1. An Introduction to Soil
Bryan McElvany
Research Coordinator
Warnell School of Forest Resources
Patrick Davies

2. This is the stuff under your fingernails
DIRT
This is the stuff under your fingernails

3. Why is soil science intimidating?
Whole new vocabulary
Hard to visualize
Somewhat complex topics like chemistry and physics

4. What is soil? Several different ways to define soil
1) Geologic definition: Loose surface of the earth as distinguished from solid bedrock (support of plant life not required).
2) Traditional definition: Material which nourishes and supports growing plants (includes rocks, water, organic material, air).

5. What is soil made of?
Mineral Matter
Organic Matter
Air
Water

6. What is the function of soil?
Plant medium
Recycling system for nutrients
Animal home
Engineering medium
System for water supply and purification

7. The ability of soil to perform those functions is dependant upon:
Soil Texture
Soil Structure
Soil Chemistry
Soil Age
Site Factors (topography, water, etc)
Etc.

8. Soil Formation Factors
There are five main factors that influence soil formation processes.
Type of Parent Material
Climate
Topography
Biotic Components
Time

10. Parent Material Two major divisions in type of parent material
Residual-Formed in place (Residuum)
Transported-Parent material was moved by:
- wind (Eolian)
- ice (Glacial)
- gravity (Colluvial)
- water:
rivers (Alluvial)
oceans (Marine)
lakes (Lacustrine)

11. Soil Formation Processes
Transformations – modification of soil constituents. Mineral weathering, organic matter breakdown.
Translocations – movement up, down, or laterally of soil constituents.
Additions – movement of soil material in from outside sources. Organic material from leaves, dust from atmosphere.
Losses – movement of soil material out of the soil. Transportation to groundwater, erosion of surface materials.

12. Soil Formation Processes

13. Soil Formation in Action
The soil formation processes, operating under the influence of the soil formation factors, is the driving force in soil genesis.
Important questions to remember are:
What transformations and translocations took place in the parent material to form this soil?
What materials were removed?
What was added?
How did the climate and topography affect those processes over time?

14. Soil Horizons As soils develop they form layers called Horizons.
These horizons have distinct characteristics produced by soil forming processes.
Master horizons include the O, A, E, B, and C horizons.
Not every soil contains every horizon based on the level of development of the soil.
Depth of each horizon varies between different soils.

15. O Horizon
Layer of accumulated organic matter such as leaves, grass, twigs
Material can be in various states of decomposition
Generally dark in color O A E B C R

16. A Horizon
Zone of incorporation of organic matter into the mineral soil
Generally it’s the most productive horizon
High biological activity
Generally dark in color
Topsoil O A E B C R

17. E Horizon E Zone of Eluviation or leaching
Eluviation is the movement of soil material out of a layer of soil
Soluble minerals and organic material move out of this horizon
Generally a lighter “washed” appearance in color O A E B C R

18. B Horizon B Zone of illuviation or deposition
Accumulation of materials “washed” from horizons above
This material is commonly clay, humus, sesquioxides, or a mixture of the three
Variable in color from reds and yellows to browns and grays O A E B C R

19. C Horizon C Parent material
The unconsolidated material that has been affected little by the soil forming processes O A E B C R

20. Soil Texture
Soil Texture is the relative proportion of the primary particles in the soil.
Sand = 2mm – 0.05mm
Silt = 0.05mm – 0.002mm
Clay = < 0.002mm

21. Soil Texture
This proportion of sand, silt, and clay can be grouped into textural classes.
Soil texture has a great deal to do with many other soil characteristics.

22. Soil Texture
Coarse textured soil – soil that has a higher proportion of larger particles. Sand, Loamy sand, Sandy loam.
Fine textured soil – soil that has a higher proportion of smaller particles. Clay, Silty clay, sandy clay.

23. Soil Structure
Soil structure is the arrangement of soil particles into aggregates.
Structureless soils do occur as either single grained or massive.

24. Pore Space
The voids between soil particles are called pores. These can either be filled with water or air.
Soil texture plays a major role in total pore space as well as size of pores.
Coarse textured soils (sandy) have larger pores, but less total pore space.
Fine textured soils (clayey) have smaller pores, but more total pore space.

25. Soil Permeability
Permeability is a measure of how fast air and water can move through a soil.
Soil texture and soil structure both pay a role in soil permeability.

26. Soil Texture and Permeability

27. Soil Structure -Permeability Granular Blocky Platy

28. Soil Water Saturated soil is when that soils pores are full of water.
Gravitational water is that water that moves out of the soil due to gravity. This water is generally in the larger Macro-pores.
Capillary water is that water that is held in the soil due to adhesion and cohesion against the pull of gravity. This water is generally held in the smaller Micro-pores and as a film around soil particles.

29. Soil Water
After a major rain event, once the gravitational water has left the soil, the soil is at Field Moisture Capacity.
The wilting point is reached when soil water levels decline to the point that all remaining water is held too tightly by soil particles to be removed by the plant.

30. Soil Water

31. Available Water Capacity
Available water capacity is a measure of the water available to plants.
Commonly defined as the difference between the amount of water at field moisture capacity and the amount at the wilting point.
This is the water a plant has a chance of utilizing.

32. Available Water Capacity

33. The End
Thanks