Introduction to Soils

Chapter 4:Soil Architecture and Physical Properties

Physical Properties

Physical Properties of the soil will deal with the particle aggregation as well as texture, colloidal properties, surface area, pore spaces, water in the soil and other physical properties.

SOIL COLOR Color plays an important role in

differentiating different conditions of the soil.

Color Classification System

Munsell color charts1) Hue – red to yellow color of the

soil2) Chroma – brightness of the color –

0 neutral gray3) Value – lightness or darkness of

the soil –0 being black

Color can give a clue to: OM content Water content Oxidation states of iron and

possibly manganese oxides.

Dark Soils Usually high in organic matter Rich soils if there is not too much

water

Light Soils Sandy High salts, gypsum or lime

Light Brown to Red Aging soils iron – oxidizing Drainage is fair

Bluish - Gray They have poor drainage Indicates no 02 in soil and could

smell rotten called - Gleying soils

Mottled Colors Indicates a period of water logged

and then drying out

SOIL TEXTURE the amount of sand silt and clay in

the soil The Soil Separates (USDA)

Sand - .05 – 2 mm Silt .05 - .002 mm

Clay less then .002mm

Soil Texture triangle allows the classification of the soil texture

into 12 classes.

Determining soil texture

Ribbon test No ribbon – sandy loam Shorter then 1” – loam, silt, silt

loam 1-2” – Sandy clay loam, silty clay

loam, clay loam 2-3” – Sandy clay, silty clay clay

Measuring the soil separates in a jar

Can one change soil texture?

Surface area considerations:

It is the surface area of the soil that has the ability to hold water and be a site for chemical activity. The greater the surface area the greater the chemical activity and possible nutrient holding ability.

 

Surface area of the soils and its affect

Water holding capacity, fertility and total pore space.

Sand SiltClay

Soil Porosity Total pore space of the soil

Sand has less than clay

Determining porosity %:Wet weight (grams) – dry weight (grams) Soil volume (cm3) X 100

= %

Permeability

How the water moves through the soil

• Sand is rapid and clay is slowTextural types and their characteristics ie

Soil Porosity, Permeabilitya) Clayeyb) Silty

c) Sandy

Soil Structure

Soil Structure is how the soil separates

combine to form larger aggregates

Types of structure

peds natural formation clods caused by tillage

a. Granular- ideal agronomic structure accomplished through correct cultural practices: in the A horizon Minimum tillage, done at the correct time incorporating organic matter and encouraging microbial action

b. Platy- Usually in the lower depths of the A horizon or compacted soils

c. Blocky- in the B horizon large aggregates that slows water penetration

e. Single grain- Structure less – the absence of soil structure.f. Massive- Structure less - concrete

Structure Formationa. Soil adhesives released by fungi and organismsb. Good soil structure can be encourage through

proper cultivationc. Organic matterd. Gypsum e. Culture practices done when soil is at proper

water retentionf. Avoid compaction

1) Equipment2) Reduction of traffic through the

field3) Keep vegetation on the soil

a.    

Soil Consistence Refers to soils ability to resist

crushing and its ability to be molded.

Such as for most soil Loose, Very Friable, Friable, Firm, Very Firm and extremely firm.

Soil Tilth Physical condition of the soil after

tillage- usually the granular form

Compaction is detrimental to good soil tilth-

Compaction causes• Reduced porosity and permeability• Reduced gas exchange• Slow percolation rate and slow drying rate.• Restricted root growth

Soil Particle Density As to the specific weight of the soil

particles.

This is a direct correlation to the parent material and should be between 2.6 to 2.75 g/cm3

Bulk Density Is the weight over volume.

This will determine the amount of pore space in the soil.

Clay – 1.15 Silt – 1.35 Sand – 1.8

Soil Temperature Germination is dependant on proper soil

temperature.

Corn needs 65-70FMost warm season crops need warm soilRoot growth is more abundant when soil temperatures

Soil Characteristics that affect temperature Dark soils warm fast Rocky soils warm fast Sandy soils warm fast Grandular loose soils warm quicker then compacted soils

Formation and Stabilization of Soil

Aggregates:

Physical Chemical Processes: Flocculation of Clays Clays will be the first to join together as

they are attracted to each others negative and positive sides.

Together with humic acid binds together the individual particles acting like glue to bind them together.

Formation and Stabilization of Soil Aggregates:

Both biological and physical processes are responsible for soil aggregates

Biological Processes: Soil organisms – help bind

particles together Earth worms Root exudates Fungal products ie

glomalin Bacteria

OMIs the principle method of soil

aggregate stabilization.