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Soil Texture and Structure
Chris Thoreau
February 24, 2012
Soil texture refers to the relative
amount of sand, silt, and clay found in
a soil
The mixture of these components
affects the feel of the soil as well as
water, nutrient, and pore space
interactions
Mineral Components
• Sand
• Silt
• Clay
Sand
• Largest soil mineral particles (.02 – 2 mm)
• Formed greatly from physical processes
• Spherical/erratic in shape
Sand = little rocks
• Larger pore spaces
Good drainage
• Does not hold a charge
• Difficult to compact
Silt
• Size between sand and clay (.002 - .02 mm)
• Usually physically formed out of sand
• Hold and releases water well
• Flat or round in shape
• Holds very little charge
• Feels soapy
• Carried in moving water
Clay • Smallest soil mineral particle (< .002 mm)
• Holds water very well
• Holds strong negative charge for mineral
adsorption
• Susceptible to compaction
• Platy-/flat-shaped particles
• Various lattice structures
Mineral ratios determine soil texture
Clay • Understanding structure of clay is important for:
Compaction
Water holding
Cation adsorption
Soil cultivation
Clays are categorized by their layer
structure Relationship of Si-tetrahedral and Al-octahedral sheets
2:1; 1:1; 4:1; 5:2
2:1 Clay Shrink and swell
1:1 Clay
No change
Shrink and Swell of Clay
• Interlayer space expands
with increasing water
content in soil
• Space contracts as water
is removed
• Clay can crack when
it shrinks
Why is Texture Important? • Water Infiltration
• Water Storage
• Fertility
• Aeration
• Trafficability
Soil texture knowledge is the key to developing an overall soil maintenance and improvement plan
Notes:
• We do not change the texture of soils
We can change the characteristics of certain textured
soils
• We change soil characteristics through:
Additions of organic matter
In soil and on top of soil
Cultivation practices
Raised beds
Attributes of Different Soil Textures
Property Sand Silt Clay
Water
Holding
Poor Medium to
high
High
Nutrient
Holding
Poor Medium to
High
High
Aeration Good Medium Poor
We can determine the texture of the
soil by feeling it
• Ribbon test
• Ball test
• Jar test
• Laboratory tests give more accurate results
Texture Questions?
Soil Structure: How the soil fits
together
• Primary particles are arranged into
secondary particles called aggregates (or
peds)
Why is Structure Important?
• Pore space
Air and water movement
Rooting space
• Nutrient storage and release
• Contributes to soil resilience
Cultivation
• Erosion resistance
How does aggregate formation occur? Flocculation + Cementation = Aggregation
Flocculation:
• Primary pulled close together (into flocs) by attractive forces (electrostatic forces, H bonding)
Cementation • Primary particles held together by cementing
agents Carbonates; clays; OM; Oxides
Soil Aggregates are classified by their
shape
• Ideal structure: Spheroidal
Typical in A Horizon
Rounded; loose
Granular (porous) or Crumb (very porous)
Greatly affected by soil management OR
mismanagement
Improved with OM additions and microbial activity
Soil structure is particularly important
in providing adequate pore space for:
• Root growth
• Water movement
• Gas exchange
• Microbial activity
• Macrobial activity
Structure can be easily observed in the
soil and structural stability, or aggregate
stability, can be measured in the lab
Structure can be improved, to a point, by
soil cultivation.
Soil cultivation is also a great way to
destroy structure
Related to texture
Very important when considering soil
cultivation
Dependent on: • Texture/clay content
• Clay type
• Soil water content
Cultivating soil when too dry
• Breaks aggregates into small pieces
De-aggregates
Can result in dust
• Very damaging to soil structure
• The drier the soil – the more it acts like
powder
Cultivating soil when too wet
• Where to start?!
Compaction
• Risk and depth of compaction increases in
wet soil
Cultivating soil when too wet
The wetter the soil - the more it acts
like water
Soil consistency, determined greatly
by water content and percentage of
clay, plays a major role in when soil
can be cultivated!
Not as crucial when hand digging…
We promote good structure in soil by: • Minimizing cultivation (especially in sandy soils) and
using appropriate cultivation methods
• Avoiding compaction (especially in clay soils) No tractor in wet soil!
Especially careful with clays
Cultivating at proper soil consistency
• Adding various types of organic matter regularly
• Maintaining a proper pH
• Promoting microbial life – especially fungi
• Always keeping the soil covered Preferably by crops
Soil texture influences soil’s ability to
aggregate • Clay soils aggregate more readily
• Sandy soils have les stable aggregates
Organic matter, plant growth, and
microbial activity all contribute to
aggregate stability
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