1. UBC Farm Soil Workshop Series
Class 1: Introduction to Soil Formation, Texture and Structure
Chris Thoreau
February 11, 2012

2. What is Soil? 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…

3. What is Soil? Physical Aspects: Minerals (from rocks) Organic Matter
Sand
Silt
Clay and Colloids
Organic Matter
Plants and Roots
Detritus (decaying organic matter)
Animal waste (including microbes)
Pore Space
Air/Gases
Water

4. Soil is the Mother of All Terrestrial Life
What is Soil?
Soil is the Mother of All Terrestrial Life

5. What is Soil? Conceptual Aspects: Habitat Micro-organisms
Bacteria, Fungi – both good and bad
Viruses
Macro-organisms
Worms, Arthropods, Detrivores and Predators
Plants
Small Mammals
Birds

6. What is Soil? 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

7. What is Soil? Habitat Providing for plant life
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?

8. What is Soil?
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…

9. What is Soil?
Questions?

10. Where does soil come from?
Soil Formation
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…

11. 5 Factors of Soil Formation
1. Parent Material 2. Climate 3. Biota 4. Topography 5. Time

12. 5 Factors of Soil Formation
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

13. 5 Factors of Soil Formation
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

14. 5 Factors of Soil Formation
4. Topography
Slope influences soil development
Water infiltration rate
Surface runoff
Vegetation
Aspect
North and South slopes develop differently
Elevation
Climate changes with altitude

15. 5 Factors of Soil Formation
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

16. Weathering of Rocks 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

17. Weathering of Rocks
Weathering of Rocks
Physical
Chemical 1
Chemical 2

18. Soil Formation Processes
Additions
Losses
Transformations
Translocations

19. Soil Horizons

20. Soil Horizons Organic (O) Horizon A Horizon B Horizon C 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

21. Soil Orders of Canada
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. 

22. Soil Orders of Canada

23. Soil Orders of Canada Podzol Gleysol Chernozem Brunisol Luvisol
Organic
Solonetzic
Vertisol
Regosol
Cryosol

24. Podzol Soil
Video here

25. Soil Formation 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

26. Soil Texture
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

27. Soil Texture
Mineral Components
Sand
Silt
Clay

28. Soil Texture
Mineral ratios determine soil texture

29. 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

30. Soil Texture 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

31. Soil Texture 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

32. Soil Texture Clay Clays are categorized by their layer structure
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

33. Soil Texture
2:1 Clay
Shrink and swell
1:1 Clay
No change

34. Soil Texture 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

35. Soil Texture 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

36. Soil Texture 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

37. Soil Texture Attributes of Different Soil Textures Property Sand Silt
Clay
Water
Holding
Poor
Medium to high
High
Nutrient Holding
Medium to High
Aeration
Good
Medium

38. Soil Texture We can determine the texture of the soil by feeling it
Ribbon test
Ball test
Jar test
Laboratory tests give more accurate results

39. Soil Texture
Texture Questions?

40. Soil Structure Soil Structure: How the soil fits together
Primary particles are arranged into secondary particles called aggregates (or peds)

41. Soil Structure Why is Structure Important? Pore space
Air and water movement
Rooting space
Nutrient storage and release
Contributes to soil resilience
Cultivation
Erosion resistance

42. Flocculation + Cementation = Aggregation
Soil Structure
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

43. Soil Structure

44. Soil Structure
Soil Aggregates are classified by their shape

45. Soil Structure 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

46. Soil Structure
Soil structure is particularly important in providing adequate pore space for:
Root growth
Water movement
Gas exchange
Microbial activity
Macrobial activity

47. Soil Structure
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

48. Soil Consistency Related to texture
Very important when considering soil cultivation
Dependent on:
Texture/clay content
Clay type
Soil water content

49. Soil Consistency

50. Soil Consistency 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

51. Soil Consistency Cultivating soil when too wet Compaction
Where to start?!
Compaction
Risk and depth of compaction increases in wet soil

52. Soil Consistency Cultivating soil when too wet
The wetter the soil - the more it acts like water

53. Soil Consistency

54. Soil Consistency
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…

55. Good Soil Structure 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

56. Soil Structure and Texture
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