1. Unit 2: Soil Physical Properties
Chapter 2

2. Unit 2 Objectives Differences in sand, silt, clay & soil textures
Understand soil structural classes
Importance of soil porosity & aeration
Knowledge of soil color and its importance

3. Soil Texture
Soil Separates – particle size groups of sand, silt, and clay
Proportion of each determines the soil texture
Texture affects water intake rates, water storage, soil tilth, aeration, fertility

4. Soil Texture Soil Textural Classes
Clay – soils that are more than 60% clay
Silt – soils with high silt content
Sand – soils with highest content of sand
Soils that don’t exhibit a dominant area in any of the three called loam
Soil Textural Triangle
Organic matter content has no bearing on these values

8. Soil Texture Particle Size Analysis
How to determine soil textural classification
Stoke’s Law
Settling rates of each of the soil separates based upon its buoyancy, gravity, and resistance to water friction
Placing a soil sample into proper solution, then allowing each soil separate to settle will help determine soil texture

9. Rock Fragments
Particles >2 mm diameter called rock fragments & can be classified by shape
Have no bearing on soil texture
Rounded fragments
Gravel, cobble, stone, boulder
Flat fragments
Channer (smallest), flagstone, stone, boulder

10. Rock Fragments
% of rock fragments in a soil may be used to help describe a soil texture
<15% by volume: no mention
15 to 35% by volume: name the dominant kind of rock fragment (ex. Stony loam)
35 to 60% by volume: add very to the description (ex. Very Stony loam)
>60% by volume: substitute extremely into description (ex. Extremely Stony loam)

11. Soil Structure
Soil Structure – arrangement of particles into aggregates
Aggregates – secondary units composed of many soil particles held together by organic matter, iron oxides, carbonates, clays, etc.
Peds – natural aggregates, vary in water stability (clod is used if soil is broken by artificial means)

12. Soil Structure Fragment – pieces of broken peds
Concretion/Shot – mass of precipitation of certain chemical dissolved in percolating waters
Soil Structural Classes
Peds described by three characteristics
Type (shape)
Class (size)
Grade (strength of cohesion)

13. Soil Structure Types Blocky (angular or subangular) Columnar Granular
Platy
Prismatic

15. Soil Structure Classes Grades Structureless Soils: no noticeable peds
Very fine, fine, medium, coarse, very coarse
Grades
Evaluated by distinctness, stability, & strength of the peds
Structureless Soils: no noticeable peds
Noncoherent mass of sand (single grain)
Cohesive mass such as clay soils around here (massive)
Especially found in lowland wet soils

16. Soil Structure Structured soils
Weak: peds can barely be distinguished
Moderate: peds visible, most can be handled without breaking
Strong: very visible peds, easily handled without breaking
Structure is very important influence on soil properties
What affect might different structures have on soil?
Infiltration of air, fertilizers, & water?

17. Soil Structure Genesis of Soil Structure
Peds form due to shrink/swell of soil & adhesive materials
Mostly 5/6 sided shapes
Prismatic structure tends to develop early in the genesis of soil w/ vertical cracking
More blocky structure will develop as the soil matures (especially in clay soils) due to horizontal cracking

18. Soil Structure Granular peds Platy structure
Tends to be influenced by: tillage, rodents, worms, frost action
Held together by organic matter
Mostly round shapes
Limited to surface horizon
Platy structure
Requires force: water, equipment, livestock

19. Soil Structure Deterioration of Aggregates
Increasing Na+ as exchangeable ions speeds deterioration of soil structure
Disperses ions in the soil, therefore, breaking natural soil bonds
Often forms when water has high salt content, and improper drainage

20. Soil Porosity & Permeability
Pore spaces – portion of the soil not occupied by mineral or organic solids
Often referred to as the soil matrix
Typically occupied by: air, water, living roots
Irregular shape, size, & direction to pores
Which soil has the largest/smallest pores?
How does that affect the soil & crops?

21. Soil Porosity & Permeability
Pore sizes are more important than total pore space
Relative amounts of air & water in pores fluctuates
Rain
Deep percolation
Transpiration
Evaporation

22. Soil Air Free oxygen must be available
Required for root growth (respiration) and by soil microbes for organic matter decomposition
Well-aerated soil is best, w/ rapid, continuous gaseous exchange
Factors affecting gas exchange rates
Pore sizes
Pore continuity
Temperature

23. Soil Air Composition of Soil Air Atmospheric air Depth in the soil
Wetting/drying
Coverings on the soil surface
Composition of Soil Air
Atmospheric air
N2 = 79%
O2 = 20.9%
CO2 = .038%

24. Soil Air Rates of O2 Exchange Soil air
Some O2 used, much CO2 produced
Soil air CO2 may be 10%
Range of O2 values from 10% to virtually none
What type of soil would be on each end of the range?
Rates of O2 Exchange
Oxygen diffusion rate (ODR) – rate at which gases in the soil exchange w/ O2 in the atmosphere

25. Soil Air Factors affecting ODR
Pore size
Water filled pores
Diffusion of CO2 gas through water is 10,000x slower through water than air
Depth in the soil
At ~3’ depth, ODR is ½ to ¼ rate of top few in.
So, how does this affect our high-clay soils?
What does is affect?
What makes the problems worse?
What might improve ODR?

26. Soil Air Oxidation-Reduction Potential (Eh or Redox)
Describes tendency for chemicals in the soil or water to be oxidized
A measure of the availability of O2 in the soil
High redox = O2 is present, low redox = O2 absent
Most plants must have O2 in the soil at root growth
Give an example of a plant that doesn’t

27. Soil Air Most plants grow best in an oxidized (aerated) soil
Free oxygen is the primary acceptor of electrons in the soil
What does this mean?
More soil nutrients stay/converted soil plant available forms
N is not lost to the atmosphere as much
Plant roots are able to respire

28. Soil Air Aeration & Energy for Plant Growth Energy obtained from sun
Stored in chemical bonds (photosynthesis)
Energy released by breaking the bonds (respiration)
w/ O2, aerobic glycolysis plus respiration makes much more energy available to the plant
~19x more than anaerobic glycolysis

29. Soil Air Anaerobic glycolysis
Results in much less energy availability
Decomposition of organic matter is much slower
How do deficient O2 concentrations occur?
Waterlogging
Compaction
High clay soils what pinch pores when wet
O2 consuming organic matter decomposers
What can we do as managers of the soil to improve O2 concentrations?

30. Consistence (Strength)
Consistence – soil’s response to mechanical forces
Resistance to rupture
Soft/hard when dry
Friable (crumbly), firm, rigid when wet
Plasticity
Tolerate considerable deformation w/out breaking
Stickiness
Ease w/ which the soil is manipulated, or even walked on

31. Soil Color Dark soils absorb more heat than light colored soils
Do you think this helps explain some planting date differences?
Just because they’re dark doesn’t mean they’re warmer
Depends on soil moisture as well

32. Soil Color Soil Color vs. Soil Properties
White colors – common w/ salts or lime deposits are present
Mottles (rust colors) – soil may have periods of inadequate aeration
Gleying (bluish, grayish, greenish) – subsoils, prolonged periods of waterlogging
Darker colors – higher levels of organic matter

33. Soil Color Munsell Color Charts
Chart used to help ID soil color accurately
Hue: dominant spectral or rainbow color
Value: relative blackness or whiteness
Chroma: purity of the color (as chroma increases, the color is more brilliant)

34. Soil Temperature Relation of Soil & Air Temp
Net heat absorbed by the Earth = heat lost in form of longwave radiation
Photoperiod – affected by latitude
Soil temp can change by soil depth & time of day
Takes significant air temp changes to change soil temp deeper than 12” (& more than just daily range)

35. Soil Temperature Factors Affecting Soil Temp
Avg. summer & winter soil 3’ rarely differ by more than 9° F
Factors Affecting Soil Temp
How much heat reaches the soil surface
Soil coverings
Plastic mulches
Sun angle
Slope face
Soil

36. Soil Temperature What happens to the heat in the soil (dissipation)
Amount of heat needed to change soil temp = heat capacity
Greatly affected by soil water content
How?
Thermal conductivity – increases w/ soil-water content increasing, decreases as air-filled pores increase
Moist soils resist temp change, but conduct heat readily
Dry soils change temp faster, but conduct heat poorly
What does this mean for the soil, which is better?

37. Soil Temperature Living w/ Existing Temps
Maximizing seed germination & growth
Wheat – 40 to 50° F
Corn – 50 to 85° F
When using anhydrous
Apply when soil 4” is 50° F or less
Reduces N losses
Freeze/thaw
May cause heaving – resulting in death of shallow rooted crops

38. Soil Temperature Modifying Temp Effects
Responsible for bringing stones to the surface in fields
Modifying Temp Effects
If you have crops that are feasible/profitable to do so
Clear plastic surface covers
Increases soil temp faster
Clear plastic mulches
Can speed growth & maturity of sweet corn & strawberries

39. Soil Physical Properties & Engineering
AASHTO & Unified Engineering Soil Classification System
Used by engineers to classify soils based on particle size to determine construction limitations
Atterberg Limits
Liquid limit – relates to the amount of water a soil can retain & not break
Plastic limit – the water content at which a thread of soil can no longer hold together

40. Soil Physical Properties & Engineering
Plasticity Index – difference between liquid limit & plastic limit
Important measures for engineers to be able to understand what the soil will do under various conditions
Helps then understand what moisture needs to be present for effective compacting (make a solid base for roadways, buildings, etc.)

41. Assignment
Assignment 2.1 on WebCT