Soils:an introduction

Organic Gardening Certification Program 2009Claudia Groth

What Do Soils Provide?

• Supports the growth of higher plants• Determines the quantity, quality, and

movement of water• Makes organic material available for re

use Recycles organic materials • Provides habitat for diverse organisms• Supports human engineering needs

How Do Soils Form?

• Parent Material• Weather• Topography• Vegetation• Time

Physics

ChemistryBiology

Soil

Soil Components

(CA Fertilizer Association)

Soil Texture

The proportion of sand, silt, and clay

How coarse or fine a soil is

Relative Size of Soil Particles

(University of Nebraska, Lincoln)

% Sand

Soil Triangle

% Clay % Silt

Soil Texture and Water

Determining Soil Texture

• Professional soil test– Laboratory analysis ~ $15

• Soil surveys– Silt loam predominates in Metro area

• Home jar test or “shake method”– Pretty accurate – easy and fun

• Ribbon test– Messy, but interesting

Soil Survey

Texture via “soil shake”

• 1/3 to 1/2 soil

• Fill with water

• Shake 5 minutes

• Measure

Mark, measure each depth

• Sand: 40 seconds

• Silt:30 minutes

• Clay: 24 hours

Soil Textures in Jar Test

The math:

Height of one layerHeight of all layers

e.g. (0.5 / 2.00) X 100 = 25%

Calculate % of each particle

X 100

Using theSoil Triangle

Sand: 45%Silt: 35%Clay: 20%

Loam

45% sand

35% silt

20% clay

Using theSoil Triangle

Sand: 33%Silt: 33%Clay: 33%

Clay Loam

33% sand

33% silt

33% clay

The Ribbon Method

Ribbon Methodsand

silt

clay

Texture via the “feel”method

(USDA)

Soil Texture and Air Spaces

Clay

Sand

Soil Texture and Water

Water-holding Capacity of Different Soils

(inches water per foot depth of soil)

Sandy soil = .75 to 1 inches/ft

Loam soil = 1.5 to 1.75 inches/ft

Clay soil = 2.0 to 2.5 inches/ft

(Adapted from Cooney and Peterson, 1955)

Water Movement in Soils

sandy loam

clay loam

Soil ChemistryEssential Elements

• From air and watercarbon (C) hydrogen (H) oxygen (O)

• Macroelements – Primary nutrientsnitrogen (N) phosphorus (P) potassium (K)

• Secondary elementssulfur (S) calcium (Ca) magnesium (Mg)

• Micronutrientsboron (B) chlorine (Cl) copper (Cu)iron (Fe) manganese (Mn) molybdenum (Mo)nickel (Ni) zinc (Zn)

Essential Elements for Growth

Nutrient Availability

• Microbes alter the form of nutrients• Competition or impact of other

nutrients– forming insoluble compounds

• Soil texture• Amount of organic matter • Temperature of soil • Acidity of soil

Nutrients, Roots and Soil

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_

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_

_

_

__

_

(CA Fertilizer Assoc)

Soil pH and Nutrient Availability

Soil Biology

BacteriaActinomycetes

FungiAlgae

ProtozoaNematodes

Mycorrhizae

Actinomycetes

Rhizobia

In One Gram of Soil – ¼ tsp.

• Bacteria 100 million – 1 billion• Actinomycetes 10 million – 100 million• Fungi 100,000 – 1 million• Algae 10,000 – 100,000• Protozoa 10,000 – 100,000• Nematodes 10 - 100

Soil Food Web

Interacting with Soils -Damaging and Improving Soils

• Can’t (really) change soil texture

• Can improve our soils– By changing structure

• What is structure?– Arrangement of sand, silt, and clay

• Plus, organic matter!!

Just Add Sand??

particle in. percentSand 2.2 20%Silt 6.6 60%Clay 2.2 20%

11.0

10’ x 10’ = 100 sq ft.

particle in. percentSand 3.2 27%Silt 6.6 55%Clay 2.2 18%

12.0

Add 1 in. sand(830 lbs.)

What About Gypsum?

Calcium Sulfate

Gypsum won’t improve drainagein Pacific NW clay-based soils

Gypsum doesn’t change soil pH

Compaction Destroys Structure

Don’t dig in soggy soil

Beware foot or equipment traffic

Interfaces are Barriers

clay soil

“good“ soil

(adapted from CA Fertilizer Assoc)

(“Sustainable Gardening”)

Change in Texture - Interface

Layering impedes water movementand root growth

Application:Don’t add “good” soil over clay

(Virginia Tech)

Adding organic matter directlyUsing organic matter mulches

Growing cover crops

Improving Soil Structure

Adding Organic Matter

• Increases water-holding in sandy soils• Reduces erosion

– increases infiltration on surface

• Mulches conserve moisture• Increases drainage by forming aggregates• Moderates soil temperature for microbes• Provides food for many microorganisms

Adding More Organic Matter

• Most alter the pH only slightly• Minimizes compaction • Adds small amounts of nutrients to the soil• Increases nutrient-holding

– Improves nutrient availability– Reduces leaching losses

Potting/Container Soils

Container vs. Natural Soils

Organic Matter

Solids(incl org mat)

Water

Solids

Air Air

Water

Water Movement in a Pot

Soil

Saturated zone

Soil:Container Relationship

Greenhousecontainer

Color bowl Seedling tray

Here’s what happensActual soil volume for plant roots is reduced

Additional ResourcesThe Soul of Soil:

A Soil-Building Guide for Master Gardeners and Farmers (4th edition)by Grace Gershuny

Soil Biology Primerwww.soils.usda.gov/sqi/concepts/soil_biology/biology.html[November 20, 2007]

Nature and Properties of Soilsby Nyle C. Brady and Ray R. Weil14th edition is most recent

Soils:an introduction

Organic Gardening Certification Program 2009Claudia Groth