Class 1: Introduction to Soil Formation, Texture and Structure

Chris Thoreau February 11, 2012

Soil is a dynamic composition of: • Minerals

• Water and its solutions

• Organic matter (detritus)

• Air and other gas mixtures...

• ...which, through interacting with each other and with plant roots, allows for the growth of photosynthesizing terrestrial plants

• …and acts as a habitat for micro- and macro-organisms …which also interact with plants and plant roots

• Or…

Physical Aspects: • Minerals (from rocks) Sand

Silt

Clay and Colloids

• Organic Matter Plants and Roots

Detritus (decaying organic matter)

Animal waste (including microbes)

• Pore Space Air/Gases

Water

Soil is the Mother of All

Terrestrial Life

Conceptual Aspects:

• Habitat

Micro-organisms

Bacteria, Fungi – both good and bad

Viruses

Macro-organisms

Worms, Arthropods, Detrivores and Predators

Plants

Small Mammals

Birds

Conceptual Aspects:

• Provider to plant life

Rooting substrate

Water holding and release

Nutrient supply and reserve

Heat sink and release

Soil gases

Symbionts

Bacterial and fungal

Insects

Habitat • What happens when we disturb this habitat? At micro and macro level?

• What happens when we make additions to, or removals from, this habitat? Carbon:Nitrogen ratio?

How do soil organisms and plants respond?

Nutrient loss or gain?

Providing for plant life

• What are the short-term and long-term results? • Are we providing for the soil as well as the plants? What is the difference?

As a habitat we need to treat soil like a

living organism, which requires:

• Food

Organic Matter and Minerals

• Water

Irrigation and Natural

• Air

• Shelter

Cover crops, Cash Crops, Mulches

• General tender loving care…

Questions?

Where does soil come from?

Soil comes (mostly) from the weathering of rocks

over long periods of time – a process highly

influenced by biological organisms,

topography, aspect and human activity…

1. Parent Material

2. Climate

3. Biota

4. Topography

5. Time

1. Parent Material • Transported

Gravity - Colluvial

Water – Alluvial, Marine, and Lacustrine

Wind – Eolian

Ice – Glacial

• Cumulose

Due to plant life and anaerobic conditions

High water table

Peat and muck soils

• Residual

In situ; long periods of weathering

2. Climate

• Temperature and rainfall are major factors

Affect intensity of weathering

• Increased T and precipitation accelerate

weathering

3. Biota

• Plants influence organic matter

• Arthropods and worms mix soil; add to OM

• Small mammals also mix soil

4. Topography • Slope influences soil development Water infiltration rate

Surface runoff

Vegetation

• Aspect North and South slopes develop differently

• Elevation Climate changes with altitude

5. Time

• Often noted as most important soil formation

factor

• Our soils in Lower Mainland are relatively

young

Since last ice age 10-12,000 years ago

Primary Minerals Sand and Silt • Formed at high T and P (at depth); anaerobic

conditions • Physically and chemically formed Quartz, Feldspars, Micas,

Secondary Minerals Clay • Come from primary minerals • Formed at low T and P (at surface) with Oxygen

present • Mostly chemically formed Silicate Clays

Weathering of Rocks

Physical

Chemical 1

Chemical 2

Additions

Losses

Transformations

Translocations

Organic (O) Horizon • High in organic residue from

plant drop

A Horizon • Mineral component plus OM

• Most fertile part of soil; location of much root activity

B Horizon

• Subsoil

• A horizon leaches here

C Horizon • Little influence by soil-forming

processes

In a given area, over the period of soil formation,

environmental conditions cause a certain set of soil

processes to occur, which leads to a distinctive set of

soil horizons at the time we observe the soil.

These soil horizons are the basis for classifying the soil

in the Canadian System of Soil Classification.

Podzol Gleysol

Chernozem Brunisol Luvisol Organic

Solonetzic Vertisol Regosol Cryosol

Video here

Any questions?

Check follow up readings and websites

for more detailed information • UBC Virtual Soil Science

• Soil Orders of Canada Videos

• Canadian System of Soil Classification

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

Mineral ratios determine soil texture

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

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