like with flattened surfaces

Blocky structure

Tend to occupy lower B and C horizons
Rectangular with long vertical dimension and flattened top

Prismatic structures

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SOIL DENSITY

Particle density

Weight of an individual soil particle per unit of volume

Considers only mineral solids

Average 2.65 grams per cubic centimeter

Bulk density

Dry weight of soil per unit of volume of soil (used for soil quality)

Considers bot solids and the pore space

Actual density of a soil is less than the PD

Higher BD = Lower pore space

above 1.6 = no root penetration

BD= weight dry soil/ volume dry soil = g/cm3

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SOIL PERMEABILITY

Soil porosity

Measure of the soil volume that holds air and water

Porosity = [wet weight (g) – dry weight (g) / soil volume (cm3) ] x 100
Porosity = BD/PD x100

Permeability

Ease with which air, water, and roots move through the soil

Cannot be measured directly

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SOIL CONSISTENCE

Behavior of soil when pressure is applied

Relates to the degree that soil particles stick to one another or other objects

Soils types and characteristics

Wet soil: stickiness and plasticity
Moist soil: loose, friable, and firm
Dry soil: determined by trying to crush an air-dried mass of soil in the hand

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SOIL TILTH

Tillage tends to cause a year-by-year decline in soil structure
Physical condition of top 3in of soil is important

Physical condition of tilled soil

Suggests how easy the soil is to till, a good seedbed can be made, how easily seedlings can come up, and the ease of root growth

Tillage

Improves soil tilth for a time, improving soil air–water relations for new seedling

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COMPACTION

Results when pressure is applied to the soil surface

Primarily alters soil traits related to pores and soil strength

Can create:

Reduced porosity and permeability
Reduced air exchange and potassium uptake
Decreased infiltration rates
Increased erosion and evolution of nitrous oxide
Reduced percolation and oxygen availability

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CAUSES OF COMPACTION

Machine weight

Tire size

Tire inflation

Soil moisture conditions

Raindrops

Natural cause that impacts soil crust

Tillage operations

Continuous moldboard plowing or disking at same depth will cause tillage pans

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PHYSICAL PROPERTY MANAGEMENT

Avoid aggregate destruction

Rototillage harms soil

Stirs oxygen into soil speeding up organic matter oxidation
Tillage tools smash the weakened peds

Avoid puddling and clods

Pressure applied to very wet soil aggregates, they fall apart
Puddling- conversion of aggregated soil into massive soil

Minimize surface crusting
Improve tilth

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SOIL CHANNELS AND PANS

Large, continuous pores extending from the surface and leading deeper into the soil

Channels

Claypans
Fragipans
Plinthite
Caliche
Duripans

Any hardened layer of soil is a pan

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SOIL TEMPERATURE

Critical for plant growth and development
Important factors

Sunlight and air energy inputs
Absorption and conductance of heat
Loss of heat at the surface

Temperature effects

Affects seed germination, root growth, as well as water and nutrient availability and biological activity

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SOIL TEMPERATURE

Managing soil temperature

Natural conditions
Watering and soil manipulations
Mulching
Frost heaving
Inorganic materials and plastic sheeting

Fire and soil temperature

Soil temperature affects soil and plants in natural ecosystems
Fire is present or necessary in many natural ecosystems

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SOIL COLOR

Indicates:

Nutrient composition

Organic matter

Drainage in soils

Color as a guide to soil use

Used to classify soils according to color

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COLOR AS A GUIDE TO SOIL USE

Dark brown to black

Color result from organic matter or dark parent materials
Two situations that dark color arises from:

OM can reach very high levels in soils that are usually waterlogged

Sour, oily smell

OM can also reach high levels in adequately aerated soils

Earthy smell

White to light gray

Color may indicate that chemicals that color soil have leached out
White color may also be due to accumulation of lime, gypsum, and other salts

Light brown, yellow to red

Colors of oxidized iron minerals, similar to rust
Color indicates adequate drainage because ther is enough oxygen in soil to form the oxides

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COLOR AS A GUIDE TO SOIL USE

Bluish-gray

Colors called gley
Color of reduced iron compounds
Presence indicated a lack of oxygen in soil layer

Mottled colors

Different colors
Suggests the soil is waterlogged for part but not all of the year

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DESCRIBING SOIL COLOR

Munsell color system

Surveyor matches soil to standard color chips

Hue- color
Value- lightness or darkness of the hue

Denoted by numbers 0 to 10

0 is black, 10 is white

Chroma- purity of dominant color

Denoted by number
Low chroma suggests muddy color

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SUMMARY

THIS CHAPTER REVIEWED SEVERAL TOPICS

TEXTURE AND TEXTURAL CLASSES

PERMEABILITY

CONSISTENCE

TILTH

COMPACTION

PHYSICAL PROPERTY MANAGEMENT

TEMPERATURE AND COLOR