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