Introduction to Soils

Compliments of:• Elizabeth Sulzman OSU Soil Science

• James Cassidy OSU Soil Science

• Teresa Matteson Benton SWCD

• Heath Keirstead Benton SWCD

Soil is …

Absolutely crucial for life on Earth!

www.pikeconservation.org/SoilEcosystem.htm

5 functions of soil

• Soil anchors and provides nutrients for plants.

• Some animals live in soil; it is a habitat.

• Soil recycles waste through decomposition.

• Soil cleans and stores water.

• Soil has engineering uses.

The five factors of soil formation

• Climate

• Organisms

• Topography/relief

• Parent Material

• Time

Soil = f(cl,o,r,p,t)

Passive Factors

Active Factors

Missoula Floods delivered parent material to Willamette Valley

Landscape positions

SoilO horizon: organic matter, plant and animal litter

A horizon: topmost mineral horizon, darker than B horizon

B horizon: mineral horizon: zone of accumulation

C horizon: weathering soil; little organic matter or life

R horizon: unweathered parent material; rock

• Minerals

• Organic matter

• Water

• Air

Soil has four components

The four components of soil:

Mineral component

• Makes up less than 50% of a “soil”• Varies in chemical composition• Contains particles of several size ranges

(small to really really small)• Depends on the underlying

geology/bedrock

Organic matter• Small constituent by weight, but huge

influence on soil properties

• Made up of partially decomposed plant & animal residues + organic compounds synthesized by soil microbes

O ni

Functions of Organic Matter

1. Stabilizes soil structure, making soil easily managed

2. Increases the amount of water a soil can hold (and availability of the water)

3. Major source of plant nutrients

4. Main food/energy for soil organisms

Soil Water

• Held to varying degrees depending on amount of water and texture of soil

• Not all soil water is available to plants

Soil Air

1. High spatial variability

2. High temporal variability

3. High moisture content (Rh 100%)

4. High CO2 content

5. Low O2 content

Soil Physical Properties

• Color

• Texture

• Structure

• Aggregate stability

Soil Color

Moisture

Mineralogy (calcite, hematite, manganese)

Coatings on particles:

O.M. darkens underlying colors Fe and Al oxides (red & yellow)

Texture–

–

Proportions of sand, silt, and clay

Determines water holding capacity, water availability, nutrient supply capacity…

Loam is a soil texture with optimal sand, silt and clay contents for growing plants.

Clay is a soil particle size and a soil texture.

% c

lay

% silt

% sand

“Big” smaller really small

Sand silt clay

Relative Size Comparison of Soil Particles (fine earth fraction)

Sand Clay

Fine clay has ~10,000 times as much surface area as the same weight of medium grain sand!

Coarse textured soils larger pores

Fine textured soils greater total pore spaceSand Clay

Clay particlemostly negative charged surface

-+

-+

-+

-+

Ca++

-

--

--

----

-- -

-

-

+

-K+

NH4+

NH4+

Ca++

--

Fe++

Influence of Texture

Sand Silt Clay

Water-holding capacity

Aeration

Drainage

Nutrient retention

Low Medium High

Good PoorMedium

Slow Very slowHigh

HighLow Medium

Types of Soil Structure

Why Are Aggregates Important?

• Increase porosity

• Increase water infiltration, drainage, decrease runoff

• Increase water holding capacity

http://www.youtube.com/watch?v=9_ItEhCrLoQ

Effect of OM on aggregate stability

Effect of OM on aggregate stability

Compaction

Uncompacted soil Compacted soil

pore space

soil particles

Pore spaces are where plants get air, water, and nutrients. Soil compaction decreases valuable pore space between soil particles.

Adapted from Sulzman and Frey, 2003

Less GreaterCompaction

Take home message

• Clay soils have a higher water-holding capacity than sandy soils

• Water in coarse textured soils is easier for plants to remove than in fine textured soils

• Much of the water in high-clay soils is unavailable to plants, while most water in sandy soils is available

A cup of soil contains...

Bacteria

Fungi

Protozoa

Nematodes

Arthropods

Earthworms

200 billion

100,000 meters

20 million

100,000

50,000

<1

The immobile ones all primarily found in the rhizosphere, the zone of soil closest to plant roots

Bacteria

Nematodes

Protozoa

Mites

Springtails

Other invertebrates

Worms

Voles!

Ant Colony!

Aggregates held together by:– Fungal hyphae– Bacterial “glues”– Organic matter

sand

silt

hyphaeclay

bacteria

Amoebae

CiliateFlagellate

Nematode

• feed on bacteria and fungi

• release plant nutrients – protozoa KEY for N

Nematode

Microfauna

Collembola (springtails)

Fungus feeding mite

• feed on fungi, protozoa, nematodes, mites• important in regulating populations of everything smaller

Nematode feeding mite

Mesofauna

Photo by Suzanne Paisley

• shred plant material

• feed on bacteria and fungi associated with organic matter

Macrofauna

Earthworms• Important component of

soil fauna (not in acid soils, not in very dry soils)

• Pass as much as 30 tons/ha of soil through their bodies each year

• Casts (poop) are higher in N, P, K, Ca, Mg, pH, and CEC than soil

• Promote good soil structure and aeration

Earthworm casts vs. soil

Characteristic Earthworm casts Soils

% silt & clay 22.2

Structural stability 65

CEC (cmolc/kg) 3.5

38.8

849

13.8

Nitrogen Fixation (bacteria)

• Take N2 from atmosphere, convert to soil NH4+

• Nodules formed on roots

• Examples include:– Rhizobia on legumes– Frankia on alder

via organic matterN N NH4+

SOIL ORGANICMATTER

LivingOrganisms:BIOMASS

Dead tissues

and wastes:DETRITUS

Non-living, non-tissue:

HUMUS

SOM: What it is

Sand Clay

Large surface area means more charge so greater ability to hold water and nutrients

Set up a soil column to see how soil purifies water.

Hands-on FUN!!!

Back to why we might care…

…plants = food

Erosion:

• A process that transforms soil into sediment– Natural (soils form over time in most

settings)– Human-induced (e.g., over-grazing, forest

harvest)

• Tied with damage to plant communities

• Wind vs. water (usually 2/3 by water)

Downward spiral of land degradation

Three steps of water erosion

Most erosion is initiated by the impact of raindrops, NOT by the flow of running water

Wind erosion

• 40% of soil transported by erosion in USA

• In six of the Great Plains states, wind erosion exceeds water erosion

• Fine particles can even be transported to other continents

Current problems

• World population is increasing rapidly

• Only 10% of the world’s land area is suitable for growing crops

• Most of the most arable land is already in production

• Soil quality is degrading world-wide

The real voyage of discovery consists not in seeking new landscapes but in having new eyes.

~Marcel Proust